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thumping rim brakes... haaalp

lph2012-07-09 19:18:43 +0000 #1
Ok so my commuter bike is an mtb with rim brakes. The rear brakes are crappy (don't spring back properly and need replacing), but the front brake isn't a joy to use either. Loud thumping and shaking ensues.

So I assume there's a wobble in the wheel that I can true out. But I have the wheel out now in the stand, and it is pretty true. There's one spot that I think is causing the thumping, not a big wobble, but an abrupt one. The adjustable measuring thingys on the wheel stand "catch" very clearly on this point. But it's not a skew to one side, the rim flares out to both sides! So how do I fix that? There's no way of tightening spokes to make the whole rim narrower at one point, is there?

I'm thinking of just grabbing the entire rim with a large pair of pliers and squeezing it together, but there seems to be a pretty big chance of just creating worse bumps. Gentle hammering?
Susan2012-07-09 19:28:30 +0000 #2
Maybe sanding paper would work? Maybe you could attach it somehow to the brakes or the measuring thingy and just turn the wheel until the bump is at least less noticeable?
ridebikeme2012-07-09 19:56:07 +0000 #3
Unfortunately that is what is known as a "flatspot'. If you're careful, you could take pliers or something to try and pull it back, but also remember that there's a potential to have the entire rim move slightly. And, you're likely to have a concave spot where you just moved the rim.

At any rate, good luck with your project!
lph2012-07-09 19:54:28 +0000 #4

The wheel is otherwise good, so I want to keep it. I took a humongous pair of pliers and squeezed the rim together as hard as I dared along the inch or two that was flared. It seemed to be a little less abrupt afterwards. A gradual flare doesn't seem to be a problem for braking, just this sudden bump. Sanded it a little afterwards to remove the burrs from the pliers, and adjusted the brake pads to catch a little lower down on the rim, since I assume the flat spot (nice term!) is most noticeable near the edge, and least pronounced towards the centre of the wheel.

I won't get to test it until I brake hard at speed, but I think I've done what I could

And I'm going to the lbs to pick up some new brakes for the rear tomorrow. I've put it off for ages, but they're not getting any better and are not reparable.



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