Pad Knockback Issue

[Sleeper68]

Depending on the design of the caliper piston seal and seal groove, 10 psi might be enough to cause brake drag. A very small amount of drag is okay, but you don't want to be in a situation where the rear brake temps go up as you're cruising down the highway for example.

Given your problem, I'd be inclined to start with the 10 psi RPV as well. As long as the car still rolls freely and the brakes don't heat up from dragging (use an IR thermometer on the rotor) you should be fine.

Interesting you bring up seal design. I believe the seals are square cross section orings, likely with a brake caliper specific "squish" specification, possibly based on the D154 GM caliper it is descendent from. I can ask Wilwood. What about the seal design in your opinion affects drag potential? Deflection of the seal and non linearity at low fluid pressures? If so that could be the cause of the lack of reaction force to piston compression and thus the knockback @ 2 psi.

The total piston area of the rear calipers is 1.99 sq in each, so lets say 2 sq in to make it easy. @ 10psig line pressure we would then have a "normal" force on the pad of 20lb. The friction coeff of the pads is about 0.43 so lets say 0.5 to be conservative. That equates to 10lb of friction on each pad. Doesn't sound like alot but maybe it is enough to see cause non trivial drag. The efective radius the pad acts at is about 5.5" or 0.46 ft. The total pad frivtion force would then be 10lb * 2 or 20lb. 20lb * 0.46ft = ~9.2lb-ft. This would be on each wheel, or ~20lb-ft total.

I would think the air movement alone over the rear calipers and the heat sinking of the rotor would hide the drag from a thermal point of view. Your idea to feel it out by pushing the car is a good one.



Will attempt 10 psi RPV after checking the slider travel.

Unfortunately my Muncie decided it was time to leave this world a few weeks ago so I need to get the new trans in before any road test anyway.

Thank you

Ryan