gkmccready

Okay, I know everybody is going to say warped rotors, but I don't think so. I've got Wilwood SL6R calipers with LGM brackets grabbing NAPA rotors. The brakes feel like they grab/slip/grab/slip when you get on them hard. This brings on the feeling of a vibration under heavy braking. Light braking doesn't have this issue. Even relatively heavy braking during the out-lap doesn't appear to show the issue, but once things get warm the grab/slip/grab/slip feel comes back to the brakes.

Wilwood H pads front and rear. Stock C6 Z51 rear calipers grabbing NAPA rotors vs Z51 sized rotors.

Data shows that even with the 275mm Toyo R888 front tires I'm getting -1.0G of decel ... but the car feels like crap doing it. Like I said, I've gone through multiple sets of front rotors, so it's not a warpage issue. Suspension looks just fine. Poly bushings all recently lubed. Wheels are CCW C140 and balance out nicely. Don't feel any play in the wheel bearings and the tie rods feel good, too...

What am I missing?

gkmccready

It's not ABS, either, I don't think. No pedal feedback...

mgarfias

You have any way to datalog the TCS/ABS? If so, log the wheel speeds under heavy breaking and see if they oscillate a bit.

gkmccready

Wheel speed sensors report a steady decline while I have negative longitudinal acceleration.

FWIW, during the grab/slip/grab/slip cycle the car shakes in a fashion *similar* to having a warped rotor...

Edit: I'll try to snap a picture of the rotors tonight.

Poor-sha

Is it possible you have something on the rotor? I know that I had that issue after running the Ferodo DS3000 where they didn't bed properly. I ended up scrubbing the rotor with a scotchbrite pad and brake kleen and then rebedding with new pads.

mousecatcher

well you can check the LRO but i doubt it will reveal anything since you say when cold there's no issue. i'd replace the bearings as a diagnostic measure (after measuring LRO).

gkmccready

Possible, I guess, but it's done this with 3 sets of rotors... same pads, though!

gkmccready

http://www.petting-zoo.net/photos/Br...-09/index.html

ErnieN85

take the rotors off and check them real close. I just did a fix of this problem on my wifes car (pontiac GP) and what I found was one rotor had a high spot that looked vaguely like a pad print. a new rotor fixed it

gkmccready

This behaviour has been consistent across multiple sets of rotors. Same set of pads, though...

ErnieN85

thats going to be a real puzzle

0Todd TCE

Pic 2 looks like pad smear from being overheated. Not so much 'in use' perhaps but post track runs from not enough cool down time. Noting the hairline cracks it's clear they are run hard also. Pulling into the pit/paddock and not moving the car can cause this. Crystallization of the pad bonding to the rotor every so lightly in that one spot. Same basic issues as the street 'warpage' but at a much higher level.

My first suggestion; full use of the cool down lap followed by some occasional 'bumping' in the paddock until temps are more stable. (or ultimately more rotor mass)


Here's what is next:


My rotors cracked. Can't figure out what happened.
You come into the pit when it happened?
Yea, how'd you know??
Just a hunch.

gkmccready

Thanks Todd. So, here's the thing, I take a good cool down lap and gradually reduce brake usage during the lap. When I come off the track I drive around the paddock for 3-5 minutes to keep air moving over the brakes. When I park the car I roll it every 5 minutes or so to keep the hot pads moving around the rotor so they don't just sit in one spot and keep it hot. This also lets me inspect the rotor to see if it has cracked. I try very hard to avoid what you're describing and I think I'm at least attempting to do all the right things.

Also note that after the event I did a 3 hour drive home on the same rotors and pads so what you see may not be entirely indicative of what it looked like fresh off the track.

I am a bit short on brake cooling, I admit. I have the C6Z brake ducts on the car after my 18x11 front wheels ate up my DRM ducts, and I haven't rigged up a hose or spindle ducts for the C6Z extensions. But it sounds like you're saying my in-use temps weren't the issue here? Forgot the mention that the SL6Rs are fitted with Thermlock pistons, too, brake fluid from the fronts look great when bled -- much better than the stock rears.

0Todd TCE

Doing the things I suggested then for sure. It doesn't look like an on track problem to me however. It's usually a sign of pads baking in one spot as we both know. I just don't feel you'd have material breakdown with those pads without some related issues such as the fluid boiling, which you say you don't have.


Using an alternate pad on the street and moving to the H pads for the track perhaps? I've seen compatibility issues with the Wilwood pads run after other compounds- the Carbon Friction stuff for one. The transfer layers don't mix well.

gkmccready

Okay. I typically swap pads and rotors and keep the H pads/rotors as a set, and my BP20 pads/rotors as a set. In the full disclosure, I thought I had warped my H rotors at my last event, so I left my street rotors on and put the H pads in for this event. But I have had this issue with two sets of my "H rotors".

I bought a new set of rotors to take as spares but I didn't want to slap brand new rotors on and head out on track. On my to-do list is to check those old rotors to see if they're warped or suffering from the same issue since the wobbly mirrors in the car gave similar feedback.

0Todd TCE

You looked at other things too? LCA/Ball joints etc? Just a thought. We've covered my ideas!

gkmccready

A good thought. I gave the suspension a once over when prepping the brakes, the usual nut and bolt. Tried rocking the wheel to check the bearing. Checked the tie rods. Not sure how to check the LBJ.

I appreciate all the ideas and it may very well be one of them that you've thrown out if I've missed something when I "checked" it. Having the "smear" as a possible explanation is helpful... now I just have to figure out how it got there. I'll also note that the issue shows up even in the first session once the driver and brakes get hot and start really working. So that's without the chance to get hot and sit in the paddock after.

Is it possible for the beefy CCW C140s holding all the heat plus the thermlock pistons keeping the heat from leaving the pads that I do end up overheating the pads without cooking the fluid? I can tell you the *wheels* get hot enough to burn you if you lean against them... just ask me how I know...

bobmoore2

Read this white paper from StopTech. It should answer your question, and provide a solution.

http://www.stoptech.com/tech_info/wp...rakedisk.shtml

As discussed in the white paper, your problem is probably caused by uneven pad deposits on your rotors. I can see it when I look at your photos too. The type of pads you're using are probably not up to the heat of track days.

DarkMastyr

I see what looks like it might be a pad deposit in your first image at 3'o'clock.

gkmccready

Wilwood H compound is very much a race compound. Now I just need to figure out why the deposits are getting left behind...