Bicycle Mechanics - Brake clunk once per revolution

I was out riding the other day on my hybrid (Trek 7100FX) and started coming down a steep hill. I applied my front and rear brakes progressivly as I was speeding up. This hill decends rapidly into a decreasing radius s-turn. I made it about half way down when I started hearing a loud clunking noise coming from my rear tire. It happens once per revolution no matter how light or hard I squeeze the rear brake. It dissappears when the brake is let go. There is no side to side movement with the wheel when it spins or is stopped. The wheel spins just as freely as it always has. The v-brake looks perfectly fine. It's not out of alignment and there is no noticeable wear on the rim.

What can be causing this? I want to fix it but I don't know what is wrong.
Thanks

Rough rim seam. The spot where the rim was joined by the manufacturer (Bontranger?). Check the spot in the rim directly oppisite of the valve stem and feel for a raised bump the height of the braking surface.

OR, you could have a dent in the braking surface of the rim. To find this, just feel around the rim until you find a depression in the braking surface.

This is assuming the wheel is true when you stated the wheel has no side to side movemnt.

Okay, what do you do about it?

I rode my wife's bike today and it does it. Cheap Mavic rims without machined braking surfaces.

If its a seam, just file it using a mill bastard file. If the rim is bulging out, take off the tire and using an adjustable wrench, grab the bulge with the wrench and bend it back as well as you can. Be sure to check both side of the rim, because the bulge is usually on both sides. If there is still a little blip left after you bend it back, file it down.

I agree with the above but would add this:

If the rim is bent, it could be because of a serious pothole or because it's worn out. If the rim is bent towards the outside, check the thickness of the rim sides. If they seem to cave in rather than being straight, you might have rims that are worn off because you used them a lot. That's only a problem on a well-used bike that could have, let's say 10 - 20 000 km depending on the amount of braking you do, weather and terrain conditions.

Another possibility is either grease or sticky stuff at one point which would make the brakes grab less or more at that point.


Unless you feel a serious bump or notch in the rim, I would leave it there. I think it is better to under-correct the problem and try it then to over-correct it. You can't "un-file" a rim, and aluminium doesn't like to be bent over and over.

I think I'd have to disagree with putting grease on a rim for any reason. Clearly the action of the brake pads will smear the grease over the rest of the braking surface. And then you're only left with the flintstone method of braking.