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#1 15-07-07 19:19:28

dog
Verified Member
Registered: 29-10-06
Posts: 813

A good explanation of ABS

I thought I would post this here for the benefit of people who don't know what ABS is, what it does or how to use it. I'm not an expert or anything but I know a fair bit and I will try to explain it as simply and understandably as possible.

What it is
ABS (Anti-lock Braking System) is a safety device for cars developed by Bosch and Mercedes. It has been offered as an optional extra on top of the range luxury cars since the late 1970's, but in recent years many manufacturers have fitted it as standard on their top of the range cars, and have had it as an optional extra on other models. Some luxury car manufacturers such as BMW, Mercedes and Jaguar have had it as standard equipment on all of their cars for the last few years. It is now appearing on ALL new cars, regardless of whether they are top of the range or not.

Emergency braking
Before ABS, emergency stops were typically performed by depressing the brake pedal as hard as possible. This would cause the pistons in the disc brake housing to push the calipers into the disc with so much force that they would stop (lock) the wheels.

The majority of road cars nowadays are built in a FF (front engine, front wheel drive) configuration, this means the driving wheels (the wheels which the engine provides the driving power to) are the same as the steering wheels (the wheels which the steering wheel controls).

While the wheels are in contact with the surface below them (usually a road, or in Tim's case, a mountain) and the car is normal motion (for example, driving along a road), they are encountering rolling resistance. It is rolling resistance which causes a rolling vehicle on a flat surface to eventually come to a halt. The amount of rolling resistance is affected by the material surrounding the wheel, for example a rubber tyre gives a lot more rolling resistance than steel. The surface which the wheels are on is also a factor.

When a wheel locks whilst the vehicle is still in motion, sliding friction is encountered. Sliding friction is the force which occurs when two solid objects slide against each other, for example a locked tyre on a tarmac road. Since the wheel is not rotating, there is no rolling resistance.

Without sliding friction between the steering wheels and the surface below them, there is no steering control. This means that locked wheels are in effect, unsteerable. So when a child runs out in front of your non-ABS equipped car and you instinctively slam on the brakes and attempt to steer around them, your wheels will lock and depending on your speed, reaction time, distance from the child and road conditions, you may not stop before you hit them. The steering has no effect because all the steering wheels are doing is sliding across the ground. When the steering wheels are locked, they are about as useful as a big square rubber block as far as steering is concerned.

There are techniques to combat this, however. One of these is cadence braking. Cadence braking is normally only performed by skilled and/or professional drivers who have fast enough reactions to do so. It involves constantly depressing and releasing the brake pedal as fast as possible.

Maximum braking effectiveness is achieved when there is approximately 11% slippage between the braked wheel's rotational speed and the road surface. At this point, rolling resistance is maximised and there is very little sliding friction. Beyond this amount of slippage, rolling resistance diminishes rapidly and sliding friction alone slows the vehicle.

In an emergency stop, the braked wheels pass beyond this point very quickly and there is only very small fraction of a second when the braked wheels are at maximum effectiveness, before they lock.

With cadence braking, the constant depressing and releasing of the brake pedal causes the wheels to follow a pattern of lock-unlock-lock-unlock. This causes the vehicle to both slow down (any time whilst the wheels are being braked and also whilst they are locked) but also allows the driver to maintain some steering control (any time whilst the wheels are spinning). Since the braked wheels are constantly passing through the point of maximum braking effectiveness, this technique is highly effective in both stopping the vehicle quickly and also allowing the driver to steer whilst doing do.

However, in a situation where a driver would need to use this technique, they may only have split seconds to react by hitting the brakes (reaction time) and then only a couple of seconds before the vehicle reaches the object it is going to collide with (braking time). Inexperienced drivers and even most experienced drivers would not be able to think quickly enough to apply the technique before they had collided with the object.

The other technique used to maintain steering control under heavy braking is threshold braking. The idea of threshold braking is to only apply a certain amount of pressure to the brake pedal, so as to hold the braked wheels at or very near to the maximum effective braking force without exceeding it and locking the wheels. As with cadence braking though, this requires skill and a less experienced driver may not think of applying the technique before it is too late.

What ABS does, and how
ABS basically just applies the cadence braking technique, but much faster and far more effectively than any human being ever could. The ABS system normally consists of wheel speed sensors on each of the braking wheels connected to a computer, known as the ABS controller. The wheel speed sensors constantly measure the speed of the wheels, and feed the information back to the ABS controller in real time.

If any one of the sensors detects that a rotating wheel is rapidly decelerating (a condition which will eventually bring it to lock), it alerts the controller which then opens valves to release pressure on the braking circuit. The newest version of the anti-lock braking system can do this multiple times per second, must faster than even the most experienced drivers can. It also reacts so fast to harsh braking that it appears to be instant, much faster than a human would before they starting applying cadence braking.

On a surface such as a dry tarmac road, the ABS can activate and work so fast it will allow the wheels to lock and then spin again so fast that the human eye would not even be able to see the wheel stop spinning until the car comes to a halt. On some surfaces though, such as loose wet gravel, snow and ice, less braking force is required to lock the wheels since there is much less rolling resistance between the tyre and the surface. ABS is still highly effective on these surfaces, although it is more possible to see the wheels briefly lock and unlock until the vehicle stops.

Thanks to ABS, during emergency braking the vehicle remains steerable without the driver having to apply any special braking techniques. This allows them to simply depress the brake pedal as hard as they can, and then concentrate on steering as the ABS will take care of stopping the car quickly and without wheel lockup.

Using ABS is simple, if you're in an ABS-equipped car then all you need to do to use it is depress the brake pedal as hard as you can and keep it down until you stop - ABS does the rest.

Remember - ABS was not designed to reduce braking distance, so always keep a safe distance. Although it can reduce stopping distance on some surfaces, on other surfaces such as snow, it can increase it. This is because the best way to stop quickly on snow is to lock the wheels so that a bank of snow forms in front of the wheels.

Further advances to vehicle safety are coming into cars nowadays such as electronic brakeforce distribution, an enhancement to ABS which detects differences in the road surface between each wheel and correctly distrubutes braking force appropriately. If a car is being driven on a road and the two left tyres are on ice and the other two are on tarmac, less braking force will be required to lock the wheels on the ice than the ones on the road. EBD recognises this and can apply more braking force to the wheels on the tarmac than to the ones on the ice, so the braking force is equal on both sides of the car at all times.

Emergency brake assist is an even newer system which uses radar technology to analyse objects around the front of the car. If EBA senses that the distance between the vehicle and an object in front is suddenly decreasing (a situation which will eventually result in a collision if the speed is maintained) and the driver is not applying sufficient braking force to stop the car before it will collide, it assists the driver by applying enough pressure on the brakes to stop the car before an accident happens. Like ABS, no extra action needs to come from the user for the system to activate.

And finally, here is a video which demonstrates the advantages of ABS and how to use it: http://www.youtube.com/watch?v=xd5KDvXbE3M

I hope this is of use.

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15-07-07 19:19:28

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Re: A good explanation of ABS



#2 15-07-07 21:27:47

posh
Verified Member
Registered: 21-01-06
Posts: 855

Re: A good explanation of ABS

very useful post dog  smile the three s,s stick in your mind stomp.stay steer.

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#3 07-10-07 11:49:59

Zipper
Verified Member
From: Darwin, Northern Territory Aus
Registered: 20-08-04
Posts: 2,695
Website

Re: A good explanation of ABS

...Although it can reduce stopping distance on some surfaces, on other surfaces such as snow, it can increase it. This is because the best way to stop quickly on snow is to lock the wheels so that a bank of snow forms in front of the wheels...

Hi dog, snow is something I'll probably never experience.
I've heard that ABS can increase stopping distance on snow.
I've also heard that ABS can increase stopping distance on ice (something else I'll never experience except in my drinks glass) - is this true too?


Zipper ("G'Day Mate!")
I'm not 65! I'm only $59.95+tax
www.drivingnt.com

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#4 07-10-07 12:05:37

Dimmy
Guest

Re: A good explanation of ABS

on soft snow, gravel, and other loose materials, you can intentionally lock the wheels which will have the effect of creating a dam in front of the wheels, allowing the wheels to "burrow" into the loose material and as a result the car will slow faster, but will have limited or no steering ability.
ABS prevents the wheels from having that same effect, the result is, better steering ability but reduced stopping ability.

ABS, when utilised correctly WILL improve stopping distances for the AVERAGE driver on icy or slippery road conditions, as their reaction time to a locked wheel scenario is typically rather poor. HOWEVER, a highly skilled and experienced driver may be, and often is, able to sense how much braking force is available on the road surface at the time and maintain as much braking ability as humanly possible but BELOW the wheel lock moment, the result is no ABS activation and shorter stopping distance when on ice or other low grip surfaces.

The average motorist only THINKS they are highly skilled and experienced, and that is a distinction that MUST be made.

#5 07-10-07 12:14:01

Dimmy
Guest

Re: A good explanation of ABS

If you want to see the effects of braking on loose surfaces, look at a motor race where gravel traps are used. The racing drivers know the effects of locking the wheels on such surfaces and will INTENTIONALLY lock the wheels if they believe they cannot control the cars steering and are going to hit an obstacle. This will slow the car VERY quickly in these gravel traps.
BUT, if the driver beleives he can regain control of the car and his speed is reduced enough, he will prevent wheel lock and take advantage of steering ability to try to get back to the racing track.

#6 08-10-07 15:56:49

dog
Verified Member
Registered: 29-10-06
Posts: 813

Re: A good explanation of ABS

ABS is banned in most motor racing though, isn't it?

Also, again, the objective of ABS is to retain steering control. Not to stop you faster as the majority of the population believe.

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#7 08-10-07 22:33:26

dog
Verified Member
Registered: 29-10-06
Posts: 813

Re: A good explanation of ABS

dog wrote:

...Although it can reduce stopping distance on some surfaces, on other surfaces such as snow, it can increase it. This is because the best way to stop quickly on snow is to lock the wheels so that a bank of snow forms in front of the wheels...

Hi dog, snow is something I'll probably never experience.
I've heard that ABS can increase stopping distance on snow.
I've also heard that ABS can increase stopping distance on ice (something else I'll never experience except in my drinks glass) - is this true too?

ABS on snow: http://uk.youtube.com/watch?v=Sq2iSXBHcyo

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#8 09-10-07 07:05:45

Dimmy
Guest

Re: A good explanation of ABS

ABS is banned in most motor racing though, isn't it?

Also, again, the objective of ABS is to retain steering control. Not to stop you faster as the majority of the population believe.

Yes it is, but my post didnt say it was allowed. the post was to hilight the DRIVERS ability/decision wether to lock wheels on the gravel or not, depending on wether he needed to steer OR stop.

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