ABS: How It Works

Having considered the history of ABS, we can take a look at how Anti-lock Braking Systems work. And there’s no need to panic: we aren’t going to start on rocket science!

In fact, though ABS systems are complicated, the principle involved can be expressed quite simply. The system has to know when a wheel slows down under braking (i.e. when it locks). When it detects this, the system reduces the hydraulic pressure in the brake at the affected wheel. When the wheel speeds up again, the system reapplies hydraulic pressure, when braking is still required.

What makes ABS systems particularly clever is that they can deal with all four wheels at once, and repeatedly cut and reapply hydraulic pressure until the wheels lock again. Moreover, some can do this up to 16 times in a second.

This is what ABS does but how does it do it? The secret of its operation lies in a combination of valves and sensors, overseen by an Electronic Control Unit (ECU). At each wheel, a toothed sector – some look very like a cog wheel – is watched over by a fixed sensor. When the teeth aren’t passing the sensor’s detecting head as expected, the sensor tells the ECU, which takes the appropriate action.

As in many microprocessor-controlled devices, the ECU’s rapid responses are the result of extensive programming. The ECU therefore knows that on a car negotiating a curve, the wheels on the inside of the curve will turn faster than those on the outside. It also knows this difference should be ignored.

Under hard braking, especially on a slippery surface, the ABS is liable to be activated. When this occurs – as most drivers know by now – a rapid pulsation of the brake pedal will be felt.

Now we know the basic workings of ABS, we can look more closely at its components. The speed sensors described above are usually placed on the wheel hub. However, they needn’t be there; provided they monitor any part that rotates at wheel speed, they’ll work perfectly well. So, the speed sensor mechanism may be part of a drive shaft or even within the differential.

The valves situated in the hydraulic brake lines aren’t always simple, on/off devices. Most have three positions. Under normal circumstances, a valve may be considered as ‘off’. Then, the valve has no effect on the pressure in the brake line. ‘Position two’ can be considered as ‘on’ – the valve blocks the brake line. So, no matter how hard you press the brake pedal, you can’t apply any more braking. ‘Position three’ represents a level of sophistication in the ABS system. In this position, the valve allows some hydraulic pressure through.

The next component is an hydraulic pump, whose presence is an obvious requirement. Why? Because when you press the brake pedal, it is you who are creating pressure in the brakes’ hydraulic system. When the ABS system needs to reapply hydraulic pressure, it can hardly ask you to press the pedal harder. Even if it could, you wouldn’t be able to react anything like quickly enough. So, the pump is what replaces the brake line pressure when the ABS is in operation.

We’ve already discussed the ECU, which controls the system so there’s only one item left. The characteristic fast on/off action of ABS, that which you feel at the brake pedal, is carried out by this final component. This is the ABS modulator, which is, naturally, actuated by the ECU.

All this begs one especially important question: does ABS work? The short answer is ‘yes’ but it’s appropriates to garnish this bald fact with a few details. Long before ABS arrived on cars, rally drivers developed a technique called ‘cadence braking’. This was, in effect, a ‘manual’ version of ABS that involved repeatedly applying and releasing the brakes. In short, the driver would release the brakes as soon as he felt the wheels skid and instantly reapply them.

Cadence braking worked well, particularly on loose surfaces but then again, not all drivers were rally drivers. In the more prosaic world of normal driving, drivers tend to lock the wheels during a ‘panic stop’. A screaming, smoking skid with all wheels locked is extremely spectacular but locked wheels don’t provide much retardation. So, no driver in a car without ABS can improve on the stopping distance given by a good ABS system…on a good surface.

Paradoxically, ABS tends to increase the stopping distance on snow, gravel or sand. On these surfaces, locked wheels ‘dig in’ and stop the vehicle sooner. Similarly, sheet ice can defeat ABS, which depends on making comparisons between the car’s four wheels. Some ABS systems can cope with slippery surfaces better than others because they are able to reduce their cycling time, the number of times per second that they operate.

In some contexts, there is a major benefit of ABS that is sometimes missed. We know that locked wheels provide very little grip and therefore very little braking. Conversely, ABS lets wheels retain their grip by the car’s being on the point of skidding but not actually skidding. Here’s where the element of grip comes in: when the car is in the grip of ABS, as it were, you still have steering. So, you can steer around an obstacle – you can’t do this in a skid.


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