Bicycle Mechanics - Brake grab--is my rim toast?
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I've got some brake grab on my front rim, right around where the joint is. Searching through other threads, I found some ideas on how to correct it. Here are some things I've tried (not necessarily in this order):
Cleaning the rim with rubbing alcohol.
Cleaning the brake pads with rubbing alcohol.
Adjusting the toe of the brakes.
Sanding the rim smooth.
Roughing up the rim with sandpaper.
Tightening the headset.
Replacing brake pads.
Truing the wheel.
The toe does not appear to be the problem. In fact, the grab sill occurs when I hold the brake and pull the bike in reverse. Tightening the headset helped some of the vibration, but I can still see the bottom of the fork vibrate and feel a sudden additional braking force in a small area when I pull on the front brake. Sanding didn't help either--the joint is extremely smooth.
This was a minor problem when I originally bought the bike used, and it has grown worse over time. It's becoming a major nuisance when I need to stop suddenly, and has even started to lift the back tire up suddenly and unexpectedly when it hits this small portion of the rim. It's almost as if the aluminum surface is different there--possibly due to heating when they join the rim?
Anyway, are there any more ideas about what to try, or should I just give up and get a new rim/wheel already?
09-16-10, 10:11 AM
Why don't you borrow a front wheel from somebody and see what happens?
My bet is that, if it's as bad as you say, your rim is warped beyond redemption.
09-16-10, 11:44 AM
Most of what you did is either steps towards curing a shudder or squeal or is intended to perk up the friction to achieve a lighter squeeze for the same stopping force at the lever. If your rim has a kink in it then depending on the kink truing the wheel could help. But if the kink is a very sharp and short bend or if it actually is the rim's joint where the kink or mismatch is then you will get a once per rotation bump from the brakes. The key is to examine the braking surface with your fingers and eyes to feel and look for a short and sudden wiggle or a step in the braking surface. If you have such a thing if it's a step then you can try to file the step dow a little. If it's a wiggle you can try to file the crests off to make it less noticable. If it's a "fat" or "pinched" spot in the rim where the rim sidewalls suddenly flare out or in (usually at the joint or damage from hitting a curb really hard) then file the wide spots down level or live with the pinched low spot or try to wedge it out a little from between the sidewall in the bead channel.
Or just live with it for now. Eventually the pads will wear down into the rim and the wiggle or step will go away. But that'll be a lot of street riding or a few muddy trail ride days to do that.
I don't have another 700c wheel to try on it (my other bikes are 26" and 650c), so that's not really an option. When I looked at the sides, they seemed similar to the rest of the rim--and yes, they were slightly out of true, but it's about as good as I could get it. Probably no more than a millimeter off. I don't see any major kinking, or divots along the sidewall, and aside from the excess grabbing and the fact that the joint is there, there's nothing remarkable about that area of the rim. It looks just like the rest of the rim, and feels the same to the touch, only the brakes seem to grab it like it's sandpaper.
I don't think living with it will help, because it has only gotten worse with use. I'll try a file, since that is more aggressive than sandpaper, and I can't think of much else to do at this point. To be somewhat scientific, I'll also test another area with the same method and see if it gets worse or better.
If you sanded the joint smooth there may not be any bump left. But the sanding would leave the surface rougher than the rest of the rim and that would certainly grab more than the smooth areas. If you can feel any bump or lip, I'd try smoothing it out with steel wool to try to get the suface roughness to match the rest of the rim better.
But if that joint is changing/moving, I'd get a new rim.
I tried filing it down, which didn't help--it made it worse. Then I ran some fine sandpaper (800-1200 grit) to get it really smooth, and that didn't help either. At this point, it would require more effort to try to fix than to just buy or build a new front wheel. So, I've ordered new stuff, and I'll just toss out the rim. Thanks for the suggestions though :thumb:.
09-16-10, 10:42 PM
Keep the hub.
Of course...it's a 36 spoke though, and my new wheel is going to be 32. Still overbuilt for me--I'm a light rider, and the new rim is a deep V. Who knows, maybe I'll build up a another 36 in the future so I have a spare wheel lying around--if I have room for it!
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