Introduction
The emergency stop driving tutorial will have introduced you to how to stop your car in an emergency and if you're taking driving lessons in hull with me we'll have plenty of practice at how to do it.
This page has been included for those of you who wish to know more about advanced braking systems and how they work.

ABS - what it does and how it works
We’ve all heard of ‘anti lock brakes’, but most people don’t know much about ABS or how the system works. I’m sure that most people know that it’s a safety feature, but there’s a danger that some drivers may think that they can rely on it to keep them safe and not have to think about braking distances and safe following positions. One thing that must be accepted is that ABS will not make you a safe (or safer) driver. You still need to apply all the rules of road safety that you’ve learned.

You don’t need too much detail to understand the general principles of how the system works so I’ve tried to make the following explanation as simple as possible.

ABS is basically an enhanced or improved version of an ordinary braking system and it was originally designed and developed for aircraft, allowing the pilot to stop the machine safely under all conditions. ABS is designed to stop the brakes from locking up and prevent the wheels from skidding when braking hard or when braking on wet or slippery surfaces. This is a significant safety feature, assisting your driving by preventing dangerous skids and allowing you to maintain steering control while trying to stop the car.

ABS doesn’t always reduce your stopping distance. In fact your stopping distance can actually increase on dry roads. The real advantage is on wet roads where ABS may reduce the stopping distance by up to 25% or even more. Think about it. . . this could be the difference between a safe stop and a very serious collision.

There are several different ABS systems in use, but they all have the ability to control or prevent wheel lockup during hard braking. The only contact that your car has with the road surface is the tyre’s. Nothing else. When you need to stop quickly and you apply the brakes hard and fast, there are tremendous forces of friction from the road surface and the rubber compound of the tyre’s. It’s these friction forces that bring the car to a stop.

It’s quite hard to imagine, but a tyre that is producing the maximum braking effect is just on the verge of skidding. A wheel that is just on the verge of skidding produces more friction with respect to the road than one which is already locked and skidding. What I mean is that once all traction is lost and the wheels are completely locked, the friction is reduced and the vehicle takes longer to stop.

There is one interesting exception to this rule. This is when a car is travelling on loose snow. A locked tyre on loose snow allows a wedge of snow to build up just in front of it which allows it to stop in a shorter distance than a rolling tyre. ABS system manufacturers are aware of this. This is why some vehicles have a switch for deactivating the anti lock braking system when driving on loose snow.

Steering control also depends on traction. A wheel that is still able to turn will move the car only in the direction it turns. Once the wheel skids, the driver loses just about all directional control and is at the mercy of the forces of nature. By preventing or minimising this loss of traction, anti lock braking helps to maintain steering control and creates a far safer driving environment.

ABS is an enhancement to the existing brake system of a vehicle that only comes into play when traction is reduced or during emergency stops. Most of the time, it has no effect on your normal driving or braking.

ABS systems are usually designed to be as safe as possible. If a problem occurs, most systems will deactivate themselves immediately. The ABS warning light will come on in these circumstances, but the vehicle should still have normal braking control. The vehicle will be safe to drive, but the ABS system won’t work in an emergency. Essentially, you’ll be driving a car with a completely normal braking system, as though ABS wasn’t fitted.

Don’t ever ignore the ABS warning light. If it comes on, have the car checked out by a garage. If the brake warning light is also on, this could indicate a serious loss of hydraulic pressure or a low fluid level and the vehicle should not be driven until the brakes can be checked.

So, how does ABS work?

Put very simply, the wheels are fitted with ‘wheel speed sensors’. These sensors send an electrical signal to a control unit. The control unit analyses the signals from each wheel sensor and decides what to do. If the control unit decides that it needs to override your braking, it will do so and it will automatically vary the braking pressure on each wheel. Anti lock braking systems control tyre skid by monitoring the deceleration rates of all the wheels during braking. If one wheel starts to slow down quicker than the other wheels, or at a faster rate than is programmed into the control module, it means that the wheel is starting to skid and could be in danger of losing traction and locking. The ABS system controls the situation by reducing the hydraulic pressure to the brake on the affected wheel for a very short time. This could be on one wheel or more, depending on how many the control unit detects as being affected.

Electrical solenoid valves and an ABS pump are used to release and reapply the pressure to the brakes. This produces a rhythmic ‘pulsating’ effect, which can usually be felt in the brake pedal during hard braking. In fact, on some cars, the pedal seems to ‘collapse’ towards the floor and very little pressure is felt by the driver as the system takes over completely. You may also hear a ‘banging’ noise from the ABS hydraulic unit. Some people find an ABS system in full activation to be quite a frightening experience!

This rapid applying and releasing of brake pressure in the brake circuit reduces the load on the wheel and allows it to keep its traction, thus preventing any lockup. It’s just the same as pumping the brakes very quickly with your foot (cadence braking - a different story!!), except that the ABS system does it all automatically for each brake circuit, and at speeds that would be completely impossible for a driver to imitate. Many systems can apply and reapply the pressure 30 or 40 times per second. Some even more.

The pressure in the braking system is released by high speed valves. After the pressure has been released, the control unit needs to reapply it. It does so by means of an ABS pump (pictured above). It then opens the valve to release the pressure, then activates the pump to reapply it, and the whole cycle goes on and on at unbelievably fast rates.

Once the braking for all the wheels returns to normal and the danger of a lockup is avoided, the car reverts to normal braking and anti lock reverts to a passive mode.

As above, ABS systems keep track of wheel deceleration rates with wheel speed sensors. On some cars, each wheel is equipped with its own speed sensor. These systems are called ‘four channel’ system. Some cars have fewer sensors. Many four-wheel ABS systems have a separate wheel speed sensor for each front wheel but use one speed sensor for both rear wheels. These are called "three channel" systems.

Another variation is the "single channel" rear-wheel only ABS system that is used on many rear-wheel drive pickups and vans. Ford’s version is called "Rear Anti lock Brakes" (RABS) while Vauxhall call theirs "Rear Wheel Anti-Lock" (RWAL). The front wheels have no speed sensors and only a single speed sensor mounted in the differential or transmission is used for both rear wheels. Rear-wheel anti lock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it’s used on pickup trucks and vans. Because the rear-wheel anti lock systems have only a single channel, they’re much less complex and costly than the three- and four-channel, four-wheel types.

How to use ABS

ABS is fitted to millions of cars, but many drivers still don’t know the "right way" to use ABS in an emergency braking situation. So here are some tips for getting the best from your ABS equipped car in an emergency:

Try to keep your foot on the brake pedal. Maintain firm and continuous pressure on the brake pedal to let four-wheel ABS work properly. The ABS on most passenger cars is designed to activate when you ‘hit the pedal’ hard to stop quickly, so there’s no need to worry about being too rough with it! Avoid pumping the brake (cadence braking), even if the brake pedal is pulsating. If you own a truck or van with rear-wheel anti lock (RWAL) brakes, apply the brake pedal with just enough force to stop without locking the front wheels. This way you can maintain steering control while the rear-wheel anti lock system prevents the vehicle from skidding.

Always allow enough distance to stop safely. Follow two seconds or more behind vehicles when driving in good conditions. Allow more time if conditions are hazardous. Double the following distance on wet roads and leave even greater safety margins on ice and snow.

Practice using your ABS and get used to the pulsations that occur in the brake pedal when ABS is activated. Car parks or other open areas are excellent places to practice an emergency stop. When you need ABS for real, you’ll know what to expect and you won’t panic when the system ‘kicks in’.

Always consult your vehicle owners manual for instructions regarding your ABS system.

Never drive an ABS - equipped vehicle faster or more aggressively than a vehicle without ABS. Driving around bends faster, changing lanes abruptly or performing other aggressive steering manoeuvres is always dangerous. Keep in mind that ABS is a safety feature for cars when driven safely. It won’t save you if you drive like an idiot and crash into a tree.

As above, try not to pump the brakes. With four-wheel ABS systems, pumping the brakes turns the ABS system on and off, which decreases braking efficiency and increases your stopping distance. ABS pumps the brakes for you automatically at a much faster rate than you could do it yourself, and allows better steering control. What’s more, it also pumps the brakes on individual wheels as needed — which is something you can’t do.

Remember to steer! Four-wheel ABS can help you steer around hazardous situations, but your vehicle won’t steer itself!!

Clicking noises, pedal pulsation’s, knocking and banging are all quite normal when braking hard. These conditions are normal when ABS is active and lets you know the system is working. There should be no ABS feedback, though, when braking normally on dry roads. In fact, for most of your driving life you shouldn’t even know that you have ABS. If your ABS system comes into play more than once in your driving life, you need to think carefully about your anticipation and observation skills. ABS should only come into play during emergency stops or when the road is wet, icy, slick or covered with loose gravel.

Note: This page has been reproduced here with the permission of the author David Hartley. To see more of his work and look at another first class driver training site go to www.dashdriving.com