High speed braking issue ??

[Zombie killer]

Seeking input from experienced road course guys on hard braking issues on the road course. At NCM during Camaro Fest 8 , on the braking zone heading into turn 1, under hard braking I would feel a slight shimmy or possible feeling of like a warped rotor in the brake pedal. I chalked it up maybe heating up a rotor and warping it, but I can find no evidence of permanent warping now. I did take off all wheels off on Friday and checked for anything amiss, including inspecting all rotors closely. Nothing stood out. Now the OEM rotors where checked and measured to make sure they were within wear tolerances before I even left for BGK. I also installed and bedded in new Hawk street/race pads the week before CamaroFest.

So I am curious if I am overlooking anything needing to be checked or if maybe this is somewhat normal “feel” when the rotors get hot from hard breaking. I was standing on the brakes pretty hard for what these pads could do but never really had mulch brake fade but maybe a handful of times abeit briefly.

Other suspension mods include upgraded sway bars and also the JPSS radial arm solid aluminum inserts and the Z28 Caster locks. However I am still on OEM rotors and the Brembo 4 pots.

Any say suggestions of what to look for? Planning to run next month at Nola Motorsports Park and would like to go back and look again for any possible issues before I go.

[Norm Peterson]

Did this happen during every session, including the first one?

I highly doubt that there's anything wrong with the rotors themselves, but there could be some unevenness in the pad deposits. Hopefully you didn't set the P-brake in paddock between your sessions.


Norm

[frankwjr]

Has the brake fluid been thoroughly bled and replaced with DOT4? I have done 2 complete bleeds since owning the car(3 years) and getting ready to do a 3rd. Each time the brake feel and performance got/get's better.


Make sure to follow the correct bleeding procedures, our cars are a bit different in that regard. I use a hand held one man bleeder and get about a oz. out of each caliper. If you want to flush the ABS you need to either go out and slam on the brakes several time to get the unit to pulse...or a Tech II would be needed.


Good luck!


peace

[Zombie killer]

Originally Posted by Norm Peterson

Did this happen during every session, including the first one?

I highly doubt that there's anything wrong with the rotors themselves, but there could be some unevenness in the pad deposits. Hopefully you didn't set the P-brake in paddock between your sessions.


Norm

It didn’t happen in the first session and not sure in the entire second session, but I started noticing it more after the break for lunch. And it was only in the very high speed braking sections. And I unfortunately have a L99 so no parking brake used between sessions.

[Zombie killer]

[QUOTE=frankwjr;10278388]Has the brake fluid been thoroughly bled and replaced with DOT4? I have done 2 complete bleeds since owning the car(3 years) and getting ready to do a 3rd. Each time the brake feel and performance got/get's better.


Make sure to follow the correct bleeding procedures, our cars are a bit different in that regard. I use a hand held one man bleeder and get about a oz. out of each caliper. If you want to flush the ABS you need to either go out and slam on the brakes several time to get the unit to pulse...or a Tech II would be needed.


Good luck!

I replaced my fluid to Dot 4 many years ago. I also flush and bleed 2x a year as I autocross quite regularly. I even had Pete go ahead and pulse my ABS and do a full flush last year at the host hotel parking lot prior to last years NCM. I had that done just for good measure as the prior year I had a master cylinder go bad and it ruined my brake booster too. I had them both replaced then and a full flush done at that time too. Going to do another thorough check of the front suspension for any wear or play and see if I can at least eliminate variables. It is just hard to generate the same type of heat in the rotors during normal street driving to check to see if it’s the new Hawk street / race style pads.

[hammdo]

Could it have been ABS pulse? I've hit the brakes hard there and that will feel like a warped rotor. Did it this last time out into turn one and I just eased off a bit.

I try to practice ABS breaking on wet roads when No one is around - just to get a feel for ABS activation...

Another time to practice is in the hot pits on the way out... Around 35 Mph to almost full stop... If spacing allows...

-Don

[olblue75]

Possibly a caliper bolt? I'll have a set of CTS-V rotors for sale soon if you decide to go 6 piston.

[AG1LE]

Like Don, I was also thinking that you may be feeling the ABS pulse, but then I’m not sure why you didn’t feel it the first session. Maybe your braking was less aggressive at first and became more aggressive as the day progressed?

Were your tires in good shape?

[SSE 4 2SS]

Prior to installing the parachute, I felt the brake pulse of the ABS every time I ran high speed passes and jumped hard on the brakes. Now i just pray every time that the clutch brake works as it should.

I'm in agreement that it is a possibility.

[h018871]

When the ABS kicks in it will feel different with the hawk pads, particularly when compared to OEM.
Uneven pad material deposition is a possibility, although as the day goes on I would expect it to start effecting most if not all braking you’re doing or to go away as the pads scrubbed the rotors.
Since you’ve checked everything, I’d lean towards a different feel from the pads.
Check for loose hardware and play/slop in the front suspension, bearings too.

When are you headed to NOLA? That’s a fast track

[Zombie killer]

Originally Posted by h018871

When the ABS kicks in it will feel different with the hawk pads, particularly when compared to OEM.
Uneven pad material deposition is a possibility, although as the day goes on I would expect it to start effecting most if not all braking you’re doing or to go away as the pads scrubbed the rotors.
Since you’ve checked everything, I’d lean towards a different feel from the pads.
Check for loose hardware and play/slop in the front suspension, bearings too.

When are you headed to NOLA? That’s a fast track

The next Track Night in America is Sept 20th at Nola. It does have a super long straightaway doesn’t it? You just have to make that decision do you stay off the stripes or cross them and carry more speed to the wall as you approach the front stretch. Since run street tires I choose the first and play it safer and keep it off the wall.

But to think more on topic, since I have never run those Hawk pad compounds and I was getting more confident in my braking work, it could be possible I was activating ABS at those speeds. I normally run RE71s for autocross and activating the ABS is not common but when I do it is more noticeable at those slower speeds. I usually experience “ice mode” more than I do ABS mode. So when I run the Michelin’s At NCM , it may be ABS that I feeling and I am just thinking is a slight wobble in the rotors or suspension. I will pay more attention to my threshold braking and see what happens. Thanks for the input guys.

[wakespeak]

You need radius arm inserts. The z/28 has delrin versions with the replacement radius arms or you can get the inserts from JPSS.

Without them the radius arms give under hard braking and begin bouncing off the outer travel limits. Could also be wheel bearings, out of balance tires, bent rims, warped rotors, pad transfer, etc that the other guys have brought up. Anything a little out will get exaggerated with the mushy (fluid filled) radius arm bushings.

[Edit I see you have the inserts].

[wakespeak]

My bet is bent wheels, wheel bearings, or rotors. After solving the shimmy with the inserts, it came back briefly. Turns out I bent 3 of 4 MRR wheels somehow. I now have forged GM 2017 1LE wheels. You can take them to a tire shop and have them spun on a wheel balancer. You can visually see if they have runout usually.

I replaced my wheel bearings at 40k miles and rears at 50k miles. The fronts groaned and when spun on the hub (caliper removed) clicked.