Armycop

Ran my first track event this past Saturday with the new (to me) setup-Wilwood 6-pots up front, 4-pots out back, 14" two-piece rotors all around and DTC 70's/40's for pads. Here's where I'm in a pickle..

I got serious knockback everywhere on the track. Several times the pedal wouldn't move, forcing me to pump the brake to get some action. Made for an interesting day. I took to giving the pedal a left foot tap on the straights so I could make the next corner. I've shimmed the calipers so they are dead nuts centered on the rotors. I'll be doing another bleed at each rotor, but given they are two piece calipers (inner/outer bleed screw), is there a system for this? Inner first, then outer, etc.?
Additionally, ABS was insisting on interfering with the fun. I pulled the ABS fuse for the second run, which helped immensely, but I don't want this to be a part of the routine for track days. Bill, you were mentioned by several Wilwood vendors as the man to consult, so please, drop in! Everyone else too.

Jamie

sperkins

Hub bearing brand?

Armycop

SKF all four corners. Two years and change.

brkntrxn

Hmmm, I have been running Wilwoods since last summer and have not had any issues. This is with five race weekends and an enduro at tracks ranging from Watkins Glen, CMP, Road Atlanta, Barber, and Roebling. Wilwood W6A fronts with 14.25" two piece rotors and W4A rears with 14" rotors, non race version of both. I also have SKF hubs on all four corners.

Hopefully someone can chime in with a solution.


As for bleeding, I do outer and then inner to follow the age-old rule of bleeding the farthest from the master cylinder and working your way inward.

-Kevin

Armycop

Yea, I've got new hubs on order. Left side front and rear hubs wiggled in my hands when I shook the wheel today. So, that could be a contributor to my issue, no? Going to do another bleed session this week as well, see if that helps any.

FASTFATBOY

The hubs are the issue.

waddisme

Knock-back was how I knew my hubs were going. Thought it was brake related, but new SKFs cured it.

0Todd TCE

Is this where I say "told ya so?" LOL!

Figured this was going to be the answer you'd find. But not being a Corvette expert and only a Wilwood expert I figured I'd defer to the masses. Either way I'm glad you found the wiggle and I suspect things will improve greatly next time out Jamie.

JRitt@essex

Bearing and upright flex are a common cause of knockback. Fresh bearings may help, and some guys switch to the ZR1 spindles, which I believe are supposed to be stiffer.

Finally, do the calipers you're running have Anti-knockback springs? This is a caliper feature designed specifically to combat the knockback problem. We include them in all of our Competition BBK's, and all serious race calipers at the higher levels of motorsport have them.

From our website:



The term "knockback" or "knockoff" describes when caliper pistons are pushed back into the caliper by the brake disc. This typically occurs when the driver goes through a series of opposing turns (esses), and is caused by compliance in the vehicle upright and hub assembly. The brake disc acts as a lever, and forces the pistons back into the caliper body. When the pistons are pushed back, slack develops in the brake pedal. An initial press of the pedal is required to bring the pistons and pad back into contact with the disc (taking up the slack) before any actual braking can occur.

To combat this issue, AKB springs are placed behind the caliper pistons. These springs resist the pressure of the disc against the pistons, and prevent the pistons from being forced back into the caliper. At the same time, the springs are light enough that they don't cause significant brake drag when combined with the appropriate piston seals.

Benefits:

If you've ever experienced knockback firsthand, it's an extremely disconcerting feeling when entering a corner. The initial pedal drop you experience can be substantial, and it may feel like brake pressure has been completely lost. AKB springs can provide a tremendous boost in driver confidence. You won't need to 'pre-tap' your brakes before each corner, and you'll find the same high, hard pedal waiting for your foot every time. The more confidence you have, the faster you will drive.

sperkins

Jeff,
What is your experience/impression with inline residual brake pressure valves? Something like the 2psi Wilwood version.

sperkins

You mean like this?
Watch the brake pedal closely. I counted 17 taps with the left foot before corner entry.

RX-Ben

watch Mike Skeens PWC video from 2011 or so where he follows the Caddy around Road Atlanta. Caddy taps before every turn, every lap.

Armycop

So it's certainly not uncommon if the pros are doing it too with their high dollar AP Racing setups. Interesting.

0Todd TCE

I believe 2lb RPVs were run on Jerry Onks car with the W4A track kit for this same reason.

Bill Dearborn

Jamie,
Didn't see this until just now but I think you have your answer. Loose hubs will do that to you especially as loose as you indicate your hubs are.

As for the bleed sequence Wilwood recommends doing the outside caliper half followed by the inside caliper half. I think GM recommends just the opposite on their Z06 calipers but it probably doesn't make much difference as long as you do them all the same way each time you bleed.

Bill

YYZ06

When I took the Ford Racing school a few years ago at Miller Motorsports Park, pretty much all of the instructors tapped prior to the brake zone especially after long periods off the brakes.

I'm sure you did but did you check the bearing mounting bolts? Remember Mid-Ohio last year...

Armycop

Yes, and they are all tight. No matter, the hubs are being replaced today anyway. After the Tudor Belle Isle race of course...

Armycop

And the hits keep comin'!
In the process of re-bleeding the calipers I managed to shear off one of the bleed screws-outboard driver's front. Once I got the screw carcass out of the caliper, I ran a thread chaser through it to clean out the threads of leftover brass. In the process of doing that, I managed to widen the threads juuuust enough so now brake fluid oozes out from between the new bleed screw and the caliper body...uugh.

So I either need to replace this caliper. It's a Wilwood 6 piston, model # 8004, driver's front left. Anybody got one extra? Just one. Don't need a whole set, vendors.

Here's some pictures so everybody can enjoy my misery.
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0Todd TCE

Message sent. There's also a very remote possibility that the hole could be tapped for 1/8npt like some of the other models, not M10-1.0 that it is now. Then thread in the npt/nipple adapter. If you can find a buddy or shop near by who has one, you got nothing to lose trying.

Bottom ones:

Addicted to track

Hey if you are going to repair it I think I have some of those bleeder screws and I also have my old SL6s. They are lug mount and the pistons are shot. (just worn out and a few scratches) but maybe you could take the outer half off and mate it up with yours?? Its not painted red its the plain anodized aluminum finish...just let me know you can have them.