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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: 2000rsv on August 31, 2012, 10:27:29 AM
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'86 K75C with 21k miles (non-ABS). The front rotor imparts a slight pulsation when applying the brakes. Feels like when the front rotor on my truck gets warped.
Put a new front skin on and had the dealer check the runout of the rotor while doing the balance. Runout was about 0.004". Still stops fine (for an anemic K75 front brake) but the pulsation bothers me.
OK, the question. Is there anywhere that turns/machines the front rotors so they're flat? Or do I need to replace it?
Thanks, J
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I took some to a machine shop once and they said there wouldn't be enough meat left after they turned them. I've never heard of anyone having them turned. YMMV.
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They don't turn them, they have to be ground. Stainless is too hard. I'd live with the pulsations as long as the rotor has the thickness in spec.
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I have access to a disc grinder and tried to do a front disc but because of it being thin in the first place it wouldnt come out true.
You have to put a small pressure on the disc face for the grinder to work but even with that pressure they warp and wont grind true.
I have ground a lot of discs from the Jap bikes on that machine and they always come out true but not the K series BMW. Many of the earlier discs were OK.
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Generally a "warped" disk is not warped. There is material from the pad embedded in the surface.
You can try Scotchbrite and water or wet sanding to clean it up. Or bed in the new pads by harsh
braking until everything is hot. That will put a new later of material on the surface.
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Generally a "warped" disk is not warped. There is material from the pad embedded in the surface.
You can try Scotchbrite and water or wet sanding to clean it up. Or bed in the new pads by harsh
braking until everything is hot. That will put a new later of material on the surface.
I had the disk runout measured while being balanced after a tire change. Warped may not be the right term, it may be uneven wear (or uneven buildup) rather than warpage. Will likely just live with it since there appears to be plenty of meat on the rotor.
However, I will test out the Scotchbrite idea the next time I put pads on.
Thanks all for the information and experience.
J
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+1 on several hard stops from 70+ mph. Bimmers and beemers probably both appreciate a good scrubbing now and then to kill pad squeal and possibly brake shudder as well. There was a time, years ago, when the 5 series had MAJOR brake shudder issues. Mostly it could be ameliorated with some hard stops. Non-sporty drivers were having a hellofa time, as were the dealers.
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Had the same problem as you earlier in the year, cured it by lightly sanding both sides of the discs with a air die grinder. I had a rust buildup on the rotors as the bike had been sitting a while.