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Suspension is the system of
tires A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
, tire air, springs,
shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most ...
s and linkages that connects a
vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...
to its
wheel A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be ...
s and allows relative motion between the two. Suspension systems must support both road holding/ handling and ride quality, which are at odds with each other. The tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the road or ground forces acting on the vehicle do so through the contact patches of the
tire A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
s. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and
rear suspension Suspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. Suspension systems must support both road holding/ handling and ride ...
of a car may be different.


History

An early form of suspension on ox-drawn carts had the platform swing on iron chains attached to the wheeled frame of the carriage. This system remained the basis for most suspension systems until the turn of the 19th century, although the iron chains were replaced with the use of leather straps called thoroughbraces by the 17th century. No modern automobiles have used the thoroughbrace suspension system. By approximately 1750, leaf springs began appearing on certain types of carriage, such as the
Landau Landau ( pfl, Landach), officially Landau in der Pfalz, is an autonomous (''kreisfrei'') town surrounded by the Südliche Weinstraße ("Southern Wine Route") district of southern Rhineland-Palatinate, Germany. It is a university town (since 1990 ...
. By the middle of the 19th century, elliptical springs might additionally start to be used on carriages.


Modern suspension

Automobiles were initially developed as self-propelled versions of horse-drawn vehicles. However, horse-drawn vehicles had been designed for relatively slow speeds, and their suspension was not well suited to the higher speeds permitted by the internal combustion engine. The first workable spring-suspension required advanced metallurgical knowledge and skill, and only became possible with the advent of
industrialisation Industrialisation ( alternatively spelled industrialization) is the period of social and economic change that transforms a human group from an agrarian society into an industrial society. This involves an extensive re-organisation of an econo ...
. Obadiah Elliott registered the first patent for a spring-suspension vehicle; each wheel had two durable steel
leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...
s on each side and the body of the carriage was fixed directly to the springs which were attached to the
axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...
s. Within a decade, most British horse
carriage A carriage is a private four-wheeled vehicle for people and is most commonly horse-drawn. Second-hand private carriages were common public transport, the equivalent of modern cars used as taxis. Carriage suspensions are by leather strapping ...
s were equipped with springs; wooden springs in the case of light one-horse vehicles to avoid
taxation A tax is a compulsory financial charge or some other type of levy imposed on a taxpayer (an individual or legal entity) by a governmental organization in order to fund government spending and various public expenditures (regional, local, o ...
, and steel springs in larger vehicles. These were often made of low-carbon steel and usually took the form of multiple layer leaf springs. Leaf springs have been around since the early
Egyptians Egyptians ( arz, المَصرِيُون, translit=al-Maṣriyyūn, ; arz, المَصرِيِين, translit=al-Maṣriyyīn, ; cop, ⲣⲉⲙⲛ̀ⲭⲏⲙⲓ, remenkhēmi) are an ethnic group native to the Nile, Nile Valley in Egypt. Egyptian ...
. Ancient military engineers used leaf springs in the form of bows to power their
siege engine A siege engine is a device that is designed to break or circumvent heavy castle doors, thick city walls and other fortifications in siege warfare. Some are immobile, constructed in place to attack enemy fortifications from a distance, while oth ...
s, with little success at first. The use of leaf springs in
catapult A catapult is a ballistic device used to launch a projectile a great distance without the aid of gunpowder or other propellants – particularly various types of ancient and medieval siege engines. A catapult uses the sudden release of stor ...
s was later refined and made to work years later. Springs were not only made of metal; a sturdy tree branch could be used as a spring, such as with a bow. Horse-drawn carriages and
Ford Model T The Ford Model T is an automobile that was produced by Ford Motor Company from October 1, 1908, to May 26, 1927. It is generally regarded as the first affordable automobile, which made car travel available to middle-class Americans. The relati ...
used this system, and it is still used today in larger vehicles, mainly mounted in the rear suspension. Leaf springs were the first modern suspension system, and, along with advances in the construction of roads, heralded the single greatest improvement in road transport until the advent of the
automobile A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...
. The British steel springs were not well-suited for use on
America The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
's rough roads of the time, so the Abbot-Downing Company of
Concord, New Hampshire Concord () is the capital city of the U.S. state of New Hampshire and the county seat, seat of Merrimack County, New Hampshire, Merrimack County. As of the 2020 United States census, 2020 census the population was 43,976, making it the third larg ...
re-introduced leather strap suspension, which gave a swinging motion instead of the jolting up-and-down of spring suspension. In 1901, Mors of
Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. Si ...
first fitted an automobile with
shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most ...
s. With the advantage of a damped suspension system on his 'Mors Machine', Henri Fournier won the prestigious Paris-to-Berlin race on 20 June 1901. Fournier's superior time was 11 hrs 46 min 10 sec, while the best competitor was Léonce Girardot in a Panhard with a time of 12 hours, 15 minutes, and 40 seconds.
Coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
s first appeared on a
production vehicle Production vehicles or production cars are mass-produced identical models, offered for sale to the public, and able to be legally driven on public roads ( street legal). Legislation and other rules further define the production vehicle within part ...
in 1906 in the Brush Runabout made by the Brush Motor Company. Today, coil springs are used in most cars. In 1920, Leyland Motors used torsion bars in a suspension system. In 1922, independent front suspension was pioneered on
Lancia Lambda The Lancia Lambda is an innovative automobile produced from 1922 through 1931. It was the first car to feature a load-bearing unitary body, (but without a stressed roof) and it also pioneered the use of an independent suspension (the front slidi ...
, and became more common in mass market cars from 1932. Today, most cars have
independent suspension Independent suspension is any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump on the road) independently of the others. This is contrasted with a beam axle or deDion axle system ...
on all four wheels. The part on which pre-1950 springs were supported is called a dumb iron. In 2002, a new passive suspension component, the inerter, was invented by Malcolm C. Smith. This has the ability to increase the effective
inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
of wheel suspension using a geared flywheel, but without adding significant mass. It was initially employed in
Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship, ...
in secrecy, but has since spread to wider motorsport.


Difference between rear suspension and front suspension

Any four-wheel-drive (4WD/AWD) vehicle needs suspension for both the front wheels and rear wheels, but in two-wheel-drive vehicles there could be a very different configuration. For
front-wheel drive Front-wheel drive (FWD) is a form of engine and transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitu ...
cars A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, people in ...
, rear suspension has few constraints, and a variety of
beam axle A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have als ...
s and
independent suspension Independent suspension is any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump on the road) independently of the others. This is contrasted with a beam axle or deDion axle system ...
s are used. For
rear-wheel drive Rear-wheel drive (RWD) is a form of engine and transmission layout used in motor vehicles, in which the engine drives the rear wheels only. Until the late 20th century, rear-wheel drive was the most common configuration for cars. Most rear-wheel ...
cars A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, people in ...
, rear suspension has many constraints, and the development of the superior, but more expensive
independent suspension Independent suspension is any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump on the road) independently of the others. This is contrasted with a beam axle or deDion axle system ...
layout has been difficult.
Four-wheel drive Four-wheel drive, also called 4×4 ("four by four") or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer ca ...
often has suspensions that are similar for both the front and rear wheels.


History

Henry Ford Henry Ford (July 30, 1863 – April 7, 1947) was an American industrialist, business magnate, founder of the Ford Motor Company, and chief developer of the assembly line technique of mass production. By creating the first automobile that ...
's
Model T The Ford Model T is an automobile that was produced by Ford Motor Company from October 1, 1908, to May 26, 1927. It is generally regarded as the first affordable automobile, which made car travel available to middle-class Americans. The relati ...
used a
torque tube A torque tube system is a power transmission and braking technology that involves a stationary housing around the drive shaft, often used in automobiles with a front engine and rear drive. The torque tube consists of a large diameter stationary h ...
to restrain this force, for his differential was attached to the
chassis A chassis (, ; plural ''chassis'' from French châssis ) is the load-bearing framework of an artificial object, which structurally supports the object in its construction and function. An example of a chassis is a vehicle frame, the underpar ...
by a lateral
leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...
and two narrow rods. The torque tube surrounded the true
driveshaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connec ...
and exerted the force to its ball joint at the extreme rear of the transmission, which was attached to the engine. A similar method like this was used in the late 1930s by
Buick Buick () is a division of the American automobile manufacturer General Motors (GM). Started by automotive pioneer David Dunbar Buick in 1899, it was among the first American marques of automobiles, and was the company that established General ...
and by Hudson's ''bathtub car'' in 1948, which used helical springs that could not take fore-and-aft thrust. The
Hotchkiss drive The Hotchkiss drive is a shaft drive form of power transmission. It was the dominant means for front-engine, rear-wheel drive layout cars in the 20th century. The name comes from the French automobile manufacturer Hotchkiss, although other maker ...
, invented by Albert Hotchkiss, was the most popular rear suspension system used in American cars from the 1930s to the 1970s. The system uses longitudinal leaf springs attached both forward and behind the differential of the
live axle A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have als ...
. These springs transmit
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
to the frame. Although scorned by many European car makers of the time, it was accepted by American car makers, because it was inexpensive to manufacture. Also, the dynamic defects of this design were suppressed by the enormous weight of U.S. passenger vehicles before the implementation of the Corporate Average Fuel Economy (CAFE) standard. Another Frenchman invented the
De Dion tube De Dion rear axle A de Dion tube is a form of non-independent automobile suspension. It is a considerable improvement over the swing axle, Hotchkiss drive, or live axle. Because it plays no part in transmitting power to the drive wheels, it is ...
, which is sometimes called "semi-independent". Like true independent rear suspension, this employs two
universal joint A universal joint (also called a universal coupling or U-joint) is a joint or coupling connecting rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges ...
s, or their equivalent from the centre of the differential to each wheel. But the wheels cannot entirely rise and fall independently of each other; they are tied by a yoke that goes around the differential, below and behind it. This method has had little use in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
. Its use around 1900 was probably due to the poor quality of tires, which wore out quickly. By removing a good deal of
unsprung weight The unsprung mass (colloquially unsprung weight) of a vehicle is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them. This contrasts with the sprung mass (or weight) supported by th ...
, as independent rear suspensions do, it made them last longer. Rear-wheel drive vehicles today frequently use a fairly complex fully-independent,
multi-link suspension A multi-link suspension is a type of vehicle suspension with one or more longitudinal arms. A wider definition can consider any independent suspensions having three control links or more multi-link suspensions. These arms do not have to be of e ...
to locate the rear wheels securely, while providing decent ride quality.


Spring, wheel, and roll rates


Spring rate

The spring rate (or suspension rate) is a component in setting the vehicle's ride height or its location in the suspension stroke. When a spring is compressed or stretched, the force it exerts, is proportional to its change in length. The ''spring rate'' or ''spring constant'' of a spring is the change in the force it exerts, divided by the change in
deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...
of the spring. Vehicles that carry heavy loads, will often have heavier springs to compensate for the additional weight that would otherwise collapse a vehicle to the bottom of its travel (stroke). Heavier springs are also used in performance applications, where the loading conditions experienced are more significant. Springs that are too hard or too soft cause the suspension to become ineffective – mostly because they fail to properly isolate the vehicle from the road. Vehicles that commonly experience suspension loads heavier than normal, have heavy or hard springs, with a spring rate close to the upper limit for that vehicle's weight. This allows the vehicle to perform properly under a heavy load, when control is limited by the
inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
of the load. Riding in an empty truck meant for carrying loads can be uncomfortable for passengers, because of its high spring rate relative to the weight of the vehicle. A race car could also be described as having heavy springs, and would also be uncomfortably bumpy. However, even though we say they both have heavy springs, the actual spring rates for a racecar and a truck are very different. A luxury car, taxi, or passenger bus would be described as having soft springs, for the comfort of their passengers or driver. Vehicles with worn-out or damaged springs ride lower to the ground, which reduces the overall amount of compression available to the suspension, and increases the amount of body lean.
Performance vehicle {{Unreferenced, date=June 2019, bot=noref (GreenC bot) A performance car is a car that exhibits above-average capabilities in one or more of the following areas: acceleration, top speed, cornering and braking. It is debated how much performance is ...
s can sometimes have spring rate requirements other than vehicle weight and load.


Wheel rate

Wheel rate is the effective spring rate when measured at the wheel, as opposed to simply measuring the spring rate alone. Wheel rate is usually equal to or considerably less than the spring rate. Commonly, springs are mounted on control arms, swing arms or some other pivoting suspension member. Consider the example above, where the spring rate was calculated to be 500 lbs/inch (87.5 N/mm), if one were to move the wheel (without moving the car), the spring more than likely compresses a smaller amount. If the spring moved , the lever arm ratio would be 0.75:1. The wheel rate is calculated by taking the square of the ratio (0.5625) times the spring rate, thus obtaining 281.25 lbs/inch (49.25 N/mm). Squaring the ratio is, because the ratio has two effects on the wheel rate: the ratio applies to both the force and the distance traveled. Wheel rate on independent suspension is fairly straightforward. However, special consideration must be taken with some non-independent suspension designs. Take the case of the straight axle. When viewed from the front or rear, the wheel rate can be measured by the means above. Yet, because the wheels are not independent, when viewed from the side under acceleration or braking, the pivot point is at infinity (because both wheels have moved) and the spring is directly inline with the wheel contact patch. The result is often, that the effective wheel rate under cornering is different from what it is under acceleration and braking. This variation in wheel rate may be minimised by locating the spring as close to the wheel as possible. Wheel rates are usually summed and compared with the sprung mass of a vehicle to create a "ride rate" and the corresponding suspension
natural frequency Natural frequency, also known as eigenfrequency, is the frequency at which a system tends to oscillate in the absence of any driving force. The motion pattern of a system oscillating at its natural frequency is called the normal mode (if all pa ...
in ride (also referred to as "heave"). This can be useful in creating a metric for suspension stiffness and travel requirements for a vehicle.


Roll rate

Roll rate is analogous to a vehicle's ride rate, but for actions that include lateral accelerations, causing a vehicle's sprung mass to roll. It is expressed as torque per degree of roll of the vehicle sprung mass. It is influenced by factors including but not limited to vehicle sprung mass, track width, CG height, spring and damper rates, roll centre heights of front and rear, anti-roll bar stiffness and tire pressure/construction. The roll rate of a vehicle can, and usually, does differ front-to-rear, which allows for the tuning ability of a vehicle for transient and steady-state handling. The roll rate of a vehicle does not change the total amount of weight transfer on the vehicle, but shifts the speed and percentage of weight transferred on a particular axle to another axle through the vehicle chassis. Generally, the higher the roll rate on an axle of a vehicle, the faster and higher percentage the weight transfer on that
axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, beari ...
. By 2021, some vehicles were offering dynamic roll control with ride-height adjustable
air suspension Air suspension is a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. Unlike hydropneumatic suspension, ...
and adaptive dampers.


Roll couple percentage

Roll couple percentage is a simplified method of describing lateral load transfer distribution front to rear, and subsequently handling balance. It is the effective wheel rate, in roll, of each axle of the vehicle as a ratio of the vehicle's total roll rate. It is commonly adjusted through the use of
anti-roll bars An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) whee ...
, but can also be changed through the use of different springs.


Weight transfer

Weight transfer during cornering, acceleration, or braking is usually calculated per individual wheel, and compared with the static weights for the same wheels. The total amount of weight transfer is only affected by four factors: the distance between wheel centers (wheelbase in the case of braking, or track width in the case of cornering), the height of the center of gravity, the mass of the vehicle, and the amount of acceleration experienced. The speed at which weight transfer occurs, as well as through which components it transfers, is complex, and is determined by many factors; including, but not limited to: roll center height, spring and damper rates, anti-roll bar stiffness, and the kinematic design of suspension links. In most conventional applications, when weight is transferred through intentionally compliant elements, such as springs, dampers, and anti-roll bars, the weight transfer is said to be "elastic", while the weight which is transferred through more rigid suspension links, such as A-arms and toe links, is said to be "geometric".


Unsprung weight transfer

Unsprung weight transfer is calculated based on weight of the vehicle's components that are not supported by the springs. This includes tires, wheels, brakes, spindles, half the control arm's weight, and other components. These components are then (for calculation purposes) assumed to be connected to a vehicle with zero sprung weight. They are then put through the same dynamic loads. The weight transfer for cornering in the front would be equal to the total unsprung front weight times the G-force times the front unsprung center of gravity height divided by the front track width. The same is true for the rear.


Sprung weight transfer

Sprung weight transfer is the weight transferred by only the weight of the vehicle resting on its springs, and not by total vehicle weight. Calculating this requires knowing the vehicle's
sprung weight Sprung mass (or sprung weight), in a vehicle with a suspension, such as an automobile, motorcycle, or a tank, is the portion of the vehicle's total mass that is supported by the suspension, including in most applications approximately half of the ...
(total weight less the unsprung weight), the front and rear roll center heights, and the sprung center of gravity height (used to calculate the roll moment arm length). Calculating the front and rear sprung weight transfer will also require knowing the roll couple percentage. The roll axis is the line through the front and rear roll centers that the vehicle rolls around during cornering. The distance from this axis to the sprung center of gravity height is the roll moment arm length. The total sprung weight transfer is equal to the
G-force The gravitational force equivalent, or, more commonly, g-force, is a measurement of the type of force per unit mass – typically acceleration – that causes a perception of weight, with a g-force of 1 g (not gram in mass measur ...
times the sprung weight times the roll moment arm length divided by the effective track width. The front sprung weight transfer is calculated by multiplying the roll couple percentage times the total sprung weight transfer. The rear is the total minus the front transfer.


Jacking forces

Jacking forces are the sum of the vertical force components experienced by suspension links. The resultant force acts to lift the sprung mass, if the roll center is above ground, or compress it, if underground. Generally, the higher the
roll center The roll center of a vehicle is the notional point at which the cornering forces in the suspension are reacted to the vehicle body. There are two definitions of roll center. The most commonly used is the geometric (or kinematic) roll center, wher ...
, the more jacking force is experienced.


Other properties


Travel

Travel is the measure of distance from the bottom of the suspension stroke (such as when the vehicle is on a jack, and the wheel hangs freely) to the top of the suspension stroke (such as when the vehicle's wheel can no longer travel in an upward direction toward the vehicle). Bottoming or lifting a wheel can cause serious control problems, or directly cause damage. "Bottoming" can be caused by the suspension, tires, fenders, etc. running out of space to move, or the body or other components of the car hitting the road. Control problems caused by lifting a wheel are less severe, if the wheel lifts when the spring reaches its unloaded shape than they are, if travel is limited by contact of suspension members (See Triumph TR3B.) Many
off-road vehicle An off-road vehicle, sometimes referred to as an overland or adventure vehicle, is considered to be any type of vehicle which is capable of driving on and off paved or gravel surface. It is generally characterized by having large tires with de ...
s, such as desert racers, use straps called "limiting straps" to limit the suspensions' downward travel to a point within safe limits for the linkages and shock absorbers. This is necessary, since these trucks are intended to travel over very rough terrain at high speeds, and even become airborne at times. Without something to limit the travel, the suspension bushings would take all the force, when suspension reaches "full droop", and it can even cause the coil springs to come out of their "buckets", if they are held in by compression forces only. A limiting strap is a simple strap, often from nylon of a predetermined length, that stops downward movement at a pre-set point before theoretical maximum travel is reached. The opposite of this is the "bump-stop", which protects the suspension and the vehicle (as well as the occupants) from the violent "bottoming" of the suspension, caused when an obstruction (or a hard landing) causes suspension to run out of upward travel without fully absorbing the energy of the stroke. Without bump-stops, a vehicle that "bottoms out", will experience a very hard shock when the suspension contacts the bottom of the frame or body, which is transferred to the occupants and every connector and weld on the vehicle. Factory vehicles often come with plain rubber "nubs" to absorb the worst of the forces, and insulate the shock. A desert race vehicle, which must routinely absorb far higher impact forces, might be provided with pneumatic or hydro-pneumatic bump-stops. These are essentially miniature shock absorbers (dampers) that are fixed to the vehicle in a location, such, that the suspension will contact the end of the piston when it nears the upward travel limit. These absorb the impact far more effectively than a solid rubber bump-stop will, essential, because a rubber bump-stop is considered a "last-ditch" emergency insulator for the occasional accidental bottoming of the suspension; it is entirely insufficient to absorb repeated and heavy bottomings, such as a high-speed off-road vehicle encounters.


Damping

Damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples i ...
is the control of motion or oscillation, as seen with the use of hydraulic gates and valves in a vehicle's shock absorber. This may also vary, intentionally or unintentionally. Like spring rate, the optimal damping for comfort may be less, than for control. Damping controls the travel speed and resistance of the vehicle's suspension. An undamped car will oscillate up and down. With proper damping levels, the car will settle back to a normal state in a minimal amount of time. Most damping in modern vehicles can be controlled by increasing or decreasing the resistance to fluid flow in the shock absorber.


Camber control

See dependent and independent below. Camber changes due to wheel travel, body roll and suspension system deflection or compliance. In general, a tire wears and brakes best at -1 to -2° of camber from vertical. Depending on the tire and the road surface, it may hold the road best at a slightly different angle. Small changes in camber, front and rear, can be used to tune handling. Some racecars are tuned with -2 to -7° camber, depending on the type of handling desired, and tire construction. Often, too much camber will result in the decrease of braking performance due to a reduced contact patch size through excessive camber variation in suspension geometry. The amount of camber change in bump is determined by the instantaneous front view swing arm (FVSA) length of suspension geometry, or in other words, the tendency of the tire to camber inward when compressed in bump.


Roll center height

Roll center height is a product of suspension instant center heights and is a useful metric in analyzing weight transfer effects, body roll and front to rear roll stiffness distribution. Conventionally, roll stiffness distribution is tuned adjusting
antiroll bar An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) wheel ...
s rather than roll center height (as both tend to have a similar effect on the sprung mass), but the height of the roll center is significant when considering the amount of jacking forces experienced.


Instant center

Due to the fact that the wheel and tire's motion is constrained by the vehicle's suspension links, the motion of the wheel package in the front view will scribe an imaginary arc in space with an "instantaneous center" of rotation at any given point along its path. The instant center for any wheel package can be found by following imaginary lines drawn through suspension links to their intersection point. A component of the tire's force vector points from the contact patch of the tire through instant center. The larger this component is, the less suspension motion will occur. Theoretically, if the ''resultant'' of the vertical load on the tire and the lateral force generated by it points directly into the instant center, the suspension links will not move. In this case, all weight transfer at that end of the vehicle will be geometric in nature. This is key information used in finding the force-based roll center as well. In this respect, the instant centers are more important to the handling of the vehicle, than the kinematic roll center alone, in that the ratio of geometric-to-elastic weight transfer is determined by the forces at the tires and their directions in relation to the position of their respective instant centers.


Anti-dive and anti-squat

Anti-dive and anti-squat are percentages that indicate the degree to which the front dives under braking, and the rear squats under acceleration. They can be thought of as the counterparts for braking and acceleration, as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and the right of the vehicle. The method of determining anti-dive or anti-squat depends on whether suspension linkages react to the torque of braking and accelerating. For example, with inboard brakes and half-shaft-driven rear wheels, the suspension linkages do not react, but with outboard brakes and a swing-axle driveline, they do. To determine the percentage of front suspension braking anti-dive for outboard brakes, it is first necessary to determine the tangent of the angle between a line drawn, in side view, through the front tire patch and the front suspension instant center, and the horizontal. In addition, the percentage of braking effort at the front wheels must be known. Then, multiply the tangent by the front wheel braking effort percentage and divide by the ratio of the center of gravity height to the wheelbase. A value of 50% would mean, that half of the weight transfer to the front wheels; during braking, it is being transmitted through front suspension linkage, and half is being transmitted through front suspension springs. For inboard brakes, the same procedure is followed, but using the wheel center instead of contact patch center. Forward acceleration anti-squat is calculated in a similar manner and with the same relationship between percentage and weight transfer. Anti-squat values of 100% and more are commonly used in drag racing, but values of 50% or less are more common in cars that have to undergo severe braking. Higher values of anti-squat commonly cause wheel hop during braking. It is important to note, that the value of 100% means, that all of the weight transfer is being carried through suspension linkage. However, this does not mean that the suspension is incapable of carrying additional loads (aerodynamic, cornering, etc.) during an episode of braking, or forward acceleration. In other words, no "binding" of the suspension is to be implied.


Flexibility and vibration modes of suspension elements

In some modern cars, flexibility is mainly in
rubber bushing A bushing or rubber bushing is a type of vibration isolator. It provides an interface between two parts, damping the energy transmitted through the bushing. A common application is in vehicle suspension systems, where a bushing made of rubber ...
s, which are subject to decay over time. For high-stress suspensions, such as off-road vehicles, polyurethane bushings are available, which offer more longevity under greater stresses. However, due to weight and cost considerations, structures are not made more rigid than necessary. Some vehicles exhibit detrimental vibrations involving the flexing of structural parts, such as when accelerating while turning sharply. Flexibility of structures, such as frames and suspension links, can also contribute to springing, especially to damping out high-frequency vibrations. The flexibility of wire wheels contributed to their popularity in times when cars had less advanced suspensions.


Load levelling

Automobiles can be heavily laden with luggage, passengers, and trailers. This loading will cause a vehicle's tail to sink downwards. Maintaining a steady chassis level is essential to achieving the proper handling that the vehicle was designed for. Oncoming drivers can be blinded by the headlight beam.
Self-levelling suspension Self-levelling refers to an automobile suspension system that maintains a constant ride height of the vehicle above the road, regardless of load. Purpose Many vehicle systems on a conventional vehicle are negatively affected by the change in attit ...
counteracts this by inflating cylinders in the suspension to lift the chassis higher.


Isolation from high frequency shock

For most purposes, the weight of suspension components is unimportant. But at high frequencies caused by road surface roughness, the parts isolated by rubber bushings act as a multi-stage filter to suppress noise and vibration better than can be done with only tires and springs. (The springs work mainly in the vertical direction.)


Contribution to unsprung weight and total weight

These are usually small, except that the suspension is related to whether the brakes and are sprung. This is the main functional advantage of aluminum wheels over steel wheels. Aluminum suspension parts have been used in production cars, and carbon fiber suspension parts are common in racing cars.


Space occupied

Designs differ as to how much space they take up, and where it is located. It is generally accepted, that
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
s are the most compact arrangement for front-engined vehicles, where space between the wheels is required to place the engine. Inboard brakes (which reduce unsprung weight) are probably avoided more due to space considerations than to cost.


Force distribution

The suspension attachment must match the frame design in geometry, strength and rigidity.


Air resistance (drag)

Certain modern vehicles have
height adjustable suspension Height adjustable suspension is a feature of certain automobile suspension systems that allow the motorist to vary the ride height or ground clearance. This can be done for various reasons including giving better ground clearance over rough terrai ...
in order to improve aerodynamics and fuel efficiency. Modern formula cars that have exposed wheels and suspension typically use streamlined tubing rather than simple round tubing for their suspension arms to reduce
aerodynamic drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding ...
. Also typical is the use of rocker-arm, push rod, or pull rod-type suspensions, that, among other things, place the spring/damper unit inboard and out of the air stream to further reduce air resistance.


Cost

Production methods improve, but cost is always a factor. The continued use of the solid rear axle, with unsprung differential, especially on heavy vehicles, seems to be the most obvious example.


Springs and dampers

Most conventional suspensions use passive springs to absorb impacts and dampers (or shock absorbers) to control spring motions. Some notable exceptions are
hydropneumatic Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
systems, which can be treated as an integrated unit of gas spring and damping components, used by the French manufacturer
Citroën Citroën () is a French automobile brand. The "Automobiles Citroën" manufacturing company was founded in March 1919 by André Citroën. Citroën is owned by Stellantis since 2021 and previously was part of the PSA Group after Peugeot acquired 8 ...
; and the
hydrolastic Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
,
hydragas Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
and rubber cone systems used by the
British Motor Corporation The British Motor Corporation Limited (BMC) was a UK-based vehicle manufacturer, formed in early 1952 to give effect to an agreed merger of the Morris and Austin businesses.Morris-Austin Merger Company Named. ''The Times'', Friday, 29 February ...
, most notably on the
Mini The Mini is a small, two-door, four-seat car, developed as ADO15, and produced by the British Motor Corporation (BMC) and its successors, from 1959 through 2000. Minus a brief hiatus, original Minis were built for four decades and sold during ...
. A number of different types of each have been used:


Passive suspensions

Traditional springs and dampers are referred to as passive suspensions — most vehicles are suspended in this manner.


Springs

The majority of land vehicles are suspended by steel springs of these types: *
Leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...
– AKA Hotchkiss, Cart, or semi-elliptical spring *
Torsion bar suspension A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end term ...
*
Coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
Automakers are aware of the inherent limitations of steel springs — that these springs tend to produce undesirable oscillations, and carmakers have developed other types of suspension materials and mechanisms in attempts to improve performance: * Rubber bushings * Gas under pressure - air springs * Gas and hydraulic fluid under pressure -
hydropneumatic suspension Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
and
oleo strut An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations. It is undesirable for an airp ...
s


Dampers or shock absorbers

Shock absorbers damp out the (otherwise simple harmonic) motions of a vehicle up and down on its springs. They must also damp out much of the wheel bounce when the unsprung weight of a wheel, hub, axle, and sometimes brakes and the differential bounces up and down on the springiness of a tire.


Semi-active and active suspensions

If suspension is externally controlled, then it is a semi-active or active suspension — the suspension is reacting to signals from an electronic controller. For example, a hydropneumatic Citroën will "know" how far off the ground the car is supposed to be, and constantly resets to achieve that level, regardless of load. However, this type of suspension will ''not'' instantly compensate for body roll due to cornering. Citroën's system adds about 1% to the cost of the car versus passive steel springs. Semi-active suspensions include devices, such as air springs and switchable shock absorbers, various self-levelling solutions, as well as systems, like
hydropneumatic Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
,
hydrolastic Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
, and
hydragas Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
suspensions. Toyota introduced switchable shock absorbers in the 1983 Soarer. Delphi currently sells shock absorbers filled with a magneto-rheological fluid, whose viscosity can be changed electromagnetically — thereby giving variable control without switching valves, which is faster and thus more effective. Fully
active suspension An active suspension is a type of automotive suspension on a vehicle. It uses an onboard system to control the vertical movement of the vehicle's wheels relative to the chassis or vehicle body rather than the passive suspension provided by large sp ...
systems use electronic monitoring of vehicle conditions, coupled with the means to change the behavior of vehicle suspension in real time to directly control the motion of the car.
Lotus Cars Lotus Cars Limited is a British automotive company headquartered in Norfolk, England which manufactures sports cars and racing cars noted for their light weight and fine handling characteristics. Lotus was previously involved in Formula One ...
developed several prototypes from 1982 onwards, and introduced them to
Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship, ...
, where they have been fairly effective, but have now been banned.
Nissan , trading as Nissan Motor Corporation and often shortened to Nissan, is a Japanese multinational automobile manufacturer headquartered in Nishi-ku, Yokohama, Japan. The company sells its vehicles under the Nissan, Infiniti, and Datsun bra ...
introduced low-bandwidth active suspension ''circa'' 1990 as an option that added an extra 20% to the price of luxury models. Citroën has also developed several active suspension models (see hydractive). A fully active system from
Bose Corporation Bose Corporation () is an American manufacturing company that predominantly sells audio equipment. The company was established by Amar Bose in 1964 and is based in Framingham, Massachusetts. It is best known for its home audio systems and spea ...
, announced in 2009, uses linear electric motors in place of hydraulic or pneumatic actuators that have generally been used up until recently. Mercedes introduced an active suspension system called
Active Body Control Active Body Control, or ABC, is the Mercedes-Benz brand name used to describe electronically controlled hydropneumatic suspension. This suspension combines a high level of ride quality with control of the vehicle body motions, and therefore vi ...
in its top-of-the-line
Mercedes-Benz CL-Class The Mercedes-Benz CL-Class is a line of grand tourers which was produced by German automaker Mercedes-Benz, produced from 1992 to 2014. The name CL stands for the German Coupé Leicht (Coupé-Light) or Coupé Luxusklasse (Coupé-Luxury). It is ...
in 1999. Several electromagnetic suspensions have also been developed for vehicles. Examples include the electromagnetic suspension of Bose, and the electromagnetic suspension developed by prof. Laurentiu Encica. In addition, the new Michelin wheel with embedded suspension functioning on an electric motor is also similar. With the help of a control system, various semi-active/active suspensions realize an improved design compromise among different vibration modes of the vehicle; namely: bounce, roll, pitch and warp modes. However, the applications of these advanced suspensions are constrained by cost, packaging, weight, reliability, and/or other challenges.


Interconnected suspensions

Interconnected suspension, unlike semi-active/active suspensions, could easily decouple different vehicle vibration modes in a passive manner. Interconnections can be realized by various means, such as mechanical, hydraulic, and pneumatic. Anti-roll bars are one of the typical examples of mechanical interconnections, while it has been stated, that fluidic interconnections offer greater potential and flexibility in improving both the stiffness and damping properties. Considering the considerable commercial potentials of hydro-pneumatic technology (Corolla, 1996), interconnected
hydropneumatic suspension Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
s have also been explored in some recent studies, and their potential benefits in enhancing vehicle ride and handling have been demonstrated. The control system can also be used for further improving performance of interconnected suspensions. Apart from academic research, an Australian company Kinetic had some success with various passive or semi-active systems ( WRC: three Championships; the
Dakar Rally The Dakar Rally (or simply "The Dakar"; formerly known as the "Paris–Dakar Rally") is an annual rally raid organised by the Amaury Sport Organisation. Most events since the inception in 1978 were staged from Paris, France, to Dakar, Senegal ...
: two Championships; Lexus GX470 2004 as the 4×4 of the year with KDSS; the 2005 PACE award). These systems by Kinetic generally decouple at least two vehicle modes (roll, warp (articulation), pitch, and/or heave (bounce)) to simultaneously control each mode's stiffness and damping by using interconnected shock absorbers, and other methods. In 1999, Kinetic was bought out by Tenneco. Later developments by the Catalan compan
Creuat
have devised a simpler system design based on single-acting cylinders. After some projects on competition, Creuat is active in providing retrofit systems for some vehicle models. Historically, the first mass-production car with front-to-rear mechanical interconnected suspension was the 1948
Citroën 2CV The Citroën 2CV (french: link=no, deux chevaux(-vapeur), , lit. "two steam horse(power)s", meaning "two ''taxable'' horsepower") is an air-cooled front-engine, front-wheel-drive, economy family car, introduced at the 1948 Paris Mondial d ...
. Suspension in the 2CV was extremely soft — the longitudinal link was making pitch softer, instead of making roll stiffer. It relied on extreme anti-dive and anti-squat geometries to compensate for that. This resulted in a softer axle-crossing stiffness that anti-roll bars would have otherwise compromised. The leading arm / trailing arm
swinging arm A swingarm, or "swinging arm" (UK), originally known as a swing fork or pivoted fork, is a single or double sided mechanical device which attaches the rear wheel of a motorcycle to its body, allowing it to pivot vertically. The main component of ...
, fore-aft linked suspension system, together with in-board front brakes, had a much smaller unsprung weight than existing coil spring or leaf designs. The interconnection transmitted some of the force deflecting a front wheel up over a bump, to push the rear wheel down on the same side. When the rear wheel met that bump a moment later, it did the same in reverse, keeping the car level front to rear. The 2CV had a design brief to be able to be driven at speed over a ploughed field, such as by a farmer transporting chicken eggs. It originally featured friction dampers and tuned mass dampers. Later models had tuned mass dampers at the front with telescopic dampers/shock absorbers front and rear.
British Motor Corporation The British Motor Corporation Limited (BMC) was a UK-based vehicle manufacturer, formed in early 1952 to give effect to an agreed merger of the Morris and Austin businesses.Morris-Austin Merger Company Named. ''The Times'', Friday, 29 February ...
was also an early adopter of interconnected suspension. A system dubbed ''
Hydrolastic Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
'' was introduced in 1962 on
Morris 1100 Year 1100 ( MC) was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar, the 1100th year of the Common Era (CE) and ''Anno Domini'' (AD) designations, the 100th year of the 2nd millennium, the 100th and ...
, and went on to be used on a variety of BMC models. ''Hydrolastic'' was developed by suspension engineer
Alex Moulton Alexander Eric Moulton (9 April 1920 – 9 December 2012) was an English engineer and inventor, specialising in suspension design. Early life and education Moulton's father, John Coney Moulton, was a naturalist working in the Far East. Alex ...
, and used rubber cones as the springing medium (these were first used on the 1959
Mini The Mini is a small, two-door, four-seat car, developed as ADO15, and produced by the British Motor Corporation (BMC) and its successors, from 1959 through 2000. Minus a brief hiatus, original Minis were built for four decades and sold during ...
) with suspension units on each side connected to each other by a fluid-filled pipe. The fluid transmitted the force of road bumps from one wheel to the other (on the same principle as Citroën 2CV's mechanical system described above), and because each suspension unit contained valves to restrict the flow of fluid, also served as a shock absorber. Moulton went on to develop a replacement for ''Hydrolastic'' for BMC's successor
British Leyland British Leyland was an automotive engineering and manufacturing conglomerate formed in the United Kingdom in 1968 as British Leyland Motor Corporation Ltd (BLMC), following the merger of Leyland Motors and British Motor Holdings. It was partly ...
. This system, manufactured under licence by Dunlop in Coventry, called ''
Hydragas Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies. Invented by British rubber engineer Alex Moulton, and first used on the 1962 BMC p ...
'', worked with the same principle, but instead of rubber spring units, it used metal spheres divided internally by a rubber diaphragm. The top half contained pressurised gas, and the lower half the same fluid as used on the ''Hydrolastic'' system. The fluid transmitted suspension forces between the units on each side, whilst the gas acted as the springing medium through the diaphragm. This is the same principle as the Citroën
hydropneumatic Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
system, and provides similar ride quality, but is self-contained, and does not require an engine-driven pump to provide hydraulic pressure. The downside is, that ''Hydragas'' is, unlike the Citroën system, not height-adjustable, or self-levelling. ''Hydragas'' was introduced in 1973 on
Austin Allegro The Austin Allegro is a small family car that was manufactured by the Austin-Morris division of British Leyland from 1973 until 1982. The same vehicle was built in Italy by Innocenti between 1974 and 1975 and sold as the Innocenti Regent. The Al ...
, and was used on several models; the last car to use it being
MG F The MG F and MG TF are mid-engined, rear wheel drive roadster cars that were sold under the MG marque by three manufacturers between 1995 and 2011. The MG F was the first new model designed as an MG since the MGB that was produced from 196 ...
in 2002. The system was changed in favour of coil springs over dampers due to cost reasons towards the end of the vehicle's life. When it was decommissioned in 2006, the ''Hydragas'' manufacturing line was over 40 years old. Some of the last post-war
Packard Packard or Packard Motor Car Company was an American luxury automobile company located in Detroit, Michigan. The first Packard automobiles were produced in 1899, and the last Packards were built in South Bend, Indiana in 1958. One of the "Th ...
models also featured interconnected suspension.


Types

Suspension systems can be broadly classified into two subgroups: dependent and independent. These terms refer to the ability of opposite wheels to move independently of each other. A ''dependent suspension'' normally has a beam (a simple 'cart' axle) or a (driven)
live axle A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have als ...
that holds wheels parallel to each other and perpendicular to the axle. When the camber of one wheel changes, the camber of the opposite wheel changes in the same way (by convention, on one side, this is a positive change in the camber, and on the other side, this a negative change). De Dion suspensions are also in this category, as they rigidly connect the wheels together. ''
Independent suspension Independent suspension is any automobile suspension system that allows each wheel on the same axle to move vertically (i.e. reacting to a bump on the road) independently of the others. This is contrasted with a beam axle or deDion axle system ...
'' allows wheels to rise and fall on their own without affecting the opposite wheel. Suspensions with other devices, such as
sway bar An anti-roll bar (roll bar, anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) whee ...
s that link the wheels in some way, are still classed as independent. ''Semi-dependent'' suspension is a third type. In this case, the motion of one wheel does affect the position of the other, but they are not rigidly attached to each other.
Twist-beam rear suspension The twist-beam rear suspension (also torsion-beam axle, deformable torsion beam or compound crank) is a type of automobile suspension based on a large H or C-shaped member. The front of the H attaches to the body via rubber bushings, and the ...
is such a system.


Dependent suspensions

Dependent systems may be differentiated by the system of linkages used to locate them, both longitudinally and transversely. Often, both functions are combined in a set of linkages. Examples of location linkages include: * Satchell link *
Panhard rod A Panhard rod (also called Panhard bar, track bar, or track rod) is a suspension link that provides lateral location of the axle. Originally invented by the Panhard automobile company of France in the early twentieth century, this device has been ...
*
Watt's linkage In kinematics, Watt's linkage (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel on a nearly straight line. It was described in ...
* WOBLink * Mumford linkage *
Leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...
s used for location (transverse or longitudinal) ** Fully elliptical springs usually need supplementary location links, and are no longer in common use ** Longitudinal semi-elliptical springs used to be common, and are still used in heavy-duty trucks and aircraft. They have the advantage, that the spring rate can easily be made progressive (non-linear). ** A single transverse leaf spring for both front wheels and/or both back wheels, supporting solid axles, was used by
Ford Motor Company Ford Motor Company (commonly known as Ford) is an American multinational automobile manufacturer headquartered in Dearborn, Michigan, United States. It was founded by Henry Ford and incorporated on June 16, 1903. The company sells automobi ...
, before and soon after
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, even on expensive models. It had the advantages of simplicity and low unsprung weight (compared to other solid-axle designs). In a front-engine rear-drive vehicle, dependent rear suspension is either "live-axle" or
deDion axle De Dion rear axle A de Dion tube is a form of non- independent automobile suspension. It is a considerable improvement over the swing axle, Hotchkiss drive, or live axle. Because it plays no part in transmitting power to the drive wheels, it ...
, depending on whether or not differential is carried on the axle. Live-axle is simpler, but unsprung weight contributes to wheel bounce. Because it assures constant camber, dependent (and semi-independent) suspension is most common on vehicles that need to carry large loads as a proportion of the vehicle's weight, that have relatively soft springs and that do not (for cost and simplicity reasons) use active suspensions. The use of dependent front suspension has become limited to heavier commercial vehicles.


Independent suspensions

The variety of independent systems is greater, and includes: *
Swing axle A swing axle is a simple type of independent (rear wheel) suspension designed and patented by Edmund Rumpler in 1903. This was a revolutionary invention in automotive suspension, allowing driven (powered) wheels to follow uneven road surfaces ...
*
Sliding pillar A sliding pillar suspension is a form of independent front suspension for light cars. The stub axle and wheel assembly are attached to a vertical pillar or kingpin which slides up and down through a bush or bushes which are attached to the veh ...
*
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
/
Chapman strut The Chapman strut is a design of independent rear suspension used for light cars, particularly sports and racing cars. It takes its name from, and is best known for its use by, Colin Chapman of Lotus. The characteristic feature of the Chapma ...
* Upper and lower
A-arm In automotive suspension, a control arm, also known as an A-arm, is a hinged suspension link between the chassis and the suspension upright or hub that carries the wheel. In simple terms, it governs a wheel's vertical travel, allowing it to move ...
(
double wishbone A double wishbone suspension is an independent suspension design for automobiles using two (occasionally parallel) wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckl ...
) *
Multi-link suspension A multi-link suspension is a type of vehicle suspension with one or more longitudinal arms. A wider definition can consider any independent suspensions having three control links or more multi-link suspensions. These arms do not have to be of e ...
*
Semi-trailing arm suspension A (semi) trailing-arm suspension, sometimes referred to as (semi) trailing-link is a vehicle axle or wheel suspension design in which one or more horizontal arms (or "links"), perpendicular to and forward of the axle, are connecting the axle or w ...
* Swinging arm ** Transverse leaf springs when used as a suspension link, or four-quarter elliptics on one end of a car are similar to wishbones in geometry, but are more compliant. Examples are the front of the original
Fiat 500 The Fiat 500 ( it, Cinquecento, ) is a rear-engined, four-seat, small city car that was manufactured and marketed by Fiat Automobiles from 1957 until 1975 over a single generation in two-door saloon and two-door station wagon bodystyles. Launc ...
, then
Panhard Dyna Z The Panhard Dyna Z is a lightweight motor car produced by Panhard of France from 1954 to 1959. It was first presented to the press at a Paris restaurant named Les Ambassadeurs on 17 June 1953 and entered production the following year. In 1959, i ...
, and the early examples of
Peugeot 403 The Peugeot 403 is a mid-size car manufactured and marketed by Peugeot between May 1955 and October 1966. A total of 1,214,121 of all types, including commercial models, were produced, making it the first Peugeot to exceed one million in sales. ...
, and the backs of AC Ace and
AC Aceca The Aceca (pronounced "A-See-Ka") is a closed coupé from the British AC Cars company, produced from 1954 until 1963. The car originally had an AC engine but the similar Bristol-engined Aceca-Bristol was also available alongside the original from ...
. Because the wheels are not constrained to remain perpendicular to a flat road surface in turning, braking, and varying load conditions, control of the wheel camber is an important issue. Swinging-arm was common in small cars that were sprung softly, and could carry large loads, because the camber is independent of load. Some active and semi-active suspensions maintain ride height, and therefore the camber, independent of load. In
sports car A sports car is a car designed with an emphasis on dynamic performance, such as handling, acceleration, top speed, the thrill of driving and racing capability. Sports cars originated in Europe in the early 1900s and are currently produced by ...
s, optimal camber change when turning, is more important. Wishbone and multi-link allow the engineer more control over the geometry, to arrive at the best compromise, than swing axle, MacPherson strut, or swinging arm do; however, the cost and space requirements may be greater. Semi-trailing arm is in between, being a variable compromise between the geometries of swinging arm and swing axle.


Semi-independent suspension

In semi-independent suspensions, the wheels of an axle are able to move relative to one another, as in an independent suspension, but the position of one wheel has an effect on the position and attitude of the other wheel. This effect is achieved through the twisting or deflecting of suspension parts under load. The most common type of semi-independent suspension is the
twist beam The twist-beam rear suspension (also torsion-beam axle, deformable torsion beam or compound crank) is a type of automobile suspension based on a large H or C-shaped member. The front of the H attaches to the body via rubber bushings, and the ...
.


Other instances


Tilting Suspension System

The Tilting Suspension System (also known as the Leaning Suspension System) is not a different type or geometry of construction; moreover, it is a technology addition to the conventional suspension system. This kind of suspension system mainly consists of independent suspension (e.g.,
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
,
A-arm In automotive suspension, a control arm, also known as an A-arm, is a hinged suspension link between the chassis and the suspension upright or hub that carries the wheel. In simple terms, it governs a wheel's vertical travel, allowing it to move ...
(
double wishbone A double wishbone suspension is an independent suspension design for automobiles using two (occasionally parallel) wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckl ...
)). With the addition of these suspension systems, there is a further tilting or leaning mechanism that connects the suspension system with the vehicle body (chassis). The tilting suspension system improves stability, traction, the turning radius of a vehicle, and the comfort of riders as well. While turning right or left, passengers or objects on a vehicle feel the G-force or inertial force outward the radius of the curvature, which is why two-wheeler riders (motorbikes) lean towards the center of curvature while turning, which improves stability and decreases the chances of toppling. But vehicles with more than two wheels, and equipped with a conventional suspension system, could not do the same until now, so the passengers feel the outward inertial force, which reduces the stability of riders and their comfort as well. This kind of tilting suspension system is the solution to the problem. If the road does not have super-elevation or banking, it will not affect the comfort with this suspension system, the vehicle tilt and decrease in the height of the center of gravity with an increase in stability. This suspension is also used in fun vehicles. Some trains also use tilting suspension (
Tilting Train A tilting train is a train that has a mechanism enabling increased speed on regular rail tracks. As a train (or other vehicle) rounds a curve at speed, objects inside the train experience centrifugal force. This can cause packages to slide ab ...
) which increases the speed at cornering.


Rocker bogie mechanism

The rocker-bogie system is a suspension arrangement, in which there are some trailing arms fitted with some idler wheels. Due to articulation between the driving section and the followers, this suspension is very flexible. This kind of suspension is appropriate for extremely rough terrain. This kind of suspension was used in the ''
Curiosity Curiosity (from Latin '' cūriōsitās'', from ''cūriōsus'' "careful, diligent, curious", akin to ''cura'' "care") is a quality related to inquisitive thinking such as exploration, investigation, and learning, evident by observation in human ...
'' rover.


Tracked vehicles

Some vehicles, such as trains, run on long
rail tracks A railway track (British English and UIC terminology) or railroad track (American English), also known as permanent way or simply track, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties (sleeper ...
fixed to the ground; and some, such as tractors, snow vehicles, and tanks run on
continuous track Continuous track is a system of vehicle propulsion used in tracked vehicles, running on a continuous band of treads or track plates driven by two or more wheels. The large surface area of the tracks distributes the weight of the vehicle ...
s that are part of the vehicle. Alhough either sort helps to smooth the path and reduce ground pressure, many of the same considerations apply.


Armoured fighting vehicle suspension

Military
armoured fighting vehicle An armoured fighting vehicle (AFV) is an armed combat vehicle protected by armour, generally combining operational mobility with offensive and defensive capabilities. AFVs can be wheeled or tracked. Examples of AFVs are tanks, armoured cars, ...
s (AFVs), including
tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful ...
s, have specialized suspension requirements. They can weigh more than seventy tons, and are required to move as quickly as possible over very rough or soft ground. Their suspension components must be protected from
land mine A land mine is an explosive device concealed under or on the ground and designed to destroy or disable enemy targets, ranging from combatants to vehicles and tanks, as they pass over or near it. Such a device is typically detonated automati ...
s and antitank weapons. Tracked AFVs can have as many as nine road wheels on each side. Many wheeled AFVs have six or eight large wheels. Some have a Central Tire Inflation System to reduce ground loading on poor surfaces. Some wheels are too big and too confined to turn, so skid steering is used with some wheeled, as well as with tracked vehicles. The earliest tanks of
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...
had fixed suspension with no designed movement whatsoever. This unsatisfactory situation was improved with
leaf spring A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, i ...
or
coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
suspensions adopted from agricultural, automotive, or railway machinery, but even these had very limited travel. Speeds increased due to more powerful engines, and the quality of ride had to be improved. In the 1930s, the
Christie suspension The Christie suspension is a suspension system developed by American engineer J. Walter Christie for his tank designs. It allowed considerably longer movement than conventional leaf spring systems then in common use, which allowed his tanks t ...
was developed, which allowed the use of
coil spring A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...
s inside a vehicle's armored hull, by changing the direction of force deforming the spring, using a
bell crank A bellcrank is a type of crank that changes motion through an angle. The angle can be any angle from 0 to 360 degrees, but 90 degrees and 180 degrees are most common. The name comes from its first use, changing the vertical pull on a rope to a h ...
. The
T-34 The T-34 is a Soviet medium tank introduced in 1940. When introduced its 76.2 mm (3 in) tank gun was less powerful than its contemporaries while its 60-degree sloped armour provided good protection against anti-tank weapons. The C ...
's suspension was directly descended from Christie designs.
Horstmann suspension Horstmann suspension, also known as Horstman, Vickers-Horstman and rarely Slow Motion, is a type of tracked suspension devised by British tank designer John Carden and worked into a production design by engineer Sidney Horstmann. First used ...
was a variation which used a combination of bell crank and exterior coil springs, in use from the 1930s to the 1990s. The
bogie A bogie ( ) (in some senses called a truck in North American English) is a chassis or framework that carries a wheelset, attached to a vehicle—a modular subassembly of wheels and axles. Bogies take various forms in various modes of transp ...
, but nonetheless independent, suspensions of
M3 Lee The M3 Lee, officially Medium Tank, M3, was an American medium tank used during World War II. The turret was produced in two forms, one for US needs and one modified to British requirements to place the radio next to the commander. In British C ...
/Grant and
M4 Sherman } The M4 Sherman, officially Medium Tank, M4, was the most widely used medium tank by the United States and Western Allies in World War II. The M4 Sherman proved to be reliable, relatively cheap to produce, and available in great numbers. It ...
vehicles was similar to the Hortsmann type, with suspension sequestered within the track oval. By
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, the other common type was
torsion bar suspension A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end term ...
, getting spring force from twisting bars inside the hull — this sometimes had less travel than the Christie type, but was significantly more compact, allowing more space inside the hull, with the consequent possibility to install larger turret rings, and thus, heavier main armament. Torsion-bar suspension, sometimes including shock absorbers, has been the dominant heavy armored vehicle suspension since World War II. Torsion bars may take space under or near the floor, which may interfere with making the tank low to reduce exposure. As with cars, wheel travel and spring rate affect the bumpiness of ride, and the speed at which rough terrain can be negotiated. It may be significant, that a smooth ride, which is often associated with comfort, increases the accuracy when firing on the move. It also reduces shock on optics and other equipment. The unsprung weight and track link weight may limit speed on roads, and can affect the useful lifetime of the vehicle's track, and its other components. Most German WWII half-tracks and their tanks introduced during the war, such as the
Panther tank The Panther tank, officially ''Panzerkampfwagen V Panther'' (abbreviated PzKpfw V) with Sonderkraftfahrzeug, ordnance inventory designation: ''Sd.Kfz.'' 171, is a German medium tank of World War II. It was used on the Eastern Front (World War ...
, had overlapping and sometimes interleaved road wheels to distribute the load more evenly on the tank's track, and therefore on the ground. This apparently made a significant contribution to speed, range and track life, as well as providing a continuous band of protection. It has not been used since the end of that war, probably due to the maintenance requirements of more complicated mechanical parts working in mud, sand, rocks, snow, and ice; as well as due to cost. Rocks and frozen mud often got stuck between the overlapping wheels, which could prevent them from turning, or would cause damage to the road wheels. If one of the interior road wheels were damaged, it would require other road wheels to be removed in order to access the damaged road wheel, making the process more complicated and time-consuming.Peter Chamberlain and Hilary Doyle, ''Encyclopedia of German Tanks of World War Two'', 1978, 1999


See also

*
Automotive suspension design process Automotive suspension design is an aspect of automotive engineering, concerned with designing the suspension for cars and trucks. Suspension design for other vehicles is similar, though the process may not be as well established. The process en ...
*
MacPherson strut The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles, and is named for American automotive engineer Ear ...
*
Multi-link suspension A multi-link suspension is a type of vehicle suspension with one or more longitudinal arms. A wider definition can consider any independent suspensions having three control links or more multi-link suspensions. These arms do not have to be of e ...
*
4-poster A 4-poster or four poster automotive test system is specifically designed for the testing of vehicles (cars, trucks). These test systems consist of 4 hydraulic actuators on top of which the wheels of the vehicle are placed. Movements of the actua ...
– a test rig *
7 post shaker The 7 post shaker is a piece of test equipment used to perform technical analysis on race cars. By applying shaking forces the shaker can emulate banking loads, lateral load transfer, longitudinal weight transfer and ride height sensitive downforc ...
– a test rig for high-speed vehicles *
Ackermann steering geometry The Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii. It was ...
*
Caster angle 250px, θ is the caster angle, the red line is the pivot line, and the grey area is the tire. 250px, Front suspension of a race car, the caster angle is formed by the line between upper and lower ball joint. The caster angle or castor angle is t ...
– self centering steering *
Coilover A coilover is an automobile suspension device. The name coilover is an abbreviation of "coil-over shock absorber". Description Coilovers are found on many vehicles, from RC cars to normal passenger cars, race cars and 4x4 vehicles. They are s ...
* Corvette leaf spring - independent suspension combined with a transverse fiber reinforced plastic leaf spring * Korres P4 — a Greek all-terrain supercar, with a unique suspension *
Magnetic levitation Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic force is used to counteract the effects of the gravitational force and any other forces. The ...
**
Maglev train Maglev (derived from '' magnetic levitation''), is a system of train transportation that uses two sets of electromagnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage ...
*
Ride height Ride height or ground clearance is the amount of space between the base of an automobile tire and the lowest point of the automobile (typically the axle); or, more properly, to the shortest distance between a flat, level surface, and the lowest p ...
- vehicle ground clearance *
Oleo strut An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations. It is undesirable for an airp ...
- design used in most large
aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...
, with compressed gas and
hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoe ...
- conceptually similar to automobile
Hydropneumatic suspension Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce ( Silver Shad ...
*
Scrub radius The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. It can be positive, negative or zero. The kingpin axis also known as steeri ...
* Short long arms suspension — also known as "unequal length A arm", one of the design parameters of double wishbone suspension * Strut bar — a form of semi-independent suspension * Other suspension


References


Further reading

* * *


External links


How Car Suspensions Work

Robert W. Temple, ''The ABCs of Chassis Frame and Suspensions'', September 1969

Suspension Geometry Calculator
{{DEFAULTSORT:Suspension (Vehicle) Automotive suspension technologies Armoured fighting vehicle equipment Vehicle technology Articles containing video clips Vehicle dynamics