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Disc Brake Pad Rubbing

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Disc Brake Pad Rubbing

Old 06-20-12, 06:06 AM
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Disc Brake Pad Rubbing

Hey guys

I recently put new wheels on to one of my bikes. Got the rear brake set up nice and dandy, but there's an issue with the front brake. Here's the credos:

I started by sticking a tire lever in between the pads to try and free them up. Then I installed the wheel, centered it and pushed the brake lever about ten times to try and free things up more so. I then set the adjusting screw so the inner (fixed) pad was as far from the rotor as possible. I then loosened the caliper mounting bolts, depressed the brake lever and tightened the caliper bolts while holding the lever.

Despite doing all this, the fixed pad was still rubbing against the rotor. So I rearranged the wheel spacers (to move things around by ~2mm) and repeated the above, but still the problem remained.

Anyone have an ideas what the problem could be? I should add that I think the bike only has about 200 miles on it.

Thanks
T
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Old 06-20-12, 09:46 AM
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try shifting the caliper it self over towards the bike centerline.

putting a shim washer behind the disc.. there are some made with 6 holes and 2 holes.

mechanical disc? pad return spring is not working, broken?
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Old 06-20-12, 06:23 PM
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Originally Posted by tommyd49
...I started by sticking a tire lever in between the pads to try and free them up...I then set the adjusting screw so the inner (fixed) pad was as far from the rotor as possible. I then loosened the caliper mounting bolts, depressed the brake lever and tightened the caliper bolts while holding the lever...the fixed pad was still rubbing against the rotor...
Since this is a mechanical brake, why did you feel the need to pry on the pads?

When you loosened the caliper bolts and pulled the lever, the outer pad effectively pulled the caliper and inner pad in the direction of the rotor. Instead, adjust the inner pad so that it hard contacts the rotor, loosen the caliper bolts, and screw the inner pad further, pull the lever (assuming this caliper does not have an adjustment for the outboard pad) and tighten the caliper bolts. Now there is room to back-off the inboard pad.
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Old 06-23-12, 05:58 AM
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Originally Posted by Werkin
When you loosened the caliper bolts and pulled the lever, the outer pad effectively pulled the caliper and inner pad in the direction of the rotor. Instead, adjust the inner pad so that it hard contacts the rotor, loosen the caliper bolts, and screw the inner pad further, pull the lever (assuming this caliper does not have an adjustment for the outboard pad) and tighten the caliper bolts. Now there is room to back-off the inboard pad.
Thanks for the tip, that helped. However, I'm still getting a lot of rub even though I've got about 4mm worth of spacers on one side of the wheel! Seems strange considering this is a front wheel, all front wheels (pretty much) are of 100mm OLD. Maybe this is quite common when changing wheels? I don't really know because my experience with discs is limited. Does seem a bit much to me though.
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Old 06-23-12, 08:23 AM
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Are you sure the caliper is square to the disc. If not mounted perfectly square, there's less working room for the disc as it passes through the gap on an angle. Also, hubs and the position of the disc are not all precisely matched on all wheels. If you switch wheels routinely, take the time to get the distance from the disc to the face of the axle (dropout) matched. Depending on the hub and disc mount you can do this by shimming betwen the disc and hub shell, or respacing the axle (you may need to re-dish the front wheel if it's more than 1mm or so). Once your wheels are matched you won't be moving the caliper or adjusting the pad spacing when you change wheels.

Go back and repeat the caliper mounting with the fixed pad set all the way in, and touching the disc. Use something wrapped around the lever to lock the brake, tighten the mounting bolts, then release the brake, and you should be able to back the fixed pad off and be good to go.
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Old 06-23-12, 08:29 AM
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Originally Posted by tommyd49
...Maybe this is quite common when changing wheels? I don't really know because my experience with discs is limited. Does seem a bit much to me though.
Your instincts are correct, if it fit without rubbing before, it should function the same regardless of wheel. Something is amiss in the caliper installation most likely. It would help to know details of the brake kit you're using; any change besides the wheel swap?

Remove the spacers, start from the beginning, again no spacers. Visit YouTube for instructions, BB5 for example https://www.youtube.com/watch?v=H5zjUaTAQmY, or search the web for how-tos, this one has an interesting approach https://bluecollarmtb.com/2006/10/30/...l-disc-brakes/.

It is possible the fork leg drop-out or mounting bracket could be defective, but this would be rare. Be sure the axle/skewer is as tight as possible, a loose axle/skewer can highlight a manufacturing tweak and a difference in tightening torque between the two wheels could be the missing link. If this has IS mounting brackets, remove paint from fork tab contact points, and reinstall. Only loosen the caliper mounting bolts enough for lateral movement before adjusting the inner pad. Remove, inspect, and reinstall the pads. Check to see if one was damaged with prying, the spring is OK, no debris in the caliper, and the pads are installed correctly. Look closely at the installation to be sure it is a pad, and not the caliper body rubbing on the rotor.

Last edited by Werkin; 06-23-12 at 08:49 AM.
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Old 06-23-12, 01:41 PM
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Are these brakes avids? bb5 or 7? I use the bb7 and the inboard pad is static when you depress the brake lever. So its only the outboard pad that moves when the brake is applied. Therefore to get correct alignment...first before you loosen any caliper bolts loosen the outboard pad dial by 5 or 6 clicks, and then tighten the inboard pad (the one nearest the wheel side) by 5 or six clicks. Now loosen the caliper bolts(so that the whole caliper unit can be wiggled side to side by hand); Now apply the front brake lever, and keep the brake applied whilst you tighten the caliper bolts back up; release the depressed brake lever at this point.

Now unscrew both the inboard AND outboard pad ajuster screws by 4 or five clicks each side. Now it should be ok if they're avids.
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Old 06-23-12, 08:08 PM
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Make sure your wheel is mounted properly in the drop outs
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Old 06-26-12, 01:39 AM
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Thanks for the help all. But I've now realised it is not the fixed pad that's rubbing against the disc, rather the disc is rubbing against the actual caliper.

BTW, this is a Shimano 'groupless' caliper I am using. The rotor is in decent condition, it is out of true no more than 1mm.
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Old 07-04-12, 04:03 AM
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Thanks for the tips guys.

I seem to have it working now. I was able to fix it by just loosening the caliper and aligning it by eye. For some reason just pressing in the lever and tightening down didn't seem to do the trick.

Only problem now, though, is that when I apply the brake, when releasing it quickly, I get this 'ping' sound. I've figured out that it's caused by the metallic part of the outside pad (near where the pin goes through) hitting the caliper hard.

Is this common? Should it be something to worry about?

Thanks.
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