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Disk Brake Rotor Repair

brakpad5.jpg (9041 bytes) In general, if there's something noticeably wrong with the rotor, you should replace it. That means if it's seriously worn, gouged, or bent, you're better off with a new one. You can, however, straighten a slightly-warped rotor.

To inspect the rotor, sight straight along the pads of the disk brake mechanism. Slowly spin the wheel while you watch the rotor.

Sighting along the rotor while turning the wheel.

Straightening a warped rotor:
The right way to true a rotor is with special tools. If you're an upper-level rider who demands precise performance from your bike, you can buy a truing gauge and drumstix. This set is from MTP (Morningstar Tooling Products). Click here for the precision rotor truing page.

Straightening a rotor to within a paper-width of perfect.

Now here's the on-the-cheap way to do it. Put the bike in a workstand and spin the wheel. Watch for side-to-side motion of the rotor compared to the brake pads.

Identify the warped area. Find the spot where the rotor most closely approaches one pad and mark it.

Marking where the rotor is most "untrue."

Rotate the wheel so the "too close" spot comes away from the brake cylinder and pads.

Clamp the disc in an adjustable open-end wrench. Tweak gently away from the direction of the warp, then rotate the wheel and watch for wobble. Continue gentle bending until the warp is minimized.

Gently bending the rotor outward to correct warp.

Fixing "ringing rotor" noise:
If you're getting a loud "honk" tone sometimes as you apply the brakes lightly, some mechanics recommend that you smooth the edges of the holes on your rotor. (This also keeps the sharp edges from "cheese-grating" your brake pads, so the pads last longer.) Beveling the rotor-hole edges reduces vibration of the rotor, preventing the ugly musical tone that can occur when you apply the brakes lightly.
You can use a rose-head countersink bit with a drill. But I think a nice sharp 1/2" drill bit does the job nicely by hand. A couple of twists for each hole and the edges are smooth, without the risk that you'll drill right through the rotor.
When to replace the rotor:
Check the rotor for concavity. This means that the gripping surface of the rotor has been eroded a significant amount away from the former surface. A concave rotor loses braking power, and becomes hard to get out of (and back into) the calipers when you're removing the wheel.
Place a ruler edge-on over the rotor. If you see more than a whisker of daylight, it's time to replace the rotor. Usually by this time, you can actually feel the depression when you run your fingers over the surface.

Also replace a rotor that has any stress-marks, dings that can't be filed smooth, bends that can't be restored completely, or a bend that shows any sign of a "fold-line."

Removing a damaged rotor:
To remove a damaged rotor, remove the wheel from the bike. Using a hex-wrench, remove the retaining bolts from the rotor.

Removing the rotor.

With the rotor off the wheel, you may be able to straighten the rotor further. You may also be able to grind out a ding.

Ideally, you should replace the rotor with a new one. Note the direction of the support struts on the rotor -- they point INTO the brake mechanism in the direction the wheel will rotate. Be sure you put the rotor back correctly, and use a bolt-locking compound ("Loctite") on the bolts.

Be sure the struts on the disc point into the brake as the wheel rotates forward.

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