fleming23

Went to a local autocross today and had the worst time I've ever experienced in the Z06. I've run with this organization, on this same surface before and was very competitive, today not so much. I have a few ideas as to the cause which I'll address in a moment. I have tracked this car in the exact same configuration without issue, but Autocross presents a slightly different set of forces on a car not experienced on a Road Course.

First, I was running 18" wheels, 18x11 front and 18x13 rears. For tires, my wheels came with Toyo 888s which I am really just burning off at this point, 315/30 fronts and 335/30 rears. I believe the front to rear percentage change is within what is considered acceptable, and as mentioned I had zero issues on track at Road Atlanta a few months ago. I do know the speedometer is off by 3-4 mph as the tire is shorter than stock.

Second, I have the DSC controller and hopefully the recommended alignment to include positive rear caster and .5mm toe out on all 4 corners. I don't believe this has any bearing BUT the car has felt really twitchy since getting the recommended alignment. Some of that is due to the 315 tire following the grooves in the road, but I also question if the alignment was actually done correctly as I had to bring my rear caster bracket and angle gauge to allow the tech to perform an accurate alignment, and explain what I was wanting. Unfortunately, I still have not found a shop that I trust to do the alignment both correctly and precisely around here.

Third, finally, and the ultimate issue, I have swapped the stock CCB brakes for Girodisc rotors and ST43 pads both front and rear. I believe the grabby nature of this pad is the ultimate issue. This combination is fantastic on the road course but may be too much for autocross. I found myself experiencing the ICE conditions when I would really get on the pedal hard. I think had I known the conditions that lead to ICE mode activation (I only found out after getting home and researching) I maybe could have entered the corner with a more gradual pedal input, but unfortunately that is not how my brain is programmed. I grab the pedal hard and then back off, as I am accustom to doing entering a hot turn on a road course.

Our autocross course today had a really long back section leading up to the timing lights where you had to get hard on the brakes to slow before making the turn off course. This was a top of 2nd gear, on the limiter stretch so carrying quite a bit of speed. Was not a great feeling not being able to slow the car at those speeds...

Just thought I'd share as I did not come across too many examples of this happening with the C7. Plenty of C5/6 examples with the Bosch ABS though.

I can share PDR video but it doesn't really show much other than me blowing the corner.

pkincy

Help me out with ICE condition? Do you mean that successive applications of hard braking doesn't allow the hydraulic piston to recycle and you get no brake pressure on the second application? If that is what you mean, that is simply (in my experience) a result of autocross requiring on some courses two hard applications of the brakes in very quick sequencing that doesn't allow the piston to cycle and give you hydraulic pressure on the second application. In those specific courses you need to figure out a way to get the car slowed without near simultaneous applications of heavy braking.

If I am way off base on your problem please ignore this post.

fleming23

Successive or simultaneous applications of the pedal has no bearing on the ICE mode. From my understanding, if you overcome the programmed ABS thresholds the car will basically limit brake pressure to all wheels resulting in a really hard pedal. This is across manufacturers with many reports of this, generally in race applications, where the car tries to outsmart you because it thinks you are actually sliding on ice with all wheels sliding. The idea is that by limiting brake pressure you will maintain the ability to steer out of the slide. It goes away as soon as you can get off the brakes and try to apply pressure again. Unfortunately, on an autocross course you usually have such little time and space to do this, you will blow the corner and have no chance to recover. If this were to happen on a road course you may be able to release and then reapply pressure. The issue I seem to be having is the bite of the ST43 pads is too great for autocross, combined with a tire that likely didn't have the best grip to begin with.

I think I may have a different pad lined up from PFC that will work better for both autocross and road course.

AzDave47

Toe out at the rear will make the car want to turn in rapidly which is probably why you feel the back end twitchy. It is sometimes done for autocross, but almost never for road courses unless there is some inherent balance handling problem that you are trying to overcome. I have not heard that the C7 Z06 suffers that condition..

fleming23

I fully understand the impact of toe and this is the first time I have ever been instructed to run rear toe out. However, check any of the DSC Sport threads and you will see that Mike Levitas recommends this rear setting as they have mapped the toe curve and apparently with his recommended .75 degrees positive rear caster, this setting maximizes suspension travel characteristics leading to the correct toe when the suspension is under compression.

Per Mike's comment, "I have found in 25mm of comp there is a + 1.5mm gain yet in reb in a 25mm stroke -.6mm"

spearfish25

The one thing with Mike's alignment recs that I keep reminding myself is that these are track settings. Yes the toe curve may move inward under compression. But if you street drive your car, you're going to be living with the effects of rear toe out almost all the time. This is the sole reason I haven't committed to an aggressive or 'full' track alignment. Fortunately my tire wear with a mix of street and track driving is doing great and is very even despite -1.5deg of camber all around and 0.5mm of toe in both front and rear.

boomerangj

Newbie question: Would these recommendations be slightly different for a different tire size? Ie; alignment specs for the taller stock tires different then for shorter tires-just slightly?

Bill Dearborn

The -.5mm toe out is a minimal amount of toe out and is well within GM's alignment specs for the C7 Rear alignment. GM specs 0.0 degree of rear toe with a plus or minus 0.2 degree tolerance. Not sure of the exact calculations but .5mm is probably somewhat more than 1/64 of total toe out or about -0.1 degrees total toe or about -0.05 toe out per wheel.

That isn't a lot of toe out.

Bill

fleming23

It doesn't show much but after the first long slalom, I attempt to brake and immediately have rock hard brake pedal. The indicator on the PDR does not really show much input but I assume that is due to the forced reduction in brake by the system.

Not sure it shows much, but this happened on the exactly same turn 3 times I believe, and at the finish twice. As you can see, there was another slalom directly after this at higher speeds but the car never had issue on that section. I was on the limiter in 1st through the second portion.

I ordered some new PFC-08 pads today to try with the Girodisc Iron rotors. Unfortunately, the circumstances will be difficult to test or induce so fingers crossed I guess.

rwheelz

I have had this happen on a short track where there was a very short straight between two very tight turns, requiring full acceleration and full braking nearly instantly... VERY scary. It caused me to go off the track once. The only thing you can do is get off the brakes and back on, which is not easy to make yourself do when you are running out of pavement. I was glad I learned the lesson when I did, as it could have been catastrophic in the wrong place at the wrong time.

AORoads

If you've driven this autocross surface before in other cars, did you drive the same way/style? And, does the way you drive this car have any relationship to the mode the car is in (touring, race, all nannies off, etc.)? Finally, is it possible to switch back to the old rotors/pads to see if that helps, or not (that would mean, same 18" tires/rims, same alignment, only difference is rotor/pad setup)?

X25

My 2 cents:

I think your tires are a bit heat-cycled, and are contributing to the problem. ICE mode happens when the brake application is too harsh, too sudden, which locks more than one tire at a time in an instant, making ABS believe that you've hit a patch of ice or whetever has low traction, and most cars usually limit the deceleration to a very low 0.6G at that point until brakes are re-applied.

Have you been using stock pads before Raybestos? These pads have a lot more bite, and you need to respect them. In other words, you need a lot less pressure on the brake pedal to make it work the same as it did before. Once you can adjust it, and swap to fresher or better tires, I think your problem will go away.

LagunaSecaZ06

What was the outcome with the PFC pads? I have 2016 Z06/Z07 that I switched Girodiscs and ST43 on to it. At Laguna Seca a few days ago I was getting ice mode going into turn 2(front straight hair pin) and going into Turn 8(90* hard left into the corkscrew). In both of these places was concerned about going off track. When I pitted after my session, my rotors were about 400*. A C6 Z06 that was in my group, his rotors were about 800* in the pits. I think the ice mode was limiting my braking pressure and hence rotor temperature. I am going to call Girodisc on Monday for their recommendation. With CCB's my car was a monster at Laguna Seca, I would brake really late and have complete confidence(the pedal never got really hard either).

NTMD8R

You aren't the only one(s).
Here's a link to another thread....

https://www.corvetteforum.com/forums...mode-help.html

fleming23

So far the different compound is working better. It is definitely less aggressive but I can still out brake most cars on track. I have also tried to be deliberately more smooth in my brake application where as previously I would jump on the pedal as hard as I could.

They definitely get warm!

fmcokc

pkincy

I don't know a thing about ICE mode and how it works and haven't autocrossed competitively in years, but do clearly remember situations where there would be not brake pressure if I was running a course that required 2 hard brake applications in very close time proximity. My feeling was that the master cylinder piston simply did not have time to cycle between the brake applications. Now I have never had that problem on my road race car as the brake applications tend to be farther apart time wise. I also would not run into that on most AutoX course designs, but every so often a mid speed corner would be immediately followed by a slow speed corner and it would take 2 hard brake applications in a row with less than 2-3 seconds between applications and on the second application there would not be any hydraulic pressure left to fully apply the brakes. The peddle would depress all the way but there would be little to no additional brake application. In those instances I simply had to learn to give up one of the two corners a bit to give the piston time to cycle properly.

village idiot

I backed my brake marker up 50-100ft and just go slower between gas and brake, allowing some weight to transfer forward. That seems to do it.

rwheelz

I have had this happen on stock pads as well as Carbotec XP10's. It will put your nuts in your tummy.