ABS?

[Irwin]

I have a problem! My friend said the car without ABS sometimes better then the car with ABS. Is it true? Can somebody tell me some example about this? Or about oposite of this? If it is possible I need some diagram also to check the truth!


Irwin

 

[stanier]

Your friend may be referring to problems with ABS that occur on dirt or gravel roads. In braking the wheel will skid and the feedback to the braking system will be somewhat different than on a sealed surface. Try your local motoring organisation website and you may find some technical details.

 

[gbent]

ABS brakes have some problems on loose surfaces such as dirt (not hard packed), gravel, and loose, new fallen snow. Maximum stopping power happens when the tire is locked, and acting like a bulldozer, rather than a tire. Most cars and trucks will go to the junkyard without ever encountering these conditions. The biggest issue with ABS is poor driver training. Most of us older drivers were trained to pump the brakes in the event of a lockup. That is very wrong with ABS brakes. Jump on them as hard as necessary, and don't let off until you have slowed enough. It makes an interesting situation for those of us with both ABS and non-ABS vehicles.

 

[GregLocock]

Concur with all the above, just remember the idea with ABS is that although the ultimate stopping distance may be a little longer than a good driver (say the top 10% at best)could achieve, it also gives you a chance to steer around the obstacle while braking effectively.

In Australia cars frequently travel on gravel roads at high speeds. Our ABS is tuned to provide longer wheel locked times than a normal ABS in recognition of this. The difference is quite remarkable. Cheers

Greg Locock

 

[Irwin]

An another friend works in the Toyota Service. He said that normaly, nobody can stop shorter way without ABS than with ABS (6th generation ABS). (The only situation is the new big snow and some same situation with other materials.) Is it right?

Irwin

 

[GregLocock]

No. In theory on dry pavement you could get lucky (or be extremely skilful) and hold the car at the exact optimum for the entire stop. The ABS cannot do that, since it oscillates around the optimum slip . If the ABS can predict or measure this optimum slip then it wouldn't need to oscillate, but so far as I am aware no available system does this.

Practically speaking he is probably right for a large majority (>90%) of drivers in most conditions.

For choice I would always have ABS on my car for driving in traffic, since in an emergency I would have to merely hit the pedal as hard as possible whilst steering. Cheers

Greg Locock

 

[ivymike]

I'd heard before that if one could improve the response rate of the ABS system by a factor of 10 compared to current (1997) technology, then the stopping distance would be far shorter than even a skilled human operator could achieve.

Another interesting phenomenon that I've observed with ABS (on a 1991 Nissan pickup, 4wd): when braking at or near the limit of the braking system, on pavement with "patchy" ice, the ABS gives a ride similar to what you'd expect of a bucking bronco. The vehicle would dive to the left, then to the right, then to the left, then fishtail, then dive again, etc. The best explanation that I've heard was that, from time to time, the ABS system was achieving better braking on one side of the car than the other, because of differing conditions at the tires, resulting in brief bursts of yaw moment about the vehicle cg.

 

[ivymike]

I decided to add that w/o ABS on similar "patchy" road, my braking feels like "whooooosh-thunk-wooosh-thunk-thunk-(whoah!)whooosh-thunk!" The thunks, of course, correspond to the tires finding traction, and the whooshes are the slippery parts. I've found that it can be very difficult to keep the vehicle on the road while braking hard in such conditions.

 

[GregLocock]

The BMW dynamic chassis control exploits this yaw effect by steering the car by braking each wheel individually while you are cornering. The improvement in stability and feel is supposed to be awesome, as are the spins you get when everything is maxed out.

I agree that ABS's aren't perfect, they're just a big improvement for the vast majority of drivers, in most conditions. Cheers

Greg Locock