Veloce Raptor

I don't know. I know it is applicable to my E36 M3 track car, and to many other BMW's as well. I know it is not applicable to the most modern 997.2 GT3's and GT3RS's. Beyond that, I really don't know how many Porsches (if any) it affects.

J richard

You have to remember that ABS actually doesn't APPLY brake pressure, it momentarily interrupts it to allow the tire to regain traction. In essence a mini me version of the classic tank slapper recovery that catches just in time to put you headlong into the wall...It can have some weird consequences in a spin, including system release and "ice mode" (that will put the fear of god in you). If you have a car with ABS you just need to understand the potential failure modes so you know what to do when it happens. That said, for 95% of situations ABS is a good tool. It took running down some wierd experiences on the track to understand what was going on.

cello

^ Yes, you are correct. And, I have experienced 'ice mode' and it is disconcerting...

I guess, then, the best that can be done is to go 'all in' and stay on top of the steering wheel as best one can..

Hopefully, I never experience all of the varieties of ABS..

Larry Herman

I wonder how the car knows that it is going backwards unless it has an accelerometer in the ABS unit (or somewhere). ABS wheel sensors cannot make a directional determination.

Veloce Raptor

No idea. But it happened to me in the famous Speed TV video of me at Barber in Grand Am Cup in an E46 330i, and it has happened to me in my E36 M3.

Larry Herman

Yeah, I remember seeing you lock up the front brakes going downhill. Something was definitely going on there.

cello

Larry - hate to quote myself, but do so only for context, - better to quote Bob R -re a discussion on 'ice mode', but see here:

https://rennlist.com/forums/7078131-post23.html & https://rennlist.com/forums/7078260-post26.html

Say the toothed speed sensor wheel is spun backwards but the programming does not allow for, or cannot read, same....

Speculating here (Porsche only).

Larry Herman

Charlie, the sensor cannot determine which way the tooth is going past it, so it has no idea which way the wheel is turning. It can only relay the pulses to the ABS computer. What Bob posts is different, as he is talking about crud filling in the teeth and causing mis-reads by the sensor.

cello

Ahhh, interesting, so it cant tell clockwise from counter-clockwise.. It just counts electrical contacts made. Thx, Larry.

J richard

It seems to me that similar to some other things that are taught for beginners safety, really late apexes, no trailbraking etc... That the "both feet in" button gets pushed way too early by advanced drivers that could recover a incipient spin but instinctively go for the binders due to reflex and never develop the ability to quickly modulate steering or power...what say you guys? When is too late really too late? A pro is going to try and save it until all hope is lost, and sometimes a little more...

924RACR

Actually... it is somewhat possible to tell forwards from backwards. Depends on what type of wheel speed sensors you have, for one - new sensors can be directional. Also, if the car has ESC (PSM/whatever) and that is functional (not disabled with sensor failure etc), it's monitoring car direction (forwards/backwards) as well.

EDIT: Of course, sideways is a bit harder to monitor.

Definitely interested in hearing what the driver diagnosis on Larry's incident reveals...

Bob Rouleau

When the toothed wheel gets crudded up, the sensor no longer sees pulses, i.e., it thinks the wheel is not turning and releases brake pressure. In a spin there is more going on however as I have seen ABS cars sliding sideways with the wheels locked. Like Larry, I suspect that when there is forward or backward motion - as opposed to going sideways, ABS might try and reduce pressure which could make the car hook up as opposed to maintaining the slide on the pavement. My rule is never give up steering and look where you want to go....

924RACR

Again - read my first post... and remember what I do for a living...

I'm not guessing - I'm stating facts.

SundayDriver

Don't you guys make mechanically actuated drum brakes? I heard that next year you are going to start working on hydraulic actuation.

Facts? Facts? We don't need no steenkin facts.

J richard

Vaughan, hypothetical question/scenario that I've wondered about:
Same setup as your described in your first post but due to countersteer or just dumb luck one wheel is pointed in a direction that does have a turning moment and does reaccelerate. For the one to two seconds that the system is figuring out what's going on, that one wheel is free and has the only traction of the four, wouldn't it steer the direction of the slide? Just a fraction of a second would seem to be enough to send you off in an unpredictable direction...