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The knowledge: ABS

Stopping on a sixpence

Ever wondered how ABS works and what it's for? Wonder no longer...

ABS warning light

ABS warning light

 
Technology that solves problems can be a great enabler, and ABS is one of the best
ABS does more than you think, you probably already know that it prevents a wheel locking up and skidding, but did you know that a modern system will actually stop the car in a shorter distance than an expert race driver could, or that it actually helps cars go faster? The technology on your car today is the stuff of dreams, well engineers dreams anyway, and these dreams started when an aeroplane skidded on an icy landing strip...

Picture the scene; the weather is terrible, the ground is frozen, but you have to land your plane as you're running out of fuel. You touch down and apply the brakes but as the wheels race over patches of ice the plane is jolted left and right, eventually loosing steering control and skidding onto the grass. This is technically known as a bad thing, and the solution is to prevent the wheels locking up so it could maintain steering, but interestingly it turns out that a very small amount of tyre slip is very beneficial, to explain here is a brief science lesson, but don't worry there is no exam at the end.

Many decades ago engineers tested tyres with special machines on dry roads, applying progressively more braking force whilst maintaining a constant speed. When there was no force transmitted through the tyre and it was just free-wheeling along, there was no slippage between the tyre contact patch and the road surface, but as the brakes were gradually applied the tyre started to very slightly slip. The tyre coped well up to a point where the slip rate became just too much for the tyre to cope with and it started to skid, peak braking force occurred when the tyre was slipping about 5%, that is to say the tyre was going 5% slower than the road. But as the brakes were applied further the tyre was unable to grip, the slip rate increased dramatically and the force generated by the tyre dwindled, this is a skid.

That is one of those great facts to drop into conversations at the bar; a locked skidding tyre can only transmit a small fraction of the braking force that it could if slip was maintained at its optimum, we have all seen this demonstrated on race tracks where a car skidding sideways carries on for quite some distance rather than coming to a sudden halt.

The other problem is a skidding tyre is not going to provide any steering force either, meaning that as well as not stopping in time you are going to hit the very thing you wanted to avoid. So clearly we need a mechanism that allows the wheel to still turn in an emergency braking situation.

And  so the first ABS systems were devised, by measuring wheel speed they could avoid a wheel locking up by relieving the braking pressure enough to let the tyre recover traction, then re-apply the braking pressure to continue slowing the vehicle. This cycle could be repeated several times a second if needed.

When applied to cars this cycling of the brakes produced a noticeable judder on the brake pedal which in a very small number of cases alarmed the driver enough for them to take their foot off the pedal and proceed to crash. Also every time it relieved the brake pressure by allowing some of the fluid to go back to the reservoir the pedal moved down a touch, so on a prolonged ABS braking event the pedal could actually sink to the floor, necessitating some quick thinking and pumping the pedal. But despite these issues the early systems maintained steering control and many thousands of accidents were avoided or their severity reduced.

However, at this early stage in development a good professional driver on a dry road could hold the brakes just at the point of maximum force and out perform an ABS system, once this became public knowledge rumours started that ABS was not as good as it was cracked up to be. But engineers are never satisfied and development continued.

More tests revealed that if a wheel starts to skid the brakes have to be backed of almost completely to allow it to regain traction, the skidding tyre generates little force to make it spin up again and the wheel and tyre inertia takes time to overcome. So the systems were refined, they analysed the wheel's speed in such a way as to react to how fast the speed was changing. The systems were calibrated on prototypes by doing a series of emergency stops in perfect test conditions, this would show the maximum braking ability of the car, and so the maximum rate at which the wheel could possibly be slowing down without skidding. Then, on the production cars, by comparing what the wheel was currently doing to the stored maximum plausible value it could deduce when the wheel was about to lock before it actually did so, yes your car looks into the future for you, isn't that nice.

This meant that the brakes could be relieved earlier and by a smaller amount, reducing the pulsations at the pedal, and because the tyre spent more time during the emergency stop generating a slowing force it also improving braking performance.

The systems were further enhanced by fitting an electric pump that pushed fluid back into the callipers after the ABS had relieved it, so then the pedal would not have to sink to the floor.

Now systems could match the performance of a reasonably talented driver and were significantly better than the average driver, but still more was to come.

Early systems treated each axle as a single unit, so if one wheel started skidding then both wheels on that axle would have their brake pressure reduced so the car didn't pull sideways. But as suspension and ABS design improved it became possible to control each wheel independently making the best use of the traction available under each tyre. These systems could cope with patchy ice or roads where mud had accumulated on one side, this system offered performance in difficult conditions that no driver no matter how expert could ever match.
The response speed was also increased so that the tyre slip rate was controlled to keep it on the point of maximum performance, so even a driver of minimal talent could brake as hard as they want and the car would sort everything out for them.

The one failing all these systems had was on snow where it is essential to lock a wheel to build up a wedge of snow, modern systems analyse the ease with which the wheel slips and can deduce that they need to provide higher slip rates, also a 'Snow' button is often provided so the driver can pre-warn the system.

Modern systems also have sensors which measure the way the car is turning, these 'yaw rate' sensors are similar to the ones fitted to aircraft and are used to balance the braking force across the car to maintain the desired direction of travel. An extension of this applies the brakes gently to the inside wheels when cornering hard in order to help the car to turn, this is stability control and can eliminate over and under steer. It is one of the innovations that has enabled extremely high powered cars to be driven by an ordinary driver with no specialised training.

Consider how far we have come, less than 20 years ago the Ford Cosworth was considered too fast to handle by many with 205bhp on tap, now an ordinary family saloon has more than that, the Land Rover Evoque has 240bhp as standard, the Audi A8 saloon is available with a super-car 500bhp but can be driven safely with ease. Technology that solves problems can be a great enabler, and ABS is one of the best.

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