LANDSKRONA, Sweden – Haldex recently posted the following overview on how a braking system functions:
Braking is based on friction. There are no other physical dimensions or forces; it’s just down to friction between the brake lining and the drum, or the pad and the disc. And, finally, what brings the brake forces to apply is friction between the tire and the road. There are two different materials touching each other and providing a certain coefficient of friction, and a braking-force results.
Generally speaking, the whole idea of brake control is to try to maintain wheel slip to give a little control, so that you can steer the vehicle. If the wheel locks, you are not able to steer. To avoid that, a control system of the brake tries to prevent complete wheel locking, to retain the ability to steer, but still provide a portion of braking force friction to come through the wheel to the street.
This is the main principle of brakes and applies to everything: passenger cars, trucks, trailers, motorbikes and now even bicycles – Bosch has developed ABS for disc brakes on the front wheel of electric bicycles.
On a bicycle, or in a car, the driver’s physical effort applies the brakes (some vehicles offer a degree of assistance). However, because trucks are so much heavier, up to 40t, there is no means for a human being to generate so much force.
Therefore, trucks and trailers use a pneumatic system instead, powered by a compressor. When the driver pushes on the brake pedal, that acts as a signal to brake and used to be transmitted by air in small cross-section piping to relay valves, which passed it on to the actuator, pushed against the piston, which moved a lever on the brake chamber to have provided the force to create friction in the brake disc or drum. The brake adjuster connects the brake drum to a mechanism that automatically expands the linings against the drum as they wear.
In previous systems, if the brakes were to be applied too forcefully, the wheels would lock, and the vehicle will skid. The rate of locking depends partly on the road surface. On a dry surface, 80% of the normal force of a brake can be transferred to friction. If it is wet, that figure reduces to 20%, and if it is icy, that goes down to less than 1%.
There is still friction at wheel lock, but the bigger problem is the loss of the ability to steer; inertia pushes the vehicle straight. You can turn the steering wheel, but the vehicle will retain the direction of its initial movement.
To prevent this, antilock braking systems (ABS) came to trucks in the mid-1970s, and to trailers in the 1990s (Modal, 1991; Modular, 1998). They compare the vehicle speed with the rotational speed of the wheel. If the wheel is determined to be locking, or moving slower than the vehicle, ABS releases the brakes for a short period – it does not apply the brakes — until the two come into sync.
This variable is called slip factor. Zero slip means that the wheel is free spinning; 100% slip factor means that the wheel is locked. Optimal braking and optimal steering require 20-30% slip. To release the brakes, ABS just releases air from the brake chamber. In an ideal world, ABS shouldn’t engage. If you drive gently observing everything in front of you, ABS does not do anything.
A few years later, electronic braking systems emerged for trailers, such as EB+ Gen1 in 2001. Compared to ABS, trailer EBS systems can apply the brakes, and do; in every brake application, EBS is in charge. That happens by receiving an electrical signal for brake application from the tractor unit.
The biggest improvement is that there is no lag between the time the brakes receive a signal and are applied. While pressure signals propagate through air at the speed of sound, electrical signals travel at the speed of light. For that reason, EBS significantly reduces stopping distance. An additional solenoid is employed to do this. There is still a relay connected to the air reservoir, which acts as braking energy storage.
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