How does brake assist work?

By Martin Pretorius

Did you know that most of the earliest motorcars (dating from the late 1800s) only had brakes on the rear wheels, and that they were operated by intricate arrangements of levers and/or cables? Brakes on the front wheels only started appearing a good decade later, and hydraulic brakes were first fitted to a car in the early 1920s.

For the next half-century, progress slowed somewhat, and apart from disc brakes coming along in the late 1940s, not much have really changed until the introduction of electronic anti-lock systems at the end of the 1970s.

Then came ABS

The advent of anti-lock brakes (ABS) signalled the start of a major revolution in automotive braking. When the first ABS systems were introduced, they did little more than keep the brakes from locking up (preventing a skid), but engineers soon realised that the new system could be adapted and improved, thus opening up some interesting new applications.

This brings us to the modern era, where it's commonplace for the brake system's electronically-controlled hardware to perform a host of secondary functions. Today's automotive brakes also act as traction- and stability control actuators, adding measures of driving safety which would have been inconceivable even a mere two decades ago. And it's all thanks to the ingenuity of the ABS and its clever network of sensors.

 Braking is still the first priority

While the ABS can be adapted to perform all manner of impressive tricks, its main focus is however still on making the car stop as quickly as possible. To this end, the engineers started looking at ways to optimise their systems to improve not only outright braking power and stability, but also the response time.

How relevant is the brake response time?

Well, it's a simple matter of maths, but we're not going to go into that in too much detail now. For the sake of this explanation, there's one thing you need to know: When a car is travelling at 100 km/h, it's covering a kilometre every 36 seconds. That means that the car travels almost 28 metres with every passing second.

Adding typical human reaction times (it takes the average driver about 2 seconds just to let go of the accelerator and press the brake pedal) to this equation means that, even at a comparatively low speed of 100 km/h, a car will usually travel more than half the length of a rugby field (about 56 metres) from the moment the driver recognises a hazard till the moment they start braking.

There's a time delay built into the car, too

Braking doesn't begin the moment the driver step on the brakes, either – there's a time delay from the moment the brake pedal is pressed until full hydraulic pressure (and thus maximum braking force) is achieved. This time delay could be as much as 0.3 seconds – or the equivalent of about 10 metres of travel.

Now there's nothing to be done about the slowest responder in this scenario (the driver), but ways have been identified with which to remove any further delays from the braking sequence.

 Computer to the rescue!

This is where the clever electronics comes in handy. By monitoring the speed with which the driver applies the brake pedal (and sometimes even by looking at how quickly the driver lets go of the accelerator), the computer instructs the ABS pump to apply maximum hydraulic pressure faster than the driver could apply full force at the pedal.

The car basically recognises when the driver needs the maximum available braking force, and applies it much quicker than the driver ever could. From that moment, the ABS takes full control, usually distributing the braking force between the wheels with the most traction, keeping the car under control and preventing unwanted skids at the same time.

Brake assist has now become commonplace

Since its introduction in the 1997 Mercedes-Benz S-Class, brake assist systems have been incorporated into most ABS-equipped cars. It uses fundamentally the same hardware and sensors as the basic ABS would use, just with a few lines of extra code in its computer, yet it can make the difference between coming to a safe stop or crumpling your car's front end like an empty chips packet. 10 metres (or more) difference in stopping distance will definitely make a big impact on your panel beater's bill, don't you agree?