Hi all,

My wife recently bought a new bike, a Trek 29er mountain bike with mechanical disc brakes. Knowing nothing of disc brakes, I wondered how removing the wheel would come into play when changing a flat. Asked the guy at the LBS, who assured me there is really nothing to it. The wheels pop right off.

Today my wife had the inevitable flat. Rear tire. I fixed it. But after popping the wheel back on, it doesn't seem quite right; I think I hear some sort of rubbing. It is very faint, but it's there. I can't see what it is. The wheel itself seems properly seated. With no rim brakes there is nothing really obvious that would rub, or at least, not within my limited experience.

Could it be something in the brake mechanism? These things are hard to diagnose by description, but can anyone tell me whether there are some obvious things to look for?

Thanks in advance.

There's no difference, except for the need to guide the disc between the disc pads on reinstallation. The brake stays on the bike, and the disc comes off with the wheel.

Note that disc clearance is narrow, so you may get some rubbing if you don't mount the wheel in the original position. I always finalize any wheel install by opening the QR with the bike vertical on the ground, letting gravity settle the bike's dropouts fully onto the axle. Gravity is a very reliable force, and I've come to trust it more than trying to center by feel or eye.

BTW- on hydraulic brakes it's important not to squeeze the brake lever with the wheel off because this moves the pads too far out. The rule doesn't apply to mechanical brakes, but you might as well develop good habits now.

You may have to loosen the caliper mounts, spin the wheel up, grab the brake lever to squeeze the caliper, hold it there, and re-tighten the caliper in order to re-center the caliper over the disc. This after centering the wheel in the dropouts as per FB's advice above.

Make sure the axle is well seated in the dropout to maintain the exact position.
Check how much brake lever motion you have, they may be set a bit tight with little clearance.
Most cable discs move on one side only and bend the rotor to contact the fixed pad. You adjust the moving pad with the cable tension and the fixed pad with a central allen bolt adjuster. Check the specs for pad spacing on your model. Some models require asymetric spacing, more on the moving side.

Be careful when the wheel is out of the drop-outs. The disc is easily bent.

Have her show you how she re installs the wheel, may not have done it as described above.

When I remove my wheel, sometimes I knock the cable out of its seat making the brakes act weird. I also have mechanical disc. Make sure the cable housing is seated properly in the caliper

Is the rubbing causing any drag? You may have gotten a little debris on the rotor or the pads. If that it the case, simply riding it or blowing some compressed air should get it back to normal.