mark kibort

I've noticed a lot of uncontrollable slides, endinding up in crashes with oncoming (edit: "Approaching" is a better word) traffic and I started to think, what if the sliding cars, indending to put both feet in to carry the momentium in the initial slide direction, cant keep the car from going back on track and into traffic, (or initially to the opposite direction where they dont want to go) because ABS doesnt allow for the wheels to lock up.
Personally, with older ABS systems, I use it on the track, about half the time (pulling the abs relay) but when it is used, is really only effective as a warning of flat spot alert.

J richard

...hmm, I think if you have crashes with oncoming traffic they need to have a discussion in the drivers meeting about pit out...

Seth Thomas

I think it doesn't effect the issue you are describing. If the driver of the vehicle has to put both feet in and is in fact spinning then the car is already out of control. If it travels back onto the track at this point it is because of the inputs the driver has added to create that situation and not because the car has ABS not allowing the wheels to lock up. A lot of drivers try to keep the car on track instead of dealing with the car going off track and limiting how much it goes off track. They force inputs into the car it doesn't like, it looses grip, and the next slide is what causes them to spin or come back onto track.

924RACR

As for the question, will ABS stop a car from sliding... no, not really.

The thing is, if you are sideways, and apply the brakes (on an ABS-equipped car), here's what's gonna happen...

The wheels will go towards lock.

The ABS will release pressure on the wheels approaching lock.

The wheels will NOT reaccelerate. This is because the car and wheels are sideways; while the torque being exerted to lock them is removed, there is no properly-oriented torque to start them rolling again. Therefore they will tend to lock anyway.

After a short period (generally within a second or two), the ABS controller will recognize this case, and allow full pressure to the wheels. This may well also be accompanied by a wheel speed sensor fault (due to implausible wheel behaviour).

So you'll experience pretty close to (if not exactly) the same behaviour, if you're applying the brakes mid-spin - assuming you're not parallel to the direction of vehicle motion.

As always, YMMV, no warranty express or implied, but that's basically about what you can expect to go down.

HTH...

M758

mark kibort

that was basically my question. of course the slide wont be helped, but im wondering , and you may have answered it, after the slide is at full tilt, the rear might be pointing backon track and then the car starts to go backward. without ABS, the tires just lock and the car becomes a projectile and follows a consistant, straight path. the reason I ask, is when BMWs or modern cars with ABS seem to spin, they always seem to come back on track in reverse. sure the slow movers are just brain dead and arent using the brakes, but im talking about the slides at high rates of speed near lockup, where ABS might allow for the direction change so that the car can come back on track to approachng traffic.

Yeah, i read that again. you know I meant approaching traffic, but on coming if you are the guy that spun and are facing the wrong direction.

mark kibort

and thats one of the scenareos. the driver gets two wheels off. (we have all seen this before) and then trying to save it rather than drive in the dirt , which usually is really easy and calm, they try and stay on track getting into a slide. now, still correcting, they are now pointed across track , usually as luck would have it, to a wall. if both feet in at the moment the slide happens, the car travels sliding, down the track. if someoe waits to lock up, the car starts to travel to the apposing wall, and even lockup doesnt avoild the inevitable contact. does the ABS, if brakes applied early enough, lock up the wheels enough to keep the cars movemet to the wall, from happening? sounds like the forces to rotate the tires once sliding, wont be enough and that answers my question if true.

924RACR

The only influence of speed is that there's that much more to kill off with the brakes before the wheels lock.

Larry Herman

Now I wonder about this as I just analysed a crash, and when we went back to the video & data there was about 1.5 seconds between when the car began to yaw, and when it hit the tire wall. And it did drivbe itself into the wall. If the brakes could have been locked early, then it should have slid straight down the track.

Phokaioglaukos

Interesting, Larry. I'm waiting to get my data from Germany, but when I spun down the Nordschleife in the VLN coming out of Miss-Hit-Miss and went both feet in in an E46 on slicks with ABS, about midway through the brake pedal went to the floor. No effect on my tajectory. brakes locked right away it seemed and did what the should have. Did your data have a brake pressure sensor reading? Was the driver really both feet in? I know I was, big time!

J richard

Conceptually speaking, the principle of both feet in is to lock the wheels to eliminate traction and steering ability of all corners of the car, ABS by it's very nature is fighting to do the opposite, it's trying to regain traction and steering. Marks original thesis is correct the system fights the age old technique of both feet in. How much really seems to be determined by the sophistication of the system and the exact conditions of the spin. Early 964s race cars were fitted with a ABS override/reset switch due to the tendency for the ABS to become confused in a spin and just release brake pressure. An experience I had rolling backwards onto the track after a spin but with my foot burried on the brake...

Veloce Raptor

In many cars with ABS, if the wheels roll backwards, even for an instant, the ABS becomes disabled, and won't work when rolling forward again w/o cycling the ignition.

cello

This is an interesting and, somewhat scary, thread. I always thought that the ABS systems were designed to acknowledge a 'panic stop', which a properly performed 2-feet-in would be; and, while not 'locking up the wheels', would maximum brake so that the most effective stopping power would be applied to the wheels regardless of attitude. That is, when 'all in' the system would attempt to stop the car in the shortest distance by locking/releasing/etc such that the car would stop in a shorter distance than if the wheels were locked up and sliding. Are you all saying that yaw/car attitude somehow over-rides that function? Could it not simply be a function of physics - traveling too fast for a properly functioning ABS system to slow a spinning car as the contact patch is in continuous flux? I guess i always thought of cars spinning across the track as a function of speed-weight-pendulum effect or of not properly 'panic braking'. Hope to never find out, BTW...

Edit - Dave - posted this at the same time you posted. Interesting. Is that applicable to Porsche?

Seth Thomas

I would like to hear more about this crash and your analysis on it.

ProCoach

+1,000,000