rustyguns

I am guessing the front tires locked up and computer went into ice mode.
SO i need to not lock up the front tires ? what the heck is ABS for ?

Towards the end of the session, everything is very hot, running stock calipers and rotors Raybestos st 47's in front and pfc 01's inback. toyo 888 275/18 front-335/18 rear.

end of straight around 100 mph, late braked hard and got the infamous hard brick brake pedal too late to pump brakes, so i went off into the dirt, made a mess but no damage.

This has happened before at different track, but i had time to pump and reset the brakes To fix it I started to do small soft pump then hard pump....ie modulate.

Only fix I can find for this is .... just don't hit the brakes hard? I just love late braking

Any ideas?

travisnd

Subscirbed... haven't discussed ice mode in awhile... I'm sure someone can chime in on what the car thinks is happening and why it occurs. Had it happen once at the back straight of VIR years ago... was scary but got the car hauled down in time.

naschmitz

Stabbing the brakes induces ice mode, as you have figured out.

Solofast

Here is how it works and why...

http://forums.corvetteforum.com/1576900761-post10.html

Basically the car is light or on a slippery surface and one wheel is locking up too quickly. One thing you can do is make sure you don't hit the brakes when the car is in the air and based on how much weight you have on the wheels determines how hard you can brake. That is a law that applies to all cars, but if you don't have an ABS system with an ice mode, your car will get over it when the traction returns.

Here's a discussion about pads, if you have pads with more initial bite it can be an issue.

http://forums.corvetteforum.com/1575427077-post12.html

froggy47

The conclusion I reached is you have to "drive around" the problem rather than mechanically/electronically try to "fix" the system.

It's track/driver/brake application specific so good heads up driving should deal with it.

FWIW

Maybe practice/train to be able to cycle the brake quickly, which is kind of funny, like we went full cycle back to the old days before abs when your foot action was the abs. I confess, I remember that method as I lived in the NE at that time, snow/ice winters.



I don't fully understand the whole process, but it's very clear when it happens. No matter how hard you push the brake you get a reduced braking capacity like 10-20%. It seems to temporarily bypass abs, when it happened to me I felt no abs pulsing. It seems to be related to having a LOT of grip in the tire as I got it when I put 335's on the back, massive rear grip. The car would stop like an anchor got thrown out, except if it stopped too good it went ice mode. Once when I was mashing the brake in a spin and again a few weeks later on a course that had a really high speed section (80+) down to a slow turn with braking more/less in a straight line.

It would be useful to have a clear technical description of what initiates ice mode but it apparently is a GM secret.

Bill Dearborn

Still haven't experienced it on the track or in an autocross. On the track I brake hard and sometimes get the ABS since I am on street tires on an autocross course I use the ABS all the time. Bury my foot on the brake as hard as I can push. Just spent two days doing that hauling the car down from 70 mph to make a hair pin turn.

Bill

davidfarmer

I've never hard it lock the brakes...but the contrary. On the street a few time, on ice/snow, the pedal modulates until the selenoids are out of capacity, and the pedal just goes to the floor without slowing the car much.

I had this happen going into turn 1 at VIR. I was off-line passing cars, entered the brake zone a bit "diagonally", and thus hit the compression bumps in the brake zone with my tires out of phase. The pedal modulated and went to the floor. I drove straight off of the track with only minor braking action. I'm now always careful to enter bumpy brake zone perpendicular to the bumps.

rustyguns

but stabbing the brakes at the last possible moment is just amazing how fast you can slowdown when the ABS works correctly but i have to stop doing that now

darn....

naschmitz

There is a difference between stabbing the brakes and squeezing the brakes. Squeezing the brakes lets you transfer weight to the front while you build up brake pressure and lets you stop even shorter.

travisnd

Yeah you can still threshold brake w/o stabbing the brakes. I found that going trom 0 to 100% full brake force (which is what they taught us at the Panoz school back in '03 on cars with manual brakes) contributed to cracking rotors. A few years back I started getting into the brakes a little easier and have had much better results. I'm still stopping as hard as I can at the latest possible point, but for a split second I ease on the brakes to get the pad in contact with the rotor before I go 100%.

naschmitz

I don't know exactly what starts ice mode, but what happens in ice mode is the car only uses the rear brakes and no abs, which is why the pedal stays hard and the car stops so poorly.

I've only experienced ice mode at an autocross braking towards a pivot cone. I've never experienced it on track but my inputs are generally smoother on-track than they are at the autocross.

SIK02SS

275 fronts and 335 rears!!? Wow...notice much understeer?

Solofast

No that is not what happens at all. Go back and read the linked page in the fourth post and get an understanding of what is happening. Suggest that you learn what the system does before making silly statments like that.

What it does is limit braking sytem line pressure to all four wheels. It absolutely does not just use the rear brakes. If you hit real ice and just used the rear brakes you would be spinning in an instant.

I would imagine that you still have ABS, but with the line pressures limited by the system, on anything other than ice you aren't going to get lockup anyway. So it doesn't matter because unless you are on a very slippery surface you can't re-lock any wheels with that low a line pressure.

naschmitz

It's the explanation Junior Johnson gave me at an Evo School when we got my car to do it. Here's how he suggested to fix it: http://forums.corvetteforum.com/1565464383-post42.html Is that BS too?

rustyguns

i read a few examples like that on other forums. I think it is good advice
thanks!

rustyguns

lol well, they are cycled out toyo 888's so it wasn't bad

i prefer 315's in front but didn't have any

rustyguns

your both right... rears lock up because they don't need as much pressure to lock with all the weight upfront

my rears did lock and i spun

so technically yes it limits all pressure, but for feel, you swear nothings upfront and only rears are working

rustyguns

This is my plan thx

trackboss

I damn near lost my car on the track because of this. Does pulling the abs fuse not allow "ice mode"?

Solofast

Brake pad bite and shock rebound stiffness can have an influence on whether or not you get into ice mode.

Ice mode isn't hard to understand. If a wheel stops faster than a specific rate, the computer invokes ice mode. The rate is based on how fast the wheel will stop if it has weight on it and traction. If there isn't any weight on the wheel, or the traction is really low, the wheel will stop very fast. If any one wheel stops too fast ice mode is invoked. When it is invoked, the brake line pressure is limited to all four wheels to correspond to a deceleration rate of (I think it was but I'm not positive and it really doesn't matter) 0.1 g's. The mode is latched in until the brakes are released then the brakes can be applied again and you have another chance at stopping at a higher rate.

Most of the times that I have gotten into ice mode it has been the front tires that did it in, because of the higher amount of braking done on the front wheels and one or both of those wheels was light or on dirt.

I don't know if changing the bias will help. It might if you are finding that you are getting into it frequently because you are hammering on the brakes and have a real aggressive setup, then a change in rear pads could possibly help.

But most of the time that I've gotten into ice mode it has been the result of the front tires being light or on a patch of sand. If a front wheel is light and you hammer the brakes, you ARE going into ice mode, giving the rear more bias isn't going to do anything to prevent it.

I also don't understand the last statement Changing the brake bias doesn't effect weight transfer. The amount of weight transferred is based on the height of the CG, the wheelbase and the deceleration rate. It is a common misconception that the bias effects how much weight is transferred, but it simply doesn't. If the thought is that changing the brake bias changes the weight is on the wheels at that end of the car and that will effect getting into ice mode, the answer is, well, no...