View Full Version : A Question

3rd Jul 2001, 20:33
Ok to settle an argument between myself and a colleague would someone answer this:

If I were driving along a dry road at 35mph and I did an emergency stop would I stop in a shorter distance with or without ABS?

Could you give me reasons why please?

Thanks, Bodge

3rd Jul 2001, 20:49
On a dry road you would probably stop just as short without ABS. The friction from locked tires on dry pavement is still very good.

On wet pavement however, ABS could save your bacon.

Same on snow and ice.

Men, this is no drill...

Big Red ' L '
3rd Jul 2001, 20:59
ABS doesn't make you stop quicker (if it does, your talking only milliseconds....) but it helps the overall control of the vehicle when hard braking and manouvering at the same time.

Winston Smith
3rd Jul 2001, 21:46
That's a difficult one:

While it's true that on the one hand a car without ABS might stop after a shorter distance because it doesn't have the short intervals during which the wheels are allowed to turn, a positive side-effect of ABS on a dry road might possibly be the fact that you do not perform the entire deceleration on the same piece of tyre surface. Without ABS, after skidding a few yards along the road some of the rubber would probably wear off and decrease the braking efficiency, whereas ABS would continually "exchange" the area which has contact with the road.

3rd Jul 2001, 22:08
My understanding is that ABS allows you to stop in the shortest distance possible in the braking conditions available at that time. Tyre condition also affects stopping distance.

When you skid, you are actually sitting on a pad of molten rubber and the stopping distance will increase.

While it is true that brake pressure is released for a milli second and re-applied during braking, this only re-measures the friction available for stopping and will still give a reduced stopping distance.

Stay cool, stay longer.

Dr Jekyll
4th Jul 2001, 00:19
ABS allows you to brake hard and steer simultaneously. Thats all.

It could be argued that on a slippery surface most drivers would not brake as hard as they might for fear of skidding, so in these circumstances an ABS equipped car might stop sooner. On the other hand,in police tests non ABS equipped cars have stopped slightly sooner.

4th Jul 2001, 01:31
Dr J, could you explain why we have such a large penalty when operating with anti skid U/S. We are not permitted to operate from wet runways at all.

To give an example. B767-200 cat "c" brakes.
Landing WT 110,000kg, 0% slope, 0 wind and runway at sea level. Landing distance required 5110 feet. With anti skid u/s we need 9750 feet. Ie it takes longer to stop because we humans can't react as quickly or tell when the wheels lock as effectivley as a computer.

I would submit that if we could apply the brakes, feel the skid start and then release the brakes in a milli second and keep doing it until we stop, we could match ABS. I don't know about you, but I most definitely can't.

Just one more thought, we don't steer with the main wheels at all so it can't be anything to do with that.

Stay cool, stay longer.

Send Clowns
4th Jul 2001, 01:51
Just a pure physicists idea:

Absolute minimum is technically the same in both cases. This is maximum break pressure applied without wheels locking - no skid, ABS does not intervene.

In practice, anti-skid/ABS shorter every time (unless cornering, when it is a liability apparently), just slam the brakes on.

Incidentally some modern systems detect emergency braking and apply full brake pressure, as people supposedly instinctively back off. Not sure about this, as I have natural distrust of machines assuming I want something (hence my hatred of Microsoft) and because I remember the first accident I was involved in. I remember when someone pulled out in front of me being able to make a conscious note that I was not in my Dad's car (ABS) but an airport courtesy car (no ABS - in the wet, the reason I wasn't taking my FAA PPL flight test at the time!) and brake accordingly, so I am not convinced people cannot brake as they think fit.

Code Blue
5th Jul 2001, 04:33
some modern systems detect emergency braking and apply full brake pressure, as people supposedly instinctively back off

I think it was Mercedes that did the study, but apparently only a minority of drivers (?45%) actually exert max pressure on the brakes in an emergency stop situation and hence the development of "emergency braking systems".

if we could apply the brakes, feel the skid start and then release the brakes in a milli second and keep doing it until we stop, we could match ABS

Many moons ago, prior to ABS, the UK Police drivers used to teach a technique called "cadence braking" which did pretty much what you described, tho' I don't recall milliseconds as the time units they used :)

edited 'cos I couldn't work out wot happened to the UBB codes

[This message has been edited by Code Blue (edited 05 July 2001).]

5th Jul 2001, 04:41
Two wheeled, in the wet, max braking for good reason... Does anyone know micrometer movements of the left hands fingers on a hydraulic braking system.

5th Jul 2001, 05:14
A car skidding with locked wheels has very inefficient braking on all surfaces. The advantage of ABS is that it unlocks the wheels when overbraking occurs (which would lead to a locked skid). The real value in the ABS braking cycle (locking and unlocking) is the braking that occurs before the wheel locks, after the brakes are automatically released and reapplied.

Maximum braking occurs, however when the most braking is applied and the wheel is still not locked. Therefore in theory, if you get used to the feel of your car and can feel maximum braking, then that is the most effective braking that can be achieved, and is unaffected by ABS (since the wheel does not lock).

Just as a tip, if you are braking in a car and feel the braking effect slacken and loud screeching noise, release the brakes a bit and reapply them (not as hard as the first time) and you will stop in a much shorter distance.

5th Jul 2001, 07:48
Do cars with ABS stop more quickly than cars without? ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as lose gravel or unpacked snow, an ABS system may actually lengthen stopping distances.


Dave Incognito
5th Jul 2001, 11:12
I'd say the difference would be around 2/5ths of bugger all.

As long as the wheels are not locked and there is a professional driver at the controls, it shouldn't really matter.

However, just to avoid fence sitting I'll back the non-ABS car.

Flying is easy - just throw yourself at the ground and miss.

5th Jul 2001, 14:23
I made this bet myself with an "instructor" who was completely unaware of my experience in this field. He maintained that the ABS would stop the vehicle in a shorter distance and I advised him that it was no contest, I could stop the vehicle in a considerably shorter distance without it.

We were on a counter-terrorist training course at the time and I was invited to put my money where my mouth was. Happy to oblige, I agreed and the equipment was set up. Electronic sensors triggered a loud "snap" as the vehicle passed a certain point and this was the signal to apply the brakes.

We used two different vehicles at 100k, 120k and 150k making six runs in all. The stopping surface was wet and well oiled and about 1.8k long. All stopping distances were marked at the appropriate points with tags denoting speed and vehicle used and that the ABS system was operating.

We then disconnected the ABS and I was on my own. In EVERY case, I stopped the vehicle in a CONSIDERABLY shorter distance than with the ABS active. The distance at 120k was over thirty five metres shorter with the ABS disconnected.

There is no real trick to dong this, it's just a matter of applying only enough brake pressure to keep the wheels just rotating. Once they lock, they skid, so you keep it just prior to lock up point. Interestingly, this is done by applying very light pressure to the pedal. If you can imagine that your hand is between your foot and the brake pedal, the amount of pressure you need would not hurt your hand at all.

Just occurs to me ..... I could win money with this ... any takers????

You want it when?
5th Jul 2001, 14:58
TowerDog - Correction.

On Snow ABS is a liability as it stops the wheels from locking up and so they don't get the chance to build up a cushion of snow in front - which actually helps to slow the car down. It's not a good drive unless the ABS cycles at least once <G>

Stop that oscillation!