Pad Knockback Issue

[stab6902]

Do think a 10psi RPV is too much for rear disc brakes? Wilwood said they sell the 10 psi version for drum brakes typically

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.

[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

[stab6902]

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.

Ryan

I'm not an expert on caliper piston seal design, but as far as I know, they are all square in profile, and the design of the seal groove has more of an impact on behavior than the seal itself. Here's a white paper on the topic if you're interested in more detail (not my paper btw, just stumbled on it looking for figures to illustrate what I'm talking about): http://www.simulia.com/download/solu...c03_delphi.pdf

[Sleeper68]

I have the same problem with my rear Wilwood brakes. I tried 10# RPV for a short period before removing them. I felt that the drag was noticeable and the rear brakes were unable to effectively shed heat.

After a series of braking test I did a "cool down" period and monitored the temps. The front brakes were substantially cooler than the rear. My theory is the RPV drag was causing the increased temperature and I removed them. I have played with trying to tighten my bearing lash on my semi-float axle, which has helped a hair...but not a lot.

I've just chosen to live with it and check-up the brakes after reasonable turns. It's habit now.

That sounds like pretty significant drag. I think I'm going to try a call with Wilwood again once I verify the silder travel. The travel is a bit tight on the inboard side, but performing a free body diagram analysis of the caliper body didn;t net an explanation for any piston compression.

You say you were able to significantly reduce axle float in your housing and you're still experiencing a lot pad knockback. This is a little concerning because to me it means the movement of the rotor is coming from the axle's deflection... Much more difficult to address. Maybe it is full floater time.

I'm not an expert on caliper piston seal design, but as far as I know, they are all square in profile, and the design of the seal groove has more of an impact on behavior than the seal itself. Here's a white paper on the topic if you're interested in more detail (not my paper btw, just stumbled on it looking for figures to illustrate what I'm talking about): http://www.simulia.com/download/solu...c03_delphi.pdf

Very interesting read. You use alot of engineering rhetoric. Are you by chance one?

My hope would be that Wilwood has the seal/groove design thing down. It is a pretty critical component of the caliper's design.

[stab6902]

Very interesting read. You use alot of engineering rhetoric. Are you by chance one?

My hope would be that Wilwood has the seal/groove design thing down. It is a pretty critical component of the caliper's design.

You caught me lol. I was a brake system vehicle test engineer for several years at a tier 1 supplier. I agree with Wilwood's point that you generally shouldn't use a 10 psi RPV with discs (production cars don't use any RPVs at all), but they might get you by for awhile until you can address the root cause at the axle. I don't think there's anything "wrong" with the design of the seal grooves or calipers - no calipers are really designed to deal with the discs moving around underneath them.

[Sleeper68]

You caught me lol. I was a brake system vehicle test engineer for several years at a tier 1 supplier.

Nice. I have also worked for a Tier 1 Automotive suppliers (JTEKT and MAHLE) as a co-op student. Very interesting line of work. We had a machine go down. Had trouble getting part out of it within BMW' spec. We then shutdown the BMW line and they set us a bill for $2M. Glad I'm not in that realm as a professional. I do medical devices now.

I agree with Wilwood's point that you generally shouldn't use a 10 psi RPV with discs (production cars don't use any RPVs at all), but they might get you by for awhile until you can address the root cause at the axle. I don't think there's anything "wrong" with the design of the seal grooves or calipers - no calipers are really designed to deal with the discs moving around underneath them.

The 4 psi RPV may be the sweet spot.

My hope was the floating caliper design would be enough to alleviate the knockback but it appears I am wrong. Do you think the movement of the rotor is mostly axle thrust play or axle bending moment from vertical and horizontal forces (mainly cornering)?

This car can generate about 1.1 g steady state. Rear static corner weights are about 650lb. I anticipate weight transfer at that acceleration of 350-450lb.

[stab6902]

Do you think the movement of the rotor is mostly axle thrust play or axle bending moment from vertical and horizontal forces (mainly cornering)?

If the amount of knockback gets proportionally worse as you corner harder, I'd guess mostly bending. If it happens even during easy street driving, I'd guess mostly axle play. I'm assuming end play etc feels/measures tight with the car on jack stands?