How ABS works
When the driver hits the brakes this pressurises the hydraulic system which causes the brake
pads to squeeze against the discs which causes the car to slow down. If the ABS system
detects that one wheel is slowing down more rapidly than the rest (a symptom of wheel-lock)
it automatically reduces the brake pressure on this wheel by opening a pressure release valve
in the hydraulic system. ABS also has the ability to build the pressure back up via the
hydraulic motor. The system reacts remarkably quickly, and compared wheel speeds many
times a second. ABS systems can act on just the front wheels (which do most of the braking
work), or all four depending on what car you're in.
Diagram 2: Wheel speed sensor (A), ABS control module (B)
hydraulic motor and pressure release valves (C), Brake pipe leading to caliper (D)
ABS in detail
Diagram 1 (above) shows the whole process in more detail. The four wheels shown are
braking heavily as a result of heavy brake pedal application, and the green wheel is about to
lock up due a low friction surface such as ice. The ABS control module (B) detects the onset
of a skid through the sensor (A) in this wheel and reacts by releasing the brake pressure
slightly by rapidly opening a pressure release valve (C). This lowers the pressure in the brake
pipe (D) which causes the brake caliper to loosen its grip on the brake disc on the locking
wheel. If this corrects the locked wheel, the hydraulic motor (C) will build up the pressure
again to the optimum braking force and the valve will revert to the closed position.
If you've ever seen a car with ABS operating, you'll notice that the wheels appear to lock and
release in jerky movements, this is because there is such a fine line between grip and traction
loss in these conditions, and the system constantly tries to keep the pressure at the point just
before lock up (which is the most effective pressure for slowing down).
Three versus four channel systems
There are two main types of modern ABS systems: three and four channel. Three channel
systems control the braking pressure on both front wheels independently, but control the rear
wheels together as a single unit. Four channel systems (as shown in Diagram 1) control the
brake force on each wheel independently (assuming you're in a four wheeled vehicle!). In
three channel systems, although both rear wheels are monitored by sensors, if wheel lock if
detected on a single wheel the hydraulic braking pressure is reduced equally on both wheels.
This does not provide the level of control of a four channel system, and thus three channel
versions are compromised and usually only fitted to cars to save on cost. Most modern cars
now use a four channel ABS systems which provide greater safety in emergency braking
conditions.
ABS software tuning
The ABS ECU contains thousands of parameters which can be tuned for the car – although
most ABS systems are supplied by companies such as Bosch, it's the software tuning which
makes individual systems different for different cars. Vehicle manufacturers must strike a
compromise between stability and the braking distance they hope to achieve, and must also
set parameters for braking on different surfaces and scenarios. Even if two different cars have
identical ABS hardware – it will have been tuned very differently for a sports car compared
with an SUV.
How can I make best use of ABS?
ABS works best with a firm, steady application of pressure to the brake pedal. During ABS
operation you're likely to experience some vibrations, which are the pressure release valves
opening and closing and an indication that the system is working correctly. Don't release the
pedal until you have safely stopped.
ABS vs no ABS
You may have heard that it's possible to slow down quicker in a car without ABS. This has
elements of truth, but in practical terms the benefits of ABS massively outweigh the slightly
longer braking distances. For road use, ABS is an absolute must as it will allow you to steer
out of the way of unexpected hazards. Diagrams 2 and 3 below explain the practical reasons
for ABS.
Diagram 2: Braking hard on a slippery surface with ABS
In the diagram above, the driver hits the brakes hard on a slippery surface (1), then steers (2),
and successfully manages to avoid the obstacle. ABS prevents the wheels from locking, and
this allows you steer.
Diagram 3: Braking hard on a slippery surface without ABS
Without ABS, as the driver applies the brakes the wheels lock (1). Despite applying steering
lock, the car continues straight ahead due to the loss of steering control (2) and a collision
results.
ABS on the track
If done correctly, threshold braking can slow a vehicle more quickly that a car fitted with
ABS (in the right conditions). Many competitive racing drivers would say that they prefer
their cars not to have the system. However, to gain maximum advantage you will need to
practice the threshold braking technique repeatedly. A good test is to find a section of test-
track, and mark out a cone which will act as the braking point. Accelerate to 60 or 70mph and
brake as hard as you can in a car equipped with ABS, then repeat without ABS. Compare the
stopping distances by having someone mark them on the side of the track, then practice
threshold braking until you notice a marked improvement. If you do have ABS fitted, don't
get into the habit of locking up wheels before entering a corner - this isn't the quickest way of
slowing down and can unsettle the car.