Bicycle Mechanics - Avid disc brake rubbing on Jamis Dakar

OK, here's another problem.

I own a 1996 (or so) Jamis Dakar Team frame. I like the frame a lot, but it was not disc compatible. So I bought an adapter made by a Taiwanese company called A2Z (you can see these on it's website, and frequently on ebay). I have the Avid mechanical disc brakes attached to the adapter. The hub is a Magura Louise Pro. So the disc seems to make a light rubbing noise--"cling, cling, cling". It does this entirely randomly. The bike will go for miles of silent operation, then this noise will start. Then stop. Then start again. The length of time that it continues varies from a few rings, to a very long period. It does it sometimes after the suspension activates over a big bump; sometimes not. It activates sometimes after a tight corner, sometimes not. The ringing noise seems to be more active when there is a little less pressure in the rear shock. It's not a bent rotor, or it would make the noise most or all of the time. I've tried positioning the rotor further away from the frame by eliminating a spacer between the rotor and the hub shell (the rotor was very close to the left chainstay). That did not eliminate the problem. The ringing noise only happens to when I'm riding the bike, but never when the rear wheel is spinning with no weight on it in the repair stand.

I realize that using an adapter on this frame is not the ideal solution, but the ringing noise is mystifying, because I cannot detect it's cause, and it is so random. The only other solution I can see is that the bushings in the rear suspension are loose, but I can't detect any play in the rear triangle either.

Anybody want to take a guess?

Sounds like that the rotor is hitting on the frame......but only when there is a load applied to the frame. When you pedal or turn, the frame flexes and its very hard to duplicate on a work bench which is probably why you only hear it when youre riding. The only noise like you are describing..a ting ting or cling cling...is the rotor making contact with something. you could try a thin washer where the hub fits into the dropout to widen hub spacing, just enough to give you like one or two mm more space, although its probably unsafe to do.

I've had similar problems with a Dakar, and I'm not the only one to have noticed this. For some reason, it is very easy for the hub to shift around in the dropouts on these bikes, but in my and others case, it was due to drivetrain torque pulling the right side of the hub forward, thus misaligning the wheel. I was able to cure it by first removing all the paint form the dropouts where the hub lock nut faces and the skewer make contact, then roughing up the surfaces, then finally started using a bolt-on skewer; haven't had the problem since.

Something to have a look at anyway. Maybe next time you notice it, stop and check if the wheel is bumped to one side or the other (look at the spacings at the front of the chainstay, right behind the pivot). Hard braking can also be the cause of a wheel shifting around in the dropout.

Make sure your frame pivots don't have any play. It doesn't take much, since Dakars have several.

sounds like just a bit of rotor rub; no big deal. can you tell formlooking at your inside chainstay, is it actually hitting that?

partly frame/pivot flex, partly sounds like you need a bit more rear brake cable/housing, esp. if it's doing it with some cornering. also may be just a very *slight* (so little it doesn't look warped, but just enough to make noise while riding) amount of rotor warp; that'll make that same sound.

do this: go for a bit longer housing/cable (say 2" longer) on the rear. then totally go through the setup directions for the avids again. you can find the info for that at www.sram.com if you no longer have the directions. may help a bit.

Have you checked spoke tension? If the spoke pattern on the brake side is reversed, the spokes will bow out and touch the caliper on occasion

That would be a rather sick wheel.

Yes, I don't see how the spokes could be the problem since they aren't very close to the caliper or the disc. If they had that much flex to them, the wheel wouldn't be rideable.

The disc is hitting something--either the frame, or the inside of caliper. It could be the housing is too tight (which it is), the wheel moving in the dropouts (I'll tighten the quick release more), or flex at the pivots, which I don't what to do about that. But thanks for your help everybody.