Does the wheel have to be perfectly centered between the break pads?
 

I just bought a Specialized Hardrock, and after taking off the front wheel with the QL level I'm finding it hard to reinstall without it being slightly off center. Even if it appears (as far as I can tell) to be centered, when I ride it there is still a very subtle scraping noise. Will riding it like this eventually cause damage? How can I fix this? Thanks

You're looking at things backward.

The wheel should be centered in the fork or frame, and the brakes adjusted to the wheel. Then as long as the brakes are roughly centered so neither side rubbing when open they're fine. Better centering allows for closer adjustment, but once the adjustment has the lever throw where you want it, more even centering isn't any better.

BTW- Get in the habit of closing the QR with the bike vertical on level ground. This ensures that the wheel is always in the same place, firmly against the tops of the dropouts. If that isn't centered in the fork or frame, see the local dealer and see if they can resolve it. (better bikes will or should be spot on, less expensive bikes aren't built as closely and might not be, but this can be corrected).

Hard to tell based on the info, but you could either 1) have a warped rim or 2) have the wheel sitting incorrectly. Both are fixable problems.

Brake, not break (unless it's broken) Sorry, pet hate of mine.

If the pads are consistently misaligned, I don't think it will be a problem with the trueing of the wheel, which would more likely manifest itself as a wobble than a dishing problem. It's possible that the wheel isn't seating correctly in both dropouts. Even a tiny bit of ingrained dirt causing a fraction of a mm difference will be amplified by the time it gets to the rim. Check the dropouts for dirt and damage, and install the bike onto the wheel, rather than the wheel onto the bike if that makes sense. Of course, it could just be that the brake pads need adjusting properly.

I had the same problem but it seemed to have been corrected by the LBS. I saw they just adjusted the very tiny screw. But I have another annoying problem: when I hit the rear brake, it makes loud noise from the friction between the rim and brake pads. It's so loud and attention-attracting that I now only use the front brake. :( I cleaned the rim well and can't find the reason and how to fix it.

^ Your rear brake pads are contaminated. You will need new ones.

to vol, try toeing in the pads and clean the pads and the rims with alcohol .

1. Wheels are to be properly dished.
2. Those wheels are to be centered between the stays when installed.
3. The brakes are to be themselves centered around the wheels. (Brakes have their own centering adjustments...)

=8-)

Thanks a lot (sorry I had forgotten this thread). I just cleaned the pads and the rims with alcohol. (Didn't remove the pads as I've never done it and don't know how and don't want to make an unsafe mistake. I just inserted a thin alcohol paper between brake pad and rim.) Apparently the pads are contiminated with a lot of tri-flow that got on the rims. Now it makes no squeaky sound any more. :thumb:

The brakes, and in particular their balancing depend on the wheel being centered in the wheel arch for good adjustment. The balance springs of brakes are only designed to handle a small range of adjustment. As others have suggested, make sure that the wheel is centered, and to do that hold it centered while the fastening is done, and make sure that the weight of the bike is applied; do not fit the wheel when it is off the ground. Then adjust the balance screws and possibly the brake gaps to get the blocks free of the rims and to the right distance.

Other things to note are whether or not any of the ferrules (the metal bits on the cables) have popped out of position, that the springs on the brake arms have not come out, and that the wheel has not developed a wobble. Eventually,a wheel takes on a slight wobble that leads to the need for wider brake gaps or the adjustment of the wheel to correct the problem.