melon

im finding that i need to recentre my 105 rear brake after almost every ride. its seems silly, but ill set it up to be perfect, ride for a while, only to find that it has become well off centre again. ill then recentre it again, but the same thing continues to happen every time i ride. im using the adjustment screw on the top of the brake.

the main bolt holding the brake to the rear triangle is certainly tight enough, and its not moving at that point.

can any explain why this is happening and how to fix it?

ivan_yulaev

Your rear wheel is shifting in the dropouts. Try loosening the QR and then tightening again - the adjustment should change. If you're using an external cam QR you should either get an internal cam one or tighten that sucker down TIGHT.

At least this is what happened to me...

CACycling

Also, to get the rim centered between the pads requires physically moving the caliper not tightening or loosening a screw. Once you get the wheel centered in the dropouts, rotate the caliper until the space between pads and rim are equal on each side.

Al1943

The wheel should not be centered to the brakes. The wheel rim should be centered as to the dropout lock nuts, chainstays, and seatstays. The brakes should then be centered to the rim.
If the brake calipers won't stay centered they need to be adjusted. Check at https://www.parktool.com and https://www.sheldonbrown.com for adjustment instructions.

Al

peripatetic

It's possible your rear brake cable is a little on the short side--any time you turn the handlebars too far, it'll tug on the caliper arms.

Centering involves tightening the brake against the frame tightly enough that it doesn't move, but it depends on the brake type as to which bolt you're supposed to tighten. Cone wrenches or the OBW wrenches from Parktools help with this. It's kind of a pain, though--you're not imagining.

melon

the frame has vertical dropouts, i dont see how the wheel can be off-centre. how do i centre the wheel? the rim is perfectly true.

i dont think the caliper needs rotating, the range is fine and can be set up perfectly with the fine adjustment screw. the problem is that is doesnt stay there. maybe the wheel is shifting, but i dont see how it can when its sits in vertical dropouts.

Al1943

The rim could be off-center due to incorrect dish, incorrect spacer washers, or incorrect hub adjustment. But these factors usually change from one ride to the next. Instead I suspect that the brake caliper is rotating slightly on the fixing bolt, a common problem. As peripatetic said, the adjustment can vary from one type of brake to another.

hernick

Had the same problem with my 105 rear brake... I took an OBW-3 offset brake wrench to the centering nut (right against the frame on the other side of the main bolt). Loosen the main bolt a bit, center with OBW-3 and tighten the main bolt.

If you don't have an OBW-3 you can try to get in with a flat 14mm wrench but you might not be able to without removing the wheel, which would obviously not help with the centering.

melon

no the wheel is perfect. i reckon it is the centreing nut, which i dont have the tools for. luckily i have a service coming up.

Cynikal 

Did you install the grip washer thingy ( I forget the actual name)? That should hold the caliper center. Also check if you cable is either too short or too long.

jccaclimber

If anybody does remember the name of that little spiked star washer, please post it here, I've been trying to remember the name for a while now.

Panthers007

Actually if you have cone-wrenches, you have the right tool for the job.

hernick

Yeah, a 14mm cone wrench would be perfect as long as it can get to the centering nut... I've seen bikes where that would be impossible without removing the wheel

DannoXYZ 

If you are not using a cone-wrench to hold the caliper centered as you tighten the fixing-nut on the rear, there's pretty much no chance you can tighten it without spinning the caliper.

peripatetic

Sometimes on older bikes, the reflectors also get in the way.

Is the star thingy you guys are asking about just the star lock washer?

OLDYELLR

The way DannoXYZ shows is the proper way to centre a sidepull brake (although the rubber gloves are a bit over the top for me). I always used to do it that way and often wondered why it often didn't stay centred. I finally started doing it the quick-and-dirty-lazy way. Unhook the spring on the side that's touching the rim, pull it outward, then hook it back again. If you've pulled too far and now the other side touches, do the same on the other side, only less.