Ideas needed on track day braking problems

[schemauer]

I have the 1993 #33 Archer Racing car that I use for open track days. The car uses the dual piston calipers in front and stock calipers in back. Full track car - no interior. Full cage and adjustable suspension.

The problem I am having is similar to the "pad knockback" that some track cars experience due to rotors that might have a bit of runout and push the pads (caliper pistons) back into the caliper.

When the brakes are first applied coming into a corner the brake pedal needs to be pushed much farther than normal to engage the brakes. If I stab or pump the pedal first before coming into the corner then the brake pedal moves it's normal distance to engage the brakes. For some reason it seems the pads are being pushed back a bit into the calipers.

I tried a bit of an experiment this past weekend. If I drive the car straight the pedal will act "normal". If I slalom the car left and right first and then apply the brakes I have the low pedal issue. This was all done at only 20-40 mph. I am thinking that maybe some part of the front suspension is flexing enough to cause the rotor to move relative to the caliper and caliper bracket,causing the pads to be pushed back in the calipers? Wheel bearings possibly? The whole front end in the car is new - bearings, tie rods ends, hubs - and the brakes are bled properly. The car has no brake booster either.

Looking for ideas or experiences that might help me out.

 

[jtmcinder]

There are gizmos available that hold about 1 psi of pressure at the caliper which are designed to help with this, although if you really have run-out issues, it might cause the rotors to heat up even more.

- Jtoby

 

[Greg Collier]

Hey Scott,

I’ve never experienced the phenomena of pad-knock (crossing fingers) with either my RRE Big Brake dbl-piston setup, or the current Stop tech’s I’m running now. I know that Rich (Slowoldpoop) was having some problems:

http://www.dsmtuners.com/forums/showthread.php?t=192583

Rich was talking about the same pressurized valving system the Jtoby has mentioned (you might PM him to see what the out come of that was)

Also, here’s an article by a Stoptech engineer shedding some light on the situation:

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