Automotive suspension design is an aspect of

automotive engineering Automotive engineering, along with aerospace engineering and naval architecture, is a branch of vehicle engineering, incorporating elements of mechanical, electrical, electronic, software, and safety engineering as applied to the design, manufac ...

, 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 entails *Selecting appropriate vehicle level targets *Selecting a system architecture *Choosing the location of the 'hard points', or theoretical centres of each ball joint or bushing *Selecting the rates of the bushings *Analysing the loads in the suspension *Designing the

spring rate A spring is an elastic object that stores mechanical energy. In everyday use the term often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel, although some non- ...

s *Designing

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 ...

characteristics *Designing the structure of each component so that it is strong, stiff, light, and cheap *Analysing the vehicle dynamics of the resulting design Since the 1990s the use of multibody simulation and finite element software has made this series of tasks more straightforward.

Vehicle level targets

A partial list would include: *Maximum steady state lateral acceleration (in understeer mode) *Roll stiffness (degrees per g of lateral acceleration) *Ride frequencies *Lateral

load transfer Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects: *the change in load borne by different wheels of even perfectly rigid vehicles during acceleration *the change in center of mas ...

percentage distribution front to rear * Roll moment distribution front to rear *Ride heights at various states of load *

Understeer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occ ...

gradient *

Turning circle The turning diameter of a vehicle is the minimum diameter (or "width") of available space required for that vehicle to make a circular turn (i.e. U-turn). The term thus refers to a theoretical minimal circle in which for example an aeroplane, a g ...

* Ackermann *Jounce travel *Rebound travel Once the overall vehicle targets have been identified they can be used to set targets for the two suspensions. For instance, the overall understeer target can be broken down into contributions from each end using a Bundorf analysis.

System architecture

Typically a vehicle designer is operating within a set of constraints. The suspension architecture selected for each end of the vehicle will have to obey those constraints. For both ends of the car this would include the type of spring, location of the spring, and location of the

shock absorbers 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 ...

. For the front suspension the following need to be considered *The type of suspension (

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 ...

double wishbone suspension 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 ...

) *Type of steering actuator (

rack and pinion A rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert rotational motion into linear motion. Rotating the pinion causes the rack to be driven ...

recirculating ball Recirculating ball, also known as recirculating ball and nut or worm and sector, is a steering mechanism commonly found in older automobiles, off-road vehicles, and some trucks. Most newer cars use the more economical rack and pinion steering ...

) *Location of the steering actuator in front of, or behind, the wheel centre For the rear suspension there are many more possible suspension types, in practice.

Hardpoints

The hardpoints control the static settings and the

kinematics Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fiel ...

of the suspension. The static settings are * Toe * Camber *

Caster A caster (or castor) is an undriven wheel that is designed to be attached to the bottom of a larger object (the "vehicle") to enable that object to be moved. Casters are used in numerous applications, including shopping carts, office chairs, ...

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 ...

height at design load *Mechanical (or caster) trail * Anti-dive and anti-squat * Kingpin Inclination * Scrub radius *Spring and shock absorber

motion ratio The motion ratio of a mechanism is the ratio of the displacement of the point of interest to that of another point. The most common example is in a vehicle's suspension, where it is used to describe the displacement and forces in the springs an ...

s The kinematics describe how important characteristics change as the suspension moves, typically in roll or steer. They include * Bump Steer *Roll Steer *Tractive Force Steer *Brake Force Steer *Camber gain in roll *Caster gain in roll *Roll centre height gain *Ackermann change with steering angle *Track gain in roll The analysis for these parameters can be done graphically, or by

CAD Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...

, or by the use of kinematics software.

Compliance analysis

The compliance of the bushings, the body, and other parts modify the behaviour of the suspension. In general it is difficult to improve the kinematics of a suspension using the bushings, but one example where it does work is the

toe control Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...

bush used in Twist-beam rear suspensions. More generally, modern cars suspensions include a

Noise, vibration, and harshness Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks. While noise and vibration can be readily measured ...

(NVH) bush. This is designed as the main path for the vibrations and forces that cause road noise and impact noise, and is supposed to be tunable without affecting the kinematics too much. In racing cars, bushings tend to be made of harder materials for good handling such as

brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wi ...

or delrin. In Passenger cars, bushings tend to be made of softer material for added comfort. In general physical terms, the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

and mechanical

hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...

(damping effect) of solid parts should be accounted for in a dynamic analysis, as well as their elasticity.

Loads

Once the basic geometry is established the loads in each suspension part can be estimated. This can be as simple as deciding what a likely maximum load case is at the contact patch, and then drawing a

Free body diagram A free body diagram consists of a diagrammatic representation of a single body or a subsystem of bodies isolated from its surroundings showing all the forces acting on it. In physics and engineering, a free body diagram (FBD; also called a force ...

of each part to work out the forces, or as complex as simulating the behaviour of the suspension over a rough road, and calculating the loads caused. Often loads that have been measured on a similar suspension are used instead - this is the most reliable method.

Detailed design of arms

The loads and geometry are then used to design the arms and spindle. Inevitably some problems will be found in the course of this that force compromises to be made with the basic geometry of the suspension.

References

Notes

Sources

*The Automotive Chassis Engineering Principles - J. Reimpell H. Stoll J. W. Betzler. - *Race Car Vehicle Dynamics - William F. Milliken and Douglas L. Milliken. *Fundamentals of Vehicle Dynamics - Thomas Gillespie. *Chassis Design - Principles and Analysis - William F. Milliken and Douglas L. Milliken. Simulation and direct equations: Abramov, S., Mannan, S., & Durieux, O. (2009)'Semi-Active Suspension System Simulation Using SIMULINK'. International Journal of Engineering Systems Modelling and Simulation, 1(2/3), 101 - 114 http://collections.crest.ac.uk/232/1/fulltext.pdf {{Automotive handling Automotive suspension technologies Automotive engineering Vehicle dynamics