Bahnzii

I picked up an '09 C6 Z06 last year and after a few track days swapped out the stock brakes for some Stoptech 6 piston frt's and 4 piston rears with Cobalt CSR pads. After getting used to the higher friction pads, I was able to start pushing harder but then I immediately noticed that after high G turns, I had no brake pedal going into the next turn

So I learned to double pump but it just sucks, it's a bit of a confidence killer.

Well, when I finally got some time and free space, I played around in an open parking lot and I can clearly feel that even at silly slow speeds (10mph) if I do a hard left or right turn, there is clearly very little brake pedal immediately after that, I have to double pump again.

Does that mean the front bearings are shot? I only have 12k miles on her...

When I lift the front and check the wheels for any "looseness", there is none, but I have to imagine that is not the end-all test for bearings.

Any advice or tips on whether my issue is knockback or bearings and how to best check for it??

Cheers

longdaddy

if you take the wheel off and rotate the hub by hand, the difference between the bearing that's on the way out and the one in good condition (i.e. the car that has not been tracked) is pretty clear.

so all you need is a friend with a "virgin" corvette willing to let you take his front wheel off for a minute

i would also take a close look at the tie rod end to see if it has any play.

ghoffman

The way to tell if is the bearings is to listen to the brakes when you come off the track. If the light noise coming off the rotors changes during a 20mph turn changes, then the bearings are bad. It sounds like you did that The stock bearings will go bad from track use due to the brake heat. The stock C6Z brakes are terrible and the calipers are most usefull after they have been recycled into beer cans. Get some SKF bearings and a proper brake system.

mcar00

I just put a new set of Stoptech front and rears on a 3,000 mile C6 Z06 and immediately noticed pad knockback. My buddy did the same exact thing on a 5,000 mile car and also had knockback. My guess is that your bearings are fine.

Falcon

Are you guys experiencing the amount of knockback "Bahnzii" speaks of? If there is that much knockback, that car would be unraceable. And at best, it would be slow on lapping days, and at worst, dangerous.

ghoffman

Some kickback happens, even with new bearings. The SKF bearings are stiffer and reduce it, but not 100%. The new Hardbar/AP brake package has a real floater rotor and the caliper has anti kickback springs behind the pistons that does eliminate this problem. When I get back Monday, I will post a video in that thread that shows how it works. See if you can sell the kit you have to a waxer that likes powdercoated calipers with aluminum pistons!

mcar00

I'm not sure how you can quantify the "amount" of knockback. Sometimes it is only in certain turns and sometimes the pedal travel isn't as severe. What I can tell you is that when you experience it for the first time it scares the crap out of you because you don't know that you need to quickly pump the pedal. There is also a function of luck involved. Did it first happen on a slower speed turn with plenty of run off room or did it first happen at the end of the back straight at VIR with another car 10 feet in front of you?

We're just trying to learn how to deal with it and probably will see how just tapping the brake pedal with the left foot going into "trouble" areas works.

Falcon

I know there's some knockback in every car, Gary, but this much:

"then I immediately noticed that after high G turns, I had no brake pedal going into the next turn."

WOW! That's seems well past an acceptable limit. That sounds like a real problem.

Bill Dearborn

That seems to work pretty well. About 6 years ago I was following Jim Walker (the guy who writes some of the stoptech white papers) around Watkins Glen in his 04Z06. As he was going down the straights the brake lights kept flashing and I was a little nervous following him as I wasn't sure what he was doing. Later in the garage he told me he was using his left foot to tap the brake pedal so the brakes were ready for the next turn. He said it worked well and got around the pad tapering problems of the C5 calipers that in conjunction with pad knockback caused an extremely long pedal.

Here is the link to his Stoptech White Paper on Pad Knockback: http://www.stoptech.com/tech_info/wp_knockback.shtml

Bill

mcar00

Thanks Bill.

ghoffman

Yes, that sounds really excessive. Please check and retorque the bearing fasteners.

fleadh

I'm glad I ran across this thread. The way pad knockback is being described (in this thread, and the stoptech white paper) is *exactly* what I experienced early this week at thunderhill (noted in my cooling thread below this one). I thought it was being caused by not bleeding the brakes before the event, and while I'm sure that didn't help the situation, this better explains why I was having brake problems *immediately* after my first warm-up lap.

I also put in an order for a Stoptech front kit. Guess I'll have to get some new wheel bearings as well (almost 30k miles on my C6 Z06, they're probably original) and read up more on possible solutions.

Thanks!

-mike

Falcon

There's a very good Porsche racer who does pretty much the same thing. Although his knockback is minimal, he always touches the brake pedal with his left foot after he exits every turn so the car will be ready for the next turn. Aftern seeing a video of his feet, I started doing the same thing. It gives me a more secure feeling when I go into the next corner.

varkwso

Very good plan. I have never had that much knockback described by the OP in any car I have driven on track - do not want to.

davidfarmer

I'm having the same issue with my PFC kit. OEM brakes never did it, PFC's do.

Left/right/left/right combinations, like going into the uphill S's do it every time.

No perceptible play in the bearings, but it can be hard to tell. I've had great results with Stoptechs, but maybe it's an issue with "floating rotors".

I have knockback springs, but haven't put them in yet.

Bahnzii

Great feedback everyone...
I'm glad (sort of) that I am not the only one experiencing it.

I do need to restate one thing, when I checked in the parking lot at low speed, when I said I have no brake, they may sound overly dramatic. What I should have said was, that right after that abrupt turn, like sa 90 degree left or right, when I apply the brakes to slow down the pedal is very-very long. Since I am going slow, I can stop but the reach is very long. If the speeds were higher, I would have to double pump to get the pads to reach the rotors and get some real braking force.

Hoffman, oddly enough I just read about your AP racing calipers and rotors. I am seriously thinking of ripping off the Stoptech's and selling them and getting something better, like your AP's. What do you run on the rears?

I am going to get her up in the air tomorrow and re-check everything I can but I gather I not find any play or so I hope.

Keep the suggestions coming, I want to solve this problem before I get her back on track

mcar00

I never experienced any knockback with the stock brakes or with running the 14 inch Stoptech 6 piston setup up front. When I went to the 15 inch fronts and 14 inch rears I started to experience it. The one place where I consistently felt it was in Turn 10 at VIR just like you did David. I'm assuming that the larger the rotor the more noticeable knockback becomes. David, are you running 14 or 15 inch front rotors?

davidfarmer

14" front, 15" rear. I wonder if the rear could be the issue in my case. Either way, when I decide to stop being lazy I'll put the knock-back springs in and take care of it.

fleadh

That's exactly what I was feeling at the track a few days ago. The car would stop, but later than I had thought AND the pedal would be so far down that I couldn't reach the gas pedal to heel-and-toe downshift. It was horrible.

However, it sounds like it's not a Stoptech problem, but a flex problem. You have too much flex/deflection somewhere..... I've ordered a set of SKF hubs for my car and will be running it immediately after they're installed. I might even run it on the stock rotors/calipers w/ the SKF hubs just to make sure... before installing the Stoptechs.

-mike

turtlevette

this is something C3 guys have been dealing with forever with the crappy old fixed calipers. Add ill fitting pistons and crappy piston seals with springs behind the pistons, and you get air pumping which is really dangerous.

You get some piston knock back from rotor wobble (runout), some from the bearings, and some we think due to spindle movement when big forces are involved. Many C3ers use dial indicators every time they replace rotors to get the runout as close to zero as possible. Use shims between the rotor and hub/spindle. We also ignore GM recommendations and preload the tapered bearings slightly in an attempt to reduce play to zero. Not much can be done about spindle movement but there were J56 add on brackets to stiffen part of it.

I think you guys are noticing this when changing from floating to fixed calipers. The floaters move around with the rotor and just roll with the punches. Something has to give when the caliper is rigidly mounted.

Someone mentioned putting springs behind the pistons but an easier fix in my opinion is to use the wilwood blue 2 pound residual pressure valve. It keeps 2 pounds of pressure on the caliper to prevent pad knock back. http://www.jegs.com/i/Wilwood/950/260-1874/10002/-1

On second thought these valves may cause your computerized braking systems to act unpredictably. Probably not a good idea
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