Bicycle Mechanics - Bottom bracket / crank clunking sound?

For the past two weeks or so, I've been experiencing a "clunk" sound from the crank / BB area of my '07 Giant Bowery (fixed gear). The cranks and BB are completely stock, and I've only had the bike for a few months, so common sense tells me that the problem can't be attributed to wear and/or improper installation of aftermarket parts.

The noise occurs each time I put moderate-heavy downward force on the cranks (both left and right) while in a sprinting-type riding mode. The clunk is also detectable through my feet. It almost feels as if something in the BB is slipping and catching itself after a small ammount of crank revolution.

Any help is much appreciated. Feel free to ask questions. Thanks!

sounds like you may have a loose cog and lock ring. make sure they're cranked down tight.

You've probably just got a loose BB cup. Worst case, your BB is a dud and you should bring it in to warranty it. Also, make sure your crankarms are tight...if not, you'll be seriously lucky if it/they are still usable.

I agree with the sloth. I would make sure that the cog and lock ring are tight before you strip the threading of the hub. I would also make sure that the crank bolts are good and tight.

At the risk of sounding like a complete ******...

How do I go about checking / tightening the lock ring?

Before posting the original question, I checked the bolts that attach the crankarms to the cog and the bolts that connect the crankarms to the BB. All were tight.

Again, Thanks!

you'll want to tighten the cog on the hub as much as possible before tightening the lock ring -- if you have a chain whip you can use that, or go find a steep hill and jam on the pedals as hard as you can (don't put any back pressure on the pedals otherwise you'll unscrew the cog, which is probably what has been happening). then use a lock ring tool to tighten the lock ring down onto the cog, remember that it's reverse threaded. your lbs might help you out for free if you don't have the tools...

Not sire what your BB / crank configuration is, but check to make sure your cranks are torqued on.

If you're going to tighten the cog, you might as well learn the rotafix method:

http://en.wikipedia.org/wiki/Rotafix

Get a good lockring tool; I like the Hozan one (~$20, but you'll be happy knowing your lockring is tight as hell).

You may have some moisture in the BB. As strang as this sounds, I have found this to be a problem before. You will need to remover the BB and then dry the inside of hte frame area. When you reinstall the BB put teflon pipe on the threads.