Commutertoddo

I do general repairs on bikes but I'm not familiar with disc brakes. A friend of mine removed his front wheel ( not sure why) however after replacing it his front disc brake squeals while riding while the brake is not applied. It quiets when the brake is applied. Any ideas of what I should look for?

Old Hammer Boy

It may need an adjustment, usually a pretty simple procedure. However, before adjusting, make sure the wheel is completely seated in the fork dropouts. I like to loosen the quick connect lever, stand over the bike, push on the center part of the handlebars with one hand and engage the locking lever with the other. Many newer bikes have lawyer tabs on the fork, so make sure you're clear of these.

If that doesn't do it, it sounds like an adjustment is in order. Search the appropriate web site to obtain directions. Various disc brakes have different adjustment requirements.

fietsbob

the adjustments instructions should be in a PDF
available from the manufacturer's website.
If you know, or can find out what brand the brake is..

igknighted

Sounds like a hydraulic disk brake... these brakes are designed to self adjust, so if the wheel was out for some time (or if the lever was squeezed without the wheel/rotor in place), the pads will adjust inwards. Pumping the brakes a few times may get them to push back out, but you may need to go through a re-centering routine (use the forum search, there are many threads on this... essentially you need to squeeze something larger than the rotor into the caliper to center the pads farther out, so they then re-center on the rotor properly when the re-adjust).

Before you drive yourself too crazy, make sure the wheel is perfectly back in place in the dropout and aligned properly. If this is off even a little bit it could cause the noise you describe.

Commutertoddo

Thanks everyone. I'll check into these things and let you know how it goes.

fietsbob

Ah , yes. they include a 'keeper' with Hydraulic disc brakes for a reason..

On my car, I pried the slave pistons apart, when I changed pads,
that may not be advisable on bicycle calipers ..

contango 

I had something very similar, I'd taken both wheels off to put the bike in the car to drive to a friend's place and when I reassembled it the back brake rubbed. In the end what I did was reposition the brake block very slightly. The way I did it (which was effective, if a little crude, and since I didn't have the user guide with me) was to slightly loosen both the allen bolts, apply the brakes hard, tighten the allen bolts, and release the brakes. The only tool needed was a 5mm allen key and it made a huge difference. Before the tweak if I lifted the back wheel and gave the pedals a fast quarter-turn push the wheel span maybe half a dozen times before stopping, after the tweak it kept spinning until I put the brake on.

As Old Hammer Boy said, make sure the wheel is properly seated first, otherwise you'll only have to readjust it when the wheel is properly seated.