froggy47

Yes you read that right. I have on occasion spun off an autox course, and have the "two feet in" down pretty well.

But there was an event late last year, where it seemed to take quite a long distance to whoa it down travelling backwards.

I was sure I'd nail a course worker or light pole & had the mirror cocked out of the way so no rear vision.

So the questions are, does abs work going backwards?

Is there generally less braking power because the weight transfer is off the front (big) brakes and onto the smaller (rear) brakes?

Any other reason?

I am going to test this out when I can find some safe place & maybe record 60 to 0 times each way, for the heck of it.

Not talking about off track grassy/dirt stuff. Same surface.

Thinking back there may have been some marbles or other crap on the lot.

Jason

Our brakes suck backwards. I've seen better drivers than I back it into a light pole from speeds they should have stopped from. Don't know why, but it happens.

davidfarmer

with 4 wheel sensor, yaw sensor, and g sensors, ABS SHOULD work in reverse. However, the transition (from forward to rearward) might send the system into a state of confusion which it can regain. Plus with the engine in the front, braking should be better in reverse.

I was talking to a friend at CMP last week, and he told me that Mustangs ABS would fail if you managed to lock ALL 4 WHEELS at the same time. Basically, the system uses wheel sensors only (no Yaw, G, etc), so if you lock all 4 wheels, the car assumed you weren't moving.

Not the same, but sort of a similar failure. Could be that once you rotate, the system just gives up. However, I'm pretty sure if you simply drive backward in a straight line, the ABS will work (will test soon)

froggy47

Of course that makes sense. Simply going straight in reverse & stopping hard is not the same as a snap spin of 180 degrees or more & then resulting in rapid backwards motion.

I do wonder if impaired braking power is possible going backwards after a snap spin. It sure felt like it to me when I did it. I am pretty sure I had tc/ah off as I always do (possible exception of rain soaked courses).

flink

The biasing is all wrong. The rears won't do much and the fronts will lock up.

You should get significant stopping out of the rears if you press the pedal like hell to get enough line pressure. If the ABS is really really good then it will prevent the fronts from locking up. I doubt if the ABS is that good. More likely the fronts will lock up causing the car to spin around again.

Test it! Post videos

blackozvet

g'day froggy47, in my job we drive abs production cars at the racetrack once a week, one thing you notice is that once your off the racing line the abs has to work harder under hard braking because of the looser surface, remember you have rubber, dirt, stones etc etc off that line.
The abs' job is to stop the brake from locking up, the harder you brake on a loose surface the harder it has to push back on your brake line pressure to stop that from happening.
I have noticed that the cars will stop quicker going backwards when you are hard on the brake pedal when you are still on the racing line (or the middle of the track).
If you slide off the that line backwards the cars seem to take forever to stop !

Solofast

I wonder if you are getting ice mode and that's the problem???

froggy47

I am really not sure what ice mode is & if I have it on my 2004.

Nothing has ever displayed on the dic that says ice mode.

Gary2KC5

it also depends on your MY...at some point GM decided the ECM should decide your brake F/R bias...I think about 2003.

Maybe they anticipated possible future demolision derby's with Vettes on figure eights driving in reverse?

Solofast

Ice mode occurs when a wheel stops a lot faster than it could if it was on pavement. The computer knows how fast you should be able to stop and what rate of deceleration is within the possible (at least it thinks it does). If you have a wheel in the air, or if you are sliding sideways, and you apply the brakes, the wheel is going to stop essentially instantaneously, or at least a lot faster than it could if it was in contact with pavement and was slowing the car down. The computer then thinks the car is on ice and it limits the amount of brake pressure to give you about .1 or .2 g of deceleration. The only way to disengage ice mode is to recycle the brake pedal, and then you will get full stopping power. If you are going sideways and have your foot on the brake it's going to stop the wheel very quickly and that could invoke ice mode. Difficult thing to do when you are sliding backwards, but if you ever feel the car isn't slowing down like it should you need to recycle the brake pedal.

Yes, you have it on your 2004, all Corvettes have had it since 1986.

Nothing is displayed on the dic, the car just doesn't stop worth a darn when it is in ice mode.

Jason

So when you're sliding at that wall sideways/backwards/sideways, take your foot off the brake then reapply. Nothing to it.

Solofast

Yup... That's about it...

You'd be surprised how fast you can cycle the brake pedal if you know that you have to do it. The hardest part is when you are closing in on something. You just have to know that it's the best thing to do and do it. After you've had it happen a couple of times, it's automatic.

froggy47

Well that certainly seems to be what happened on my spin, I don't know if I locked all 4 up in the midst of the spin but at least a couple.

And I do recall just pressing as hard as I could on the brake, I know I never lifted.

I will have to add that to what I tell students when instructing, I imagine other cars have it besides Vettes?

Would the same apply in a spin that you come out of forwards? Say a 360 as opposed to a 180?

And just to be clear this happens even with tc/ah OFF?

Any way to disable ice mode?

Here is a link to more discussion.

http://www.z06vette.com/forums/f5/c4...ce-mode-17684/

Anyone know how to disable abs?

Solofast

I only know that it applies to Corvettes with ABS post 1986. Don't know if it applies to other makes or models.

It happens because the wheels stop more quickly than is normally possible. The ABS system doesn't know if you are going backwards or forwards, it is only counting teeth as they go by. If you spun 360 degrees and the car was rolling forward and you weren't in danger of hitting something you would likely let off the brake anyway...

Yup... It's programmed into the ABS system, has nothing to do with tc/ah

Only by disabling abs, which is, to my mind a really bad idea, live with it. It doesn't happen very often.

If you had access to the ABS program you could disable that part of the code..

I've had ice mode be a performance issue only two times. That is, it has only been a consistent limiting factor that happened every lap on two occasions. Once when we had to do a real tight turn just off of rise in a parking lot and we were essentialy braking in the air, and once where the pavement was dirty and breaking up just where I needed to put my right front wheel on a course that was made up of access roads at a fairgrounds, but in over 20 years of autocrossing Corvettes those are the only times it's been an issue.

Solved the jump and turn issue in my 89' by "blowing" the abs fuse (a trick that has since made to be illegal by the SCCA) and on the other case I could not get the ABS to fail by pulling fuses on my 94'. I yanked every fuse and breaker that I could find on that car related to the ABS and it still worked! That was darn frustrating...

It seems that it happens most of the time if you are in the air or on some dirt just as you first hit the brakes. If you are already braking and have been decelerating at a good clip and three other wheels aren't stopping that fast it doesn't seem to trip into ice mode, which is why it's really a pretty rare thing.

Once you have it happen to you a few times you realize it pretty quickly and cycle the brake pedal. I've had it happen on the track once in a great while and it tends to add about a half a second to a run, but that's the only issue.

froggy47

Good info, thanks.

I am remembering a time or two (over 8 year period) when it seemed that I lost a ton of braking power going forward on the marbles & I chalked it up to simple loss of traction even with abs working.

Could be it was ice mode then.

I tend to get into abs a lot with race pads & slicks and maybe have gotten used to it instead of threshhold braking, when it's working right it really is an excellent tool.