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Sunday October 22nd 2017

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Evolution of braking

The principle of braking is to dissipate the vehicle’s kinetic energy into heat through friction. This needs to be dissipated efficiently after each braking event. The ideal brake would generate consistent brake torque under all conditions – hot or cold, wet or dry; from high speed or low speed. Unfortunately, all brakes fall short of this ideal, but some more than others.

First systems

Horse drawn vehicles had wooden wheels with steel tyres. Their brakes usually consisted of blocks of wood, mounted on pivoted brackets which could be forced against the steel tyres. Early motor cars with solid rubber tyres retained this type of braking. As speeds increased and vehicles became heavier this braking system used to destroy the tires due to heat and abrasion.

The first improvement was to provide a metal cylinder on a rotating part of the axle against which the external brake blocks could operate. Because of the reduced radius, greater force was required to generate adequate braking torque. Increasing the area of contact was necessary; a method was to use a pair of pivoted brake blocks with a mechanical linkage to clamp their free ends together. Wood was replaced by composite materials incorporating asbestos to cope with higher temperatures. These external-contracting brakes were often mounted inboard of the road wheels to provide space for the external blocks and mechanism without restricting the effective radius too much. They suffered reduced effectiveness in wet weather and high wear rates due to contamination (due to particles that would get between the two surfaces).

 Drum Brakes

When the blocks (brake shoes) where mounted inside the cylinder (drum) a significant improveme nt in braking torque (because a greater effective radius was achieved). Drum brakes had just been invented! Drum brakes shelter the friction material of the elements; however water can still enter around the edge of the drum, particularly during wading through deep water. Once inside, it does not quickly escape. Drum brakes cannot dissipate the heat as well as an open brake system. The man credited with the development of the drum brake was the French manufacturer Louis Renault, in 1902.

Drum brakes use a rotatable double-sided cam, an axially moving wedge or an internal hydraulic expander to force the shoes against the braking surface of the drum. Drum brakes changed very little in the second half of the 20th century apart from the addition of automatic adjustment mechanisms in the 1970’s and adoption of non-asbestos friction materials around 1990.


Disk Brakes

Disc brakes where first patented by Lanchester in 1902 but it was not until the series of spectacular Jaguar wins at Le Mans in the 1950’s that their advantages were clearly demonstrated to designers and users alike. Their comparatively less tendency to “fade” compared to the Drum Brakes when braking from high speeds enabled the Jaguar drivers to brake later at each corner and thereby improve their lap times. Adoption of disc brakes for cars was initially inhibited by the high costs of low volumes production, aggravated by the fact that a booster often became necessary because disc brakes produce less torque for a given input effort. Consistency of performance remains the principal reason for the widespread adoption of disc brakes.
Actuation
The actuation of brakes was mechanical; cable-actuated brakes were prone to sticking and seizing when corroded. Next came the hydraulic actuation, which was easier to modulate, and provided more force per square inch than mechanical systems. Hydraulic systems are broadly used nowadays.

The future actuation method will be the “brake by wire” (sensor in the brake pedal and a servo-electric motor applying the braking torque). It is much simpler than the hydraulic system. However it presents several challenges like, how to graduate the amount of braking effort and the safety related issues in case of system failure.

Safer braking

ABS (Anti-Lock System): Prevents the brakes from locking up. Speed sensors in the car help to determine if a wheel is going to lock up. Then a series of hydraulic valves limit or reduce the braking on that wheel. This enables to driver to maintain control of the car and it prevents the car from going into a spin.

EBD (Electronic Brake-force Distribution): Regulates the brake distribution between the front and the rear axle to provide the best braking performance and avoid locking the rear wheels.

Brake assist or booster: Amplifies the available foot pressure applied to the brake pedal so that the pressure required to stop the vehicle is minimal. Power for the booster comes from engine vacuum.


 

What is to come?

Nowadays, in order to improve the efficiency of the vehicles regenerative braking is starting to be implemented into vehicles. This method transforms the kinetic energy of the vehicle and transforms it into another form of energy (electric (batteries), kinetic (flywheels) …) in order to be able to feed it back into the vehicle in when acceleration is needed. These transformations are not 100% efficient. By law, all the vehicles still need to fit friction based braking systems.

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