A transmission brake or driveline parking brake is an inboard vehicle brake that is applied to the

drivetrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) or transmission system, is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the driv ...

rather than to the wheels. Historically, some early cars used transmission brakes as the normal driving brake and often had wheel brakes on only one axle, if that. In current vehicles, these brakes are now rare. They are found in some makes, notably

Land Rover Land Rover is a brand of predominantly four-wheel drive, off-road capable vehicles, owned by British multinational car manufacturer Jaguar Land Rover (JLR), since 2008 a subsidiary of India's Tata Motors. JLR builds Land Rovers in Brazil ...

, usually for light off-road vehicles. Simple transmission brakes could be found in large vehicles too, such as the 16 inch single disc parking brake used in the

M19 tank transporter The M19 tank transporter ( US supply catalog designation G159) was a heavy tank transporter system used in World War II and into the 1950s. It consisted of a 12-ton 6×4 M20 Diamond T model 980 truck and companion 12-wheel M9 trailer. Over 5,00 ...

of World War II. Such a system was also used on the

HMMWV The High Mobility Multipurpose Wheeled Vehicle (HMMWV; colloquial: Humvee) is a family of Military light utility vehicle, light, four-wheel drive Military vehicle#Military trucks, military trucks and utility vehicles produced by AM General. It ...

. The transmission brake is provided solely as a

parking brake In road vehicles, the parking brake, also known as a handbrake or emergency brake (e-brake), is a mechanism used to keep the vehicle securely motionless when parked. Parking brakes often consist of a pulling mechanism attached to a Wire rope, ...

or handbrake. Normal wheel brakes are still provided for use when driving. Driver's manuals usually caution against using the transmission brake when driving, as it is neither powerful enough nor robust enough and so will not work effectively and may even be damaged by trying to stop a moving vehicle. Transmission brakes use

drum brake A drum brake is a brake that uses friction caused by a set of Brake shoe, shoes or Brake pad, pads that press outward against a rotating bowl-shaped part called a brake drum. The term ''drum brake'' usually means a brake in which shoes press o ...

s, rather than

disc brake A disc brake is a type of brake that uses the #Calipers, calipers to squeeze pairs of #Brake pads, pads against a disc (sometimes called a

rake Rake may refer to: Common meanings * Rake (tool), a horticultural implement, a long-handled tool with tines * Rake (stock character), a man habituated to immoral conduct * Rake (poker), the commission taken by the house when hosting a poker game ...

rotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive f ...

s, as they are intended as a static parking brake, rather than a high performance dynamic brake. Drum brakes allow simpler adjustment with cable-actuated hand lever mechanisms. The brake is mounted to the rear output shaft of the

transfer box A transfer case is an intermediate gearbox that transfers power from the transmission (mechanics), transmission of a motor vehicle to the driven axles of four-wheel-drive, all-wheel-drive, and other multi-axled on- and off-road machines. A part o ...

. As the transmission brake is mounted inboard of the

final drive A drivetrain (also frequently spelled as drive train or sometimes drive-train) or transmission system, is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivet ...

and its reduction gearing, the brake rotates relatively faster, but with less torque, than a wheel brake. The apparently undersized transmission brake thus has more holding ability than its small size might suggest, but is less suitable for driving loads. The braking forces would also be passed through the final drive and axle

drive shaft 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 (physics), power, torque, and rotation, usually ...

s, with possible risk of overloading them. One advantage of a transmission brake is that it locks the entire drivetrain, including all four wheels of a

four wheel drive The Four Wheel Drive Auto Company, more often known as Four Wheel Drive (FWD), was a pioneering American company that developed and produced all-wheel drive vehicles. It was founded in 1909 in Clintonville, Wisconsin, as the Badger Four-Wheel Dr ...

vehicle. However any differential action, either within an axle or front-to-back on an

all wheel drive An all-wheel drive vehicle (AWD vehicle) is one with a powertrain capable of providing power to all its wheels, whether full-time or on-demand. Types The most common forms of all-wheel drive are: ;1x1 : All unicycles Reflects one axle with ...

(permanent 4×4) vehicle can still allow movement. For this reason a transmission brake is convenient as a parking brake, but should not be relied upon if a mechanic is to be working beneath the vehicle and wheel chocks should be used instead. A second advantage is that they remove the need to provide cable connections to the wheel brakes, on off-road vehicles where such may be prone to damage.

Automatic transmissions

A form of transmission brake is commonly fitted to

automatic transmission An automatic transmission (AT) or automatic gearbox is a multi-speed transmission (mechanics), transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 ...

s. These brakes use a

dog clutch A dog clutch (also known as a dog box, dog gears, dog ring, clutch dog, or positive clutch) is a type of clutch that couples two rotating shafts or other rotating components by engagement of interlocking teeth or dogs rather than by friction. T ...

pawl A pawl is a movable lever that engages a fixed component to either prevent movement in one direction or restrain it altogether. As such, it is a type of latch and can also be considered a type of dog. It typically consists of a spring-loaded ...

system, rather than friction linings. They thus provide a simple and positive lock when stationary, but cannot (or at least, ''should'' not) be engaged when moving. In the US, this is the conventional form of parking brake and leads to the handbrake being almost entirely ignored and thought of only as the 'emergency' brake.

References

{{Reflist Vehicle braking technologies

Automatic transmissions

See also

References