Newbie issue: how to align disc brakes when attaching wheels....
So I recently got a new bike with Magura disc brakes. Hydraulic disc brakes are such an upgrade/improvement over my old V-brakes its not even funny! BUT, I am wondering if there is a trick to setting the wheel back on so that there isn't any noise between the pads when the wheel spins. Case in point: I had to change my rear tire and when putting the wheel back on I can hear a slight contact point at one part when spinning the wheel. Is this something to worry about? Will disc brakes sort of self calibrate to the wheel when used? Is there a trick to getting the brakes/wheels perfectly aligned? Thanks for any help, any tips for a newbie disc brake user would be most appreciated.
how to align rotors with calipers
The best way to align the rotors with the calipers is to keep the mounting bolts of the caliper slightly loose, just enough so that the caliper moves, then squeeze and hold the lever. While holding the lever tighten the bolts. You may have to try this a few times.
If that doesn't work and the rotor is only hitting in one spot, you probably have a slightly bent rotor. This isn't really a bad thing just annoying. You can try to bend the rotor back to true but its really hard to get it right.
Originally Posted by mojobeer
This plus spinning the wheel and then applying the brake.
You better take care of me, Lord. If you don't you're gonna have me on your hands.
The best way?
Re-aligning a (non-IS) caliper after refitting a wheel can be done using the loosen bolts/pull lever/tighten bolts method (LPT), but only if the caliper was correctly aligned beforehand. I'll explain why:
If the caliper is aligned by the LPT method, it assumes that both (or all) pistons are moving equally; however, it's not unusual for a new brake to have one piston moving at a different speed to the other until the seals soften/settle a little. The result of the LPT method when one piston moves less than another, or even not at all, is that the rotor end up running uncentered through the caliper, usually closer to one pad than the other, or even with only one piston doing the braking - if the other hasn't actually extended at all. If equal piston extension is not achieved in the first place, brake squeal; lack of power; and reduced modulation will be the result. The brake pads will also tend to wear at a different rate, as one will be touching the rotor before the other when the brake is applied; thus future caliper alignment will always be affected.
For optimum brake performance, both pads should touch the rotor at the same time, and both/all pistons shoudl be moving at an equal speed.
Initial caliper set-up should, in my opinion, always be done visually, or even with the aid of feeler gauges (thin metal strips of a specific thickness). With the pads removed and both/all pistons pushed back flush with the caliper, the caliper should be positioned over the rotor so that the rotor runs perfectly through the center of the slot. This will place the rotor an equal distance from the piston(s) on each side of the caliper. A few minutes taken to ensure perfect centering will greatly reduce the chances of brake issues in the future. Thinking ahead: it's also a good idea to remove the chain when setting a rear caliper as the sound of the freehub can cover the sound of the rotor catching/rubbing the pads.
With the caliper and rotor aligned properly, it's time to align the pistons/pads. The pads can be refitted and the lever pulled to bring the pistons out into operation. Two or three gentle pulls should be enough to get the pads pushed firmly against the rotor. Once you feel that the pads have fully extended, take a look at the caliper as you pull the lever. Do both pads look to be moving the same amount? Does the rotor appear to flex when the pads grab it?
Lift the wheel and spin, listening for catching/rubbing. Remember that the caliper has been perfectly aligned over the rotor, so any noise is a result of one piston moving at a different rate to the other, thus bringing one pad into contact first. The nature of the piston seals means that - if the difference is great enough - one pad will be left further out than the other when the pistons retract.
To ensure equal pad movement, the rotor can be pushed/pulled (using both hands; clean 'em first, though!) against the side which is moving quickest, while somebody else pulls the lever for you to extend the slow-moving piston until it matches the other. It's trial and error, to a extent, and might take a few minutes to balance them out.
The result of all this should be a brake which has the rotor running bang-on through the center of two pads that move at the same speed and contact the rotor at the same time as on another. An untrue rotor (zing....zing...zing...) can be gently bent using a rotor truing tool (Park, Morning Star) or an adjustable wrench.
Now that the pistons move equally, should the wheel/rotor be refitted in a slightly different position, the LPT method can be used. However, I prefer to just lift the wheel, spin it and move the caliper manually - visually aligning it - so that the pads clear the rotor. The open-top design of Hope brakes allows me a good enough view of the rotor/pads to do this, but for other brakes the LPT method may have to do. Once you understand how the caliper needs to be aligned for best performance, you can find the best way to re-align your specific brake.
What use is a philosopher who doesn't hurt anybody's feelings? -
it sounds like you're talking about the difference in clamping force at the qr before and after removing the wheel, causing the caliper/rotor alignment to be a bit off.
Originally Posted by Withywindle
i've had good luck with the locking qr nuts from 1upusa.