One thing to remember is not everyone is forth coming with their problems. I shouldn't assume people don't have the problem. I can't imagine that folks wouldn't want to share their problems with the world.:D I can't believe DSE silicones in their axles between events. No way I would put up with that crap! Matt, what master cyinder and is there any booster or hydraboost?

Can we clarify what "knockback" actually is or feels like?

I've got manual brakes, a Wilwood 7/8" bore master cylinder to be precise.

Todd, I'm a bit surprised they just use silicone too. I guess for them it is the best solution for a street car. Since they race what they sell, and they predominantly sell to people who will almost never drive on the track, their setup makes sense. There would be no point in running a full floater on their cars and then selling the Torino Bearing 9" setup to the public. I do wish they sold a setup that was overkill though, I know I would buy it.

As for what knock back is, I think Greg might have posted a good link explaining it somewhere. Basically, under hard side loading, going around turns, there is enough stress on the bearings, and axle components that everything moves a bit causing deflection. What this does to the rotors could be compared to an outside wheel on an old car gaining positive camber around a turn. In other words the rotor flexes and is no longer perfectly parallel to the brake caliper, thus pushing the pads and pistons back. Then, when you need to brake and you step on the pedal, you get a really light feeling i.e. the pedal might fall halfway to the floor, or feel like it isn't bled properly. If you tap the brakes once and push the pistons back in place everything will feel normal again. I think, and correct me if I'm wrong, power boosters, like what DSE runs, will cut down on the amount of noticeable knock back feel, but doesn't address or cure the problem. On manual brakes like a bunch of us are running it is very noticeable. I've got a 7/8" Wilwood manual setup, and it does not inspire confidence before you tap the brakes up.

Here is a picture of one of our car's rear rotors. I think the reason I never noticed bad knockback sooner was because the rear calipers weren't evenly spaced and whenever I'd go around a hard turn, the rotors would grind against the retaining pins between the padlets on the C6 Z06 brakes. Because of this knockback was limited, and less apparent. The downside though, was that my rear brakes weren't ever working 100 percent.

You can see the ring caused by the deflection, where the rotors ground against the retaining pins:
http://i366.photobucket.com/albums/o...A/DSC_0651.jpg

Matt

What it is....

http://www.stoptech.com/tech_info/wp_knockback.shtml

knock back is generally only seen on radial mount calipers or lug mount. floating calipers like c5 etc will never get knockback because they float. radial mounts are generally run in race applications where larger/better bearings are used as well as full floaters in the rear.

That's exactly why I may still install a C5 caliper out back until someone comes up with a economical solution.

Matt, hope to have time to work on mine this weekend and will give you my thoughts on the 1" master cylinder with no residual pressure.:unibrow:

my 69 has 4 piston alcons up front with 9012 hawk pads and single piston c5 rears on a 13 inch rotor in the rear with hp plus pads. never got any brake fade, at any track, nhms,watkins glen,njmp,gingerman etc and the car is no slouch

So Todd, why is it that a power assisted setup knocks down this issue---greater initial hit on the first pump of the brake pedal? It's moving the pads further out (and away) from the calipers with the first hit?

Doug

Doug, I think the power booster reduces the apparent feel of knockback because it has a larger bore, meaning that less fluid is displaced as pads/pistons get pushed around.

Matt

Larger bore master cylinder means more fluid volume to the calipers with less pedal stroke. I got my 1" master back on tonight. All I need to do is bleed all four corners and go for a ride. Hang in there a few days.