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  1. #1
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    XT Disc Brake Install Problems

    Just got a set of used XT disc brakes for my 2008 Fuel EX 8 to replace my Avid juicy 5s.

    The front brake went on without a problem. However, I cannot get the rear brake to stop rubbing. I have the caliper to the extreme outboard position and it is still rubbing. I don't know what to do because I have no more outboard adjustment.

    I am using a brand new shimano 160mm rotor with a shimano 160mm rear adapter. I am about to take it to the LBS, but it seems like I am missing something obvious. Any thoughts?

  2. #2
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    Is the piston out too far? If so, you can simply push the piston back into the caliper.

    If not, you could try using the old adapter from your avids. The shimano adapter could be a tad wider. Also make sure the rotor is flat - it could be slightly dished.

    Someone in a recent thread had to machine his adapter face to get it to work on a fork because of a tab problem. I wouldn't be afraid to enlarge the mount holes on a caliper with a round file by a very small amount. Others might not want to do that.
    Last edited by rlouder; 04-05-2011 at 07:07 AM.

  3. #3
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    Since you have the brakes adjusted fully outboard, I am assuming that the outboard pad is still rubbing? If that's the case, take the bike to the LBS and have them face the rear disc mounting tabs. Avid brakes tend to have a bit more lateral adjustment than many brakes out there, add to that the cps washer system and they have quite a bit of room for error. Most frame manufacturers, if they face the tabs at all, will face them before painting. That and the facing, if done, is by machine rather than by hand. Not quite as precise. A little extra paint or metal can through things off, and it doesn't take much. The LBS should be able to easily tell if that's your problem and get you set up.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  4. #4
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    Quote Originally Posted by rlouder
    Is the piston out too far? If so, you can simply push the piston back into the caliper.

    If not, you could try using the old adapter from your avids. Someone in a recent thread had to machine his adapter face to get it to work on a fork because of a tab problem. I wouldn't be afraid to enlarge the mount holes on a caliper with a round file by a very small amount. Others might not want to do that.
    Rlouder - No, the piston is almost all the way in. The rotor is actually almost touching the caliper housing. I thought about taking a file to the caliper mount holes, but I am a bit concerned doing that..would be a last resort Machining the adapter would be better, but that is not within my capability.

    Squash - Yes, the outboard pad is still rubbing. I am glad you mentioned the rear mounting tabs on the frame. I was looking at them and they don't seem to be finished (not a flat surface). They are very rough as if they were hand ground; they are also not painted on the mounting surface. If the LBS can take a little off or smooth them out, I think that would solve the problem. My only concern is if grinding the tabs would void the frame warranty? I am taking it to an official Trek store, so they should know.

  5. #5
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    Facing brake tabs is common and doesn't void the warranty
    Check out my SportTracks plugins for some training aid software.

  6. #6
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    As Cil said, facing the tabs won't void the warranty and is a common practice. I work as a mechanic at a Trek dealer and it's one of the first things that we do when upgrading a customer to disc brakes. It avoids or solves a whole world of headache before it has a chance to start.

    Just one word of caution. Filing can work, however it is not very precise. You not only want the tabs a bit thinner, but you want them concentric to one another. It is almost impossible to get them even by simply filing. Park Tool and a couple of others make tools specifically for facing disc brake tabs. But they are quite expensive so most home mechs likely won't have them. They're not worth the cost unless you use them quite often. Any LBS worth spit should have facing tools though.

    I would suggest heading to your LBS with the bike, brakes mounted so they can have a look. Tell them what's going on and what you want them to do. They should be able to hook you up. And it shouldn't be to expensive.

    Machining the brake adapter or enlarging the mounting slots in the caliper, as dmr31078 suggested, should ONLY be used as a last resort.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  7. #7
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    So, what's the verdict?

    Did you have a store smooth up the tabs or did you do it yourself with a file?

  8. #8
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    I just got back from the LBS which is an official Trek store (ie. the store name is Trek). They decided to file the holes on the brake caliper. I asked them about re-facing the brake tabs or brake adapter and they said they would have to do it manually and it would be difficult to do correctly. They don't have the machines to do it properly.

    I am not thrilled about what they did, but they didn't have take much off. Plus I would have been more unhappy if the F'd up the brake tabs, so I am kinda glad they didn't attempt that. Although filing the holes probably voids any kind of warranty there was on the brakes, and it might be difficult to sell them some day.

    In the end, the brakes work.

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