[Swift016]

Originally Posted by TrackClub

Agree, the PTM is an excellent system. Yet, I don't think it would play a role here. Competitive Mode (if this is what Bill ran) should have given him the same benefit of ESC as PTM Sport 1, while possibly making the car much looser on exits due to lack of TC. But, not under braking.

But, a good point overall and i also consider the PTM an absolute cat's meow

Track club and others :

The Ps4s were same dimentions as 3r’s and I have run both with no similar issues. These Ps4s had good lateral and accel grip.. the braking issue was always straight line, moderate braking from high speed down to 40-50 mph... I have an A10 so let’s rule out my pitiful blip as a culprit

I meant to say that I was in Competitive mode, but got there from a Sport selection... Track and Sport 1, 2 were similar.

The tires are not 3r, grip wise but are balanced in corner and accel... trail braking was too low decel to show the problem.

The fishtail started in straight line braking with moderate effort... threshold braking would have made the car uncontrollable... no noticeable front dive under braking, and braking distance was at least doubled.

Felt exactly like way too much rear brake bias on my race car....I’ll recheck front pads, but my mech said they were good on pre event inspection..

I’m at a loss, but thanks for all the thoughts and suggestions....no external fluid leaks , but I suppose an internal master cylinder leak could cause it.

Too bad We don’t have f and r brake pressure sensors like on my racecar... that would at least point me in the right direction !

Thanks again,
Bill

[Swift016]

Forgot to mention:
Goodrich steel lines
Motul 600 fluid
Track camber
Tiny bit of f and rear toe in, dont need toe out for turnin and Dont like tram lining for street driving

[TWG1]

I find the downhill braking for Barber T5 made my ZL1 waggle a bit. I would stay a little further from right edge to account for it. At the time, I suspected my rear toe started going out (positive). I had my alignment checked afterwards. I lost a little less than 1/16” toe when I remeasured. Have you checked alignment post-track to verify it didn’t shift?

[Swift016]

Originally Posted by TWG1

I find the downhill braking for Barber T5 made my ZL1 waggle a bit. I would stay a little further from right edge to account for it. At the time, I suspected my rear toe started going out (positive). I had my alignment checked afterwards. I lost a little less than 1/16” toe when I remeasured. Have you checked alignment post-track to verify it didn’t shift?

Rear toe out would do it..... I have an appt with Gran Turismo East on Thursday... they are excellent and will check brake plumbing, pads and alignment..

Rear toe out doesnt really explain locking the rears and diminished front brake effectiveness... I’ll post an answer if I find it ,
Thanks !

[JT Camaro]

My money is on alignment

[R3ctivision]

My thoughts are that it's one of the following: alignment, track layout (e.g. downhill braking), or maybe even air in the brake lines. Maybe some air bubbles could cause a change in bias?

[TrackClub]

Originally Posted by Swift016

Forgot to mention:
Goodrich steel lines
Motul 600 fluid
Track camber
Tiny bit of f and rear toe in, dont need toe out for turnin and Dont like tram lining for street driving

You may want to consider upgrading from 600 to something better. Not suggesting this is related to your issue, but a few folks have cooked the 600...Personally, SRF is numero uno, but i am also very happy with Willwood EXP. Cheers!

PS If i were you id have the master cylinder checked asap.

[TrackClub]

Originally Posted by R3ctivision

My thoughts are that it's one of the following: alignment, track layout (e.g. downhill braking), or maybe even air in the brake lines. Maybe some air bubbles could cause a change in bias?

Air in the brakes will cause soft pedal not locking. Downhill braking in a straight line will offload the rears. Ive driven a car with a messed up alignment in the rear (my 20 1LE had an issue from a factory) and there was zero instability under braking. Remember, it is the fronts that do about 70% of the work here. UNLESS bias gets changed a la locked up rears due to effed up downshift (no chance here with A10), a driver doing it (a la race cars), or a master cylinder malfunctioning. Note the OP is talking about instability during LIGHT braking. Not even threshold braking. Something is broken imo. Could be brake, or suspension related.

[SFV1LE]

I agree that tires make all the difference, and in surprising ways... the ZL1 is more sensitive to the wrong tires than other cars, I don't know why.

In my stock ZLE last month at Auto Club, I went out on very good condition ~2 heat cycle PSC2 in 315-f/345-r and it was like I was on roller skates. I put on Pirelli DH take-offs in 315-f/325-r and did a 1:47 in my first session, and then a few 1:48s. My friend in a ZLE did 1:47 on day old SC3Rs.

On new Pirelli DH or GY3R, I think I'm capable of 1:45 so a great driver (ie. not me) could do 1:43 or better.

[TrackClub]

Originally Posted by SFV1LE

I agree that tires make all the difference, and in surprising ways... the ZL1 is more sensitive to the wrong tires than other cars, I don't know why.

In my stock ZLE last month at Auto Club, I went out on very good condition ~2 heat cycle PSC2 in 315-f/345-r and it was like I was on roller skates. I put on Pirelli DH take-offs in 315-f/325-r and did a 1:47 in my first session, and then a few 1:48s. My friend in a ZLE did 1:47 on day old SC3Rs.

On new Pirelli DH or GY3R, I think I'm capable of 1:45 so a great driver (ie. not me) could do 1:43 or better.

Dont know that's a car being sensitive per se. Ive tried Cup2s on my 1le and couldn't get them to fire up just right. But sure as heck didn't have any braking issues. I think Cup2s are very pressure sensitive, much more so than other tires. Having said that, there is a member here with a ZL1 who runs them and he is on a fast side. So, go figure

[Max2075]

Double check alignment. I switched out my rear tow links because the factory ones were hard to adjust and seemed to move. Look for air in front brake lines. Assuming stock pads or staggered set with more aggressive pat up front. I like stop tech brake fluid. Great high temp boiling point. I also run low 30's psi in the rear hot.

[Swift016]

Update :

Pulled the wheels and all pads were worn evenly to 6mm.. that confirms my thoughts that the front brakes are not braking properly.. the front pads should definitely be worn more with track useage..

Too much rear bias, but haven’t found the cause yet..

I’m running 34psi hot
-3.4 f camber
-2.4 r camber
.04 toe in f & r
Even tire wear across the contact patch at all corners

-2.7/-2.0 camber gave me too much outer edge wear

Thanks,
Bill

[Swift016]

Originally Posted by Swift016

Update :

Pulled the wheels and all pads were worn evenly to 6mm.. that confirms my thoughts that the front brakes are not braking properly.. the front pads should definitely be worn more with track useage..

Too much rear bias, but haven’t found the cause yet..

I’m running 34psi hot
-3.4 f camber
-2.4 r camber
.04 toe in f & r
Even tire wear across the contact patch at all corners

-2.7/-2.0 camber gave me too much outer edge wear

Thanks,

Bill

Any other thoughts ???

[Krowser]

Originally Posted by Swift016

Any other thoughts ???

I have a theory...

Make sure your brake fluid has just enough space to expand in the reservoir tank.

If the heated/overheated brake fluid is overfilled in the reservoir, the fluid can intermittently activate your brakes causing premature wear.

Additionally, when you are track racing, you put an extraordinary amount of braking to the front of the vehicle.
If you have traction control on with overheated and overfilled fluid, your ABS can slightly pulse your brakes.

Overheated and overfilled brake fluid with pulsing ABS can cause premature pad wear and slower track course times.