Disc brake issue - possible bent rotor?

J

Jubas

Likes Dirt
I've come across a problem with my rear disc brake making a rubbing sound, and i'm having some difficulty diagnosing the problem, so i thought i'd put out a question to the experts here on Farkin!

Basically, i went up over a curb on Wednesday last week, and without thinking didn't hop my backwheel up - i was carrying a bit of speed which led to my rear tube rupturing (silly i know - I've learned my lesson!).

A problem arose when i replaced the tube and reaffixed the rear wheel. Now, i can hear a rubbing which i believe is the disc on the pads. When spinning the wheel slowly (bike upside down for a quick service), i can hear a slight rub on one section of the disc/wheel, followed by a more noticeable one a short part of the spin later. What's got me stumped however is that i've visually inspected the disc, and can't notice any bend at all - i've got good eyesite (;)) so it's really starting to bug me. The rear brake still stops fine, and the disc appears fairly clean (although i'm going to grab some metho this afternoon for a proper clean). I've also run a thin cloth through the caliper/pads in case there's anything major clogged up there, to no avail.

I'm 85% sure that the noise is coming from the disc/caliper/pads, but i'm not sure whether there's any chance it could be the hub? I'm riding a Giant Alias 09, with stock Shimano M486 hydraulics and rotors.

Has anyone got any idea how i might find out the actual issue, or steps that i could take to eliminate others? It doesn't seem to affect braking at all, so it's not a critical issue, but the sound is annoying me!!

Thanks in advance..
 
Hi

I'm thinking that the wheel may not be in the bike correctly, also did you pull the lever while the wheel was out of the frame. This sometimes causes the pads to move in and stay in.

Hope this helps
 
What he said. Rear wheel's probably not sitting in the dropouts properly.
Undo quick-release. Re-tighten. Spin wheel.
Still rubbing?
Adjust brake calliper.
Search or check your manual if you need further instructions.
 
I've reseated the wheel a couple of times to try and see if that's the problem - i'll track down a way to readjust the brake calipers, although it seems strange that it would only be affected by part of the rotor..
 
usually disks warp due to excessive heat or faulty manufacturing. i can't imagine a mountain bike disk ever gets that hot cause there's not that much weight involved. if the wheel wasn't straight in the frame though it'd be rubbing all the time, not just in one or two places. i assume they're single piston calipers so i'd try backing off the non moving pad a little to see if that fixes the problem.
 
I've got that same brake set up and have experienced the exact same thing you've described. They work a treat but there's sometimes this rubbing noise. Annoying!!

Usually taking the wheel out and resitting seems to fix it although for the life of me I don't see why it should as I'm sure it's sitting right in the first place. What I've had to do twice is loosen the bolts that attach the brake to the mount, get someone to pull the brake on, and then tighten the bolts again. This puts them in the right spot with no more rubbing. Follow this --> http://www.utahmountainbiking.com/fix/ and look under Aligning the caliper.

Should be all fixed. Must be something about the 486 brakes.

Cheers,

Coaster
 
hey mate.
its common to have a small bend in discs if they take a thrashing but often you can't see it with just your eyes. try taking the disc off and placing it on a flat (very clean) piece of wood or something and push down around the edges (like push on one side and move around). if it doesn't move then flip it and try that way, if it does move then its bent.
To straighten them i find a fairly big shifter does the trick. just have the disc on the wheel and push beyond where you want it slightly. then there you go. a straight disc.:eek:
 
If you want a quick flat surface to check the disc then try a sheet of glass, wood can be all over the place and has give in it as well.

I found that i could true my disc (Avid) by hand without any tools, I think if you try to use a shifter you may do more harm than good.

Make sure your dropouts are clean as well as it doesn't take much to tilt the wheel and disk.
 
Probably what Coaster said, i'm religeous about readjusting my calipers constantly ever since i first did it and they worked a dream afterwards. my mate had an 08 alias and i did the same to his bike to stop some brake noises, and my mates '06 demo 8 had the same problem, that fixed them up as well.

keeping the clean and aligned does wonders to limiting brake noise.
 
Bent Disc continued...

Hey guys,

I'm a newbie here and searched for this problem.

I've got a Haro Extreme X6, which does the job (for now), I used to have a giant ATX-One DH, which was the good stuff back in the day (but ran formula discs that were shockingly bad - but atleast strong).

I have ridden Outlook last friday night, bent the rear Hayes 7" disc.....
I rode Wimps on Saturday and bent the front Hayes 7" disc (the stack was ok tho, but didn't even scratch the friggin' forks!).

It seems that the Hayes stainless steel disc is quite malleable (soft) and I'm wondering whether an aftermarket rotor or different brand might be a better choice?

I've never bent a disc before in my life until last damn week!!!

Cheers, any advice on similar events or upgrades you performed would be most appreciated!
 
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