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Old 09-18-2009, 12:13 PM
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Default Official: Brake Pad Knock Back Thread

As some of you know I and a few others have fought brake pad knock back issues. With a fixed caliper like many of the large manufacturers use, any slack in the bearings under load cause the pads to recede the pistons into the caliper. This isn't so great the next time you hit the brake pedal since you experience more pedal travel than the previous stroke.

The first time I autocrossed my car I had to pump the brakes between the corners. I determined I had .020.021 play end play in each axle. I bought some different retainer plates and got it down to .005/.006. While that has improved it tremendously, I'm still not satisfied on the street or track. I know it's not the front because I have C6 bearing packs and they only net .001 play. At this point I called Wilwood and they don't offer a floating caliper so I would need to switch brakes if I go that route. My number two thought is to go to a tapered bearing. I called moser and they said most circle track and autocrossers run a tapered bearing but he couldn't guarantee that would fix my problem. He also said .005 was tight enough for a roller bearing.

I can't really get anybody to cough up a spec on how tight the axle end play should be. Wilwood is actually sending me some shims but how much tighter can I go and will .002-.003 really help that much. Has anyone changed from a roller to a tapered bearing and has it helped? I'm experiencing a change in brake pedal even on the street under normal drving conditions. While it's managable on the street and track. I want a consistant brake pedal. I had a brief conversation with Frank at Prodigy yesterday and it's happening with other brake manufacturers as well. He said he knows a guy that is running a 10psi residual valve in the front and back. While I'm sure that would fix my issue, I feel it will cause excess heat and pad wear on a cruise that could potentially cause brake failure.
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Old 09-18-2009, 12:39 PM
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Do you think 10 psi residual valves would put that much heat into the rotors? I don't know, but I kind of doubt it. I do think it would make for a lot more brake dust, though.

It would be an interesting test to put your car on jackstands, run it in gear for X amount of time then check the rotor face temp with an infrared thermometer, then install the residual valves and repeat the test (noting the ambient temps).

The amount of energy absorbed and dissapated through the rotors from regular daily driving just makes the 10 psi residual valves seem underwhelming. Again, I don't know, just sayin.
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Old 09-18-2009, 02:35 PM
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Sounds like a 12 pack experiement. I'd want to run the car at highway speeds for a reasonable distance. I just feel like that's a bandaid covering up the actual problem. I put a 2lb in the rears but it's not enough to move the pistons back. I'll check with Frank and see if the individual he knows is running a tapered bearing.
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Old 09-18-2009, 03:17 PM
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lol...

I'm running tapered bearings in my 9"... I'd hate to say that there's no runout, but I can't feel any when I've messed with it.
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Old 09-18-2009, 03:36 PM
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Are you running a fixed caliper out back Scott?
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Old 09-18-2009, 03:41 PM
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Yep, they're fixed.
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Old 09-18-2009, 05:30 PM
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Quote:
Originally Posted by Vegas69 View Post
It just causes variations in pedal travel. On the street maybe an inch or so, on the track up to 2-3 inches I would say. It's more of a nuisance than anything. On the street it's not a huge deal, on the track, it's to much travel and causes late braking. I can see circle track racing not causing the issue an autocross would. You are loading the axle one way most of the time.
Thanks. I deleted my post because I think the PBR is technically a floating caliper and I didn't want to confuse people.
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Old 09-18-2009, 05:38 PM
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What rear end are you running. I would not think .005 would be noticeable unless the pistons are actually moving the axle under pressure farther than you can move by hand
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Old 09-18-2009, 05:53 PM
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It's a Moser 12 bolt, True Trac, 33 spline, big ford end with roller bearings. The .005 is definitely bearing clearance. It's not enough to cause a problem in my opinion. It's the side load on the bearing causing the issue.
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Old 09-18-2009, 06:26 PM
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I've noticed the pad knock back with my car too. It was far worst with the factory c clip 12bolt, better with the moser but still there.
It's manageable, but very annoying.

I've thought about the residual pressure valve. I was hoping 2# would be enough. I with you Todd, 10# seems excessive for pad wear and dust....

I'll be following this one.
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