Rim Problem -- Uneven Braking -- Replace rim??

[Tommasini53]

I've got a moderately used Bontrager MTB rim that has an uneven brake "grab". It is round, no dents or dings. When the brake is applied with steady preasure it is a very spastic and jerky braking action or grab.

I've cleaned the rim, replaced brake pads, and adjusted the brakes with no success.

So I assume there is an imperfection in the braking surface.

Is there a way to buff the braking surface to smooth out the braking??? Or is the only fix a new rim??? :beer:

[cnighbor1]

I noticed a slight bludge in my MTB rim No problem I just use clamps and push it back to were it should be.
worked fine for a ride or two
Seems all that braking had worn rim thickness to a point pressure from air in tires was pushing rim out. rim was too thin after a lot of use

[mpetry]

I won't presume to second guess the opinions of my good buddy Charles but it sounds like there's something wrong with that rim.

Sometimes the joint (180 degrees from the valve hole) can creep or shift and that will cause the brake pad to grab. This only happens in rims that are "pinned". Welded rims should not exhibit that problem.

Can you take the wheel off, remove the tire, and put it in a truing stand? Spin it real carefully and eyeball the braking surface. Are there any imperfections, or things you see that could cause the pad to grab? Shine a light on it and look at the reflection as you spin the wheel. If there is a bulge or dent, sometimes you can use a C clamp very carefully and squeeze it so that it evens out. It's dicey.

Take some steel wool and go around the rim, abrade the braking surface to make sure there's no grease, oil or anything else that might interfere with smooth braking.

As Charles says, it could be that wear from abrasion has caused the rim sidewall to become thin and bulge out. I've seen a whole 6" section of rim sidewall tear out on rims that are very worn. Usually this happens from hard braking when the rim sidewall or pad has dirt or grit contamination.

Give the brake pads a good working over with sand paper, and make sure there's no metal splinters or other crud embedded in the pads.

If the problem persists, try the rim on a different bike. Does the problem go away? If so, it's probably not the wheel.

From what you've described, I'm betting you have some irregularity in the rim, either at the joint, or from a dent, and you'll have to change it.

Mark Petry
Bainbridge Island, WA