How ABS works

Kevin Ash

(click on images)


It`s still commonly thought that ABS works by comparing the speeds of the front and rear wheels, releasing brake pressure if these should differ significantly. This is wrong.

Fairly obviously it couldn`t work, as if you happened to lock both wheels together the system would not be triggered, yet this could happen in a panic situation.

In fact it`s not the speed of a wheel - each is controlled independently - which matters to the computer at the heart of an ABS system, but its deceleration. On each wheel is a serrated ring (which looks a bit like a gear) or a disc with holes around it, which rotates close to a sensor. Each of the serrations generates an electrical impulse as it passes, which is sent back to the computer. The computer measures the time it takes for, say, 10 pulses to arrive and stores this piece of information. Then it measures the time it takes for the next 10 pulses to arrive, compares it with the previous time and keeps doing this with every successive batch of 10 pulses.

KTM SM-T ABS sensor ringKTM SM-T ABS sensor ringWhen one batch of 10 pulses takes longer than the previous batch, it means the wheel is decelerating. In normal circumstances, no problem, that`s what brakes are for. But as well as comparing successive times, the computer also calculates the deceleration rate and compares it with predetermined figures stored in its permanent memory.

These are arrived at by engineers during the bike`s development process, who have to find out by trial and error how much wheel deceleration is acceptable (when you`re slowing down normally) and how much is not (when the wheel is locking up). Once they have these numbers they`re programmed into the computer, and if this detects a current wheel deceleration which matches or exceeds the predetermined rates, it sends out a signal to a valve in the brake lines, which opens and releases the brake pressure, to stop the lock-up just at the point it`s starting to happen.

Compact and light Bosch ABS 9 pump and valve unitCompact and light Bosch ABS 9 pump and valve unitThis pressure is restored again by a high pressure hydraulic pump, maybe 1/10th of a second later, so the brakes are reapplied. With the brakes on again the computer continues to monitor the deceleration rate of the wheel, and if it decides this is still too fast it will once again send out a signal to release then reapply the brake pressure, and it will keep doing this until the wheel deceleration returns to an acceptable level.
What the rider feels is pulsing through the brake lever and some lurching of the bike as the pressure is constantly switched on and off. What he doesn`t feel is the abrasion of gravel on his legs.

The latest Bosch ABS 9 system adds some refinement by also looking at brake line pressure. A measure of this - how hard the rider is squeezing or pressing on the levers - provides additional information on the likelihood of imminent wheel lock-up, especially useful to know when the bike is being braked hard as it enters a corner. In this situation, the brake pressure will be reducing but the likelihood of lock-up is increasing as the bike leans over further. This is a step towards the ability to brake hard enough to activate the ABS while pitching into a corner and increasing the angle of lean.

Bosch ABS 9 system layoutBosch ABS 9 system layoutABS 9 does make a wheel speed comparison, but only to help control rear wheel lift and not directly to aid the ABS. If the back wheel is slowing more than the rear and the pressure measurements show the front brake is on very hard, it can the computer that the rear wheel is off the ground and there`s a danger of looping. So the front brake pressure is released just enough to prevent this and the bike remains stable with the rear tyre only dipping up and down.

That`s the theory, in practice the system slows down the tendency to loop over the front, but isn`t always effective in stopping it completely, so if it starts to happen the rider still needs to release the front brake.

See Comment: ABS

Testing ABS at the Bosch Proving Ground