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Discussion Starter #1
So I have a Wilwood kit on my car... Street setup calipers with dust boots. I have been getting a fair amount of pad knockback from what I can only rationalize as the rear calipers, which is weird because I have an 8" in the car.

The solution from Wilwood is to pull the dust boots, pull the aluminum pistons and replace them with their steel pistons and some anti-knockback springs.

Anyone else have this issue with fixed calipers that dont have the springs in them? Did you come up with any cheaper solutions? It looks like Im going to end up spending around $125 per rear caliper to retrofit them.
 

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All traditional style 8"/9" housing new or old are susceptible to pad knock back when cornering hard. The axle bearing isn't designed for lateral loads. Springs behind the pistons will help to keep pressure on the pistons and may work if they are strong enough to push the pads back to the rotor before the next turn.

Of course, the only real way to fix it is with a full floating rear end. Our hubs have tapered roller bearings inside. No pad knock back. Checkout the video we did with Mustang Monthly:

 

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Discussion Starter #3
Shaun, thank you for the info. Your stuff looks awesome and I’m sure it works better than it looks. I wish I had the coin for it.

I don’t suppose you sell a kit to convert an 8” over to be a full floater, do you?

Besides the knock back springs, how do you feel about a 2lb residual pressure valve for the tears to help keep them closed up? I know it will add 2lbs of drag to the system but that seems minimal at best.

Thanks.

All traditional style 8"/9" housing new or old are susceptible to pad knock back when cornering hard. The axle bearing isn't designed for lateral loads. Springs behind the pistons will help to keep pressure on the pistons and may work if they are strong enough to push the pads back to the rotor before the next turn.

Of course, the only real way to fix it is with a full floating rear end. Our hubs have tapered roller bearings inside. No pad knock back. Checkout the video we did with Mustang Monthly:

 

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All traditional style 8"/9" housing new or old are susceptible to pad knock back when cornering hard. The axle bearing isn't designed for lateral loads. Springs behind the pistons will help to keep pressure on the pistons and may work if they are strong enough to push the pads back to the rotor before the next turn.

Of course, the only real way to fix it is with a full floating rear end. Our hubs have tapered roller bearings inside. No pad knock back. Checkout the video we did with Mustang Monthly:

Shaun - how prevalent is this issue? Is it track related only or also a street issue? While rear discs are a far future possibility, I've always had this issue in the back of my mind due a similar problem with a old Volvo 142 (brake pedal would go to the floor in order to move the rear pads back to the disc). Thanks.
 

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If you are a track noob, it make take a while until you are driving hard enough to experience it. I haven’t yet on my 69 with single piston floating calipers. I certainly can’t see it happening on the street.
However the fix is a floater axle like Shaun said.
 

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Surprising that using a small RPV is not a "must do"
I might say there is no real "fix" when Ferrari and Porsche can have the same problem, maybe its more likely on huge rotors.
While coming to top speed for any straight section a light tap on the brakes is the Pro way to mitigate it.
 

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If you are a track noob, it make take a while until you are driving hard enough to experience it. I haven’t yet on my 69 with single piston floating calipers. I certainly can’t see it happening on the street.
However the fix is a floater axle like Shaun said.
I never noticed it when I had a floating caliper set up. Switched to a fixed 4 piston set up and have noticed excessive petal travel on a couple of tracks that have a series of high speed switch backs leading up to a heavy breaking zone. Uphill leading to Oak Tree turn at VIR is the worst.
 

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I have had this issue bad since I installed my Baer system. 13" fronts, 11" rear. Manual with 15/16" MC. They feel great except after I turn hard. I have tried everything to fix it. Shimmed the rear so there is no play in the axles. Added pre-load to my front hubs. have a 4lb residual valve after the master cylinder on the fronts and rears lines. No luck. Every time I turn hard the following brake pedal press goes halfway down before it engages. A little unsettling going into a corner. It's bad enough for me that I would replace my whole brake system if I new if would fit the problem.....

Honestly at wits end on this one. It's literally the only thing I have not been able to debug on my car.
 

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Discussion Starter #10
Do you have anti-knock back springs? The engineer at Wilwood suggested I go that route above all others to start.
 

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Can I add them to the Baer system I bought from you? Pistons on one side only...

Seems like a pretty serious project taking apart calipers reinstall, bleed, and test, would hope to not have to iterate too many times....do you know how much preload and what spring constant would be good to start at?

Should I do front and back or just back?

Thanks
 

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Mcmaster sells these in a variety of sizes
762944
. Figure, I'll do the rear first and see if makes a different.
 

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Conical springs did not work. They are all too thick and don't allow the pistons to move far enough back into the bore to get the discs in after assembled....Going to try some wave springs as they are a bit thinner. This thinnest conical spring had a compress length of .12"....
 

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Discussion Starter #14
Conical springs did not work. They are all too thick and don't allow the pistons to move far enough back into the bore to get the discs in after assembled....Going to try some wave springs as they are a bit thinner. This thinnest conical spring had a compress length of .12"....
I know when I talked to Wilwood, theyinformed me that I would have to replace my aluminum pistons with their steel cup pistons to be able to accommodate the springs.
 

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That makes total sense after playing with the PBR rears. The pistons seat all the way back when the pads are new and installing over the discs. Even the 0.12" compressed height of the conical spring caused issue. The stacked wave springs are shorter so maybe will work.

Another nuance is that the piston seal has a pretty high level of static friction that needs to be overcome before the spring has an effect, so for the springs to work then the force they apply must be higher than the piston to seal static friction. This amount of load may then be too high a force to apply to the pads and may cause too much drag. I won't be able to check this until I get a working set of spring installed and tested out. I purchased 3 versions of the wave spring which will apply different level of loads.....

Outside of changing the rear to full floater, any other suggestions. Are there any rear brake setups that will not be affected by this issue? The PBR is a single piston setup, if I switched to the 4 piston setup like the wildwood or baer newer brakes would the pad knockback issue be less prevalent? A new set of rear brakes is a lot less expensive than the full floater setup but not worth it unless it will for sure solve the problem.

Thanks for the help Shaun
 

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Ok, so spent a good part of day working on this issue.....

I capped my rear brake lines and removed my rear calipers. Drove the car and took some hard corners. Pad Knock-back still there, so it was the front brakes that is causing the issue, not the rears so something is flexing under stress.

I went for the simplest solution first and cranked down my hub nuts 2 more castle nut grooves to the point that there was some drag on the wheels. They are much tighter then I expected they needed to be but after driving the knockback issue is now much improved.

I reinstalled my rear brakes and was able to use some 12lb wave springs behind the rear pistons. So with the springs in the rear, 4lb residual valves on front and rear, and cranked down hub nuts I think the brakes are finally good enough for the track. At some point I may move to some heavy duty spindles with big bearings and some different brakes but things are finally good enough for Willow Springs at the end of the month!
 
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How tight are you talking ? I worry that tight bearings and high speeds may not go together well. It seems to me if you can feel the bearing drag at the tire, it's too tight.
 

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Yes there is some drag but when I spoke with the Baer tech he said some drag is ok.
 

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I feel like I get some knock back with my Granada front brakes during autox( but never on the street) I've thought about doing a residual valve for the front . I already have one for the rear
 
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