How can I fix my fixed gear bike whose rear cog rotates with the chain when I pedal?

August 23, 2009 3:58 AM

How can I fix my fixed gear bike whose rear cog rotates with the chain when I pedal?

Recently my fixed gear bike has developed a problem where the rear cog spins freely. So pedalling gets me nowhere :( If possible I'd rather try and fix it myself as bike repairs here in London seem to cost an arm and a leg.

Is it simply a case of pedaling backwards to somehow re-tighten the cog? I can't see this being a solution as surely it would just become lose again with all the forward pedalling? How are cogs even secured to hubs?

posted by Hates_ to sports, hobbies, & recreation (5 comments total)

I'm not very experienced in the fixed gear side of things, but I do know that cogs are designed to actually TIGHTEN as you pedal them, so it diodn't come loose from that. You may be able to find some helpful information here:
http://www.sheldonbrown.com/fixed-conversion.html

Also, until recently fixed gear bikes were almost all home-made, so you can probably find a knowledgeable person within the local fixed gear community. I find a lot of bike people are very quick to help other cyclists out.

Also, check more bike-specific forums, like http://www.bikeforums.net/.

Good luck! Have fun riding (once your 'fixed' is fixed that is.)
posted by toekneebullard at 4:43 AM on August 23

Sounds like the lock ring is loose. Don't ride it until you've got it fixed.
posted by scruss at 4:48 AM on August 23

You may have trashed the hub, but it sounds like the lockring is either very loose or missing.

Take off the wheel, then look at the drive side of the hub. Throw a chain whip on the sprocket, hold the wheel with the other hand, and turn the whip and sprocket clockwise. If it moves, try to spin it off. This should fail -- the sprocket should jam against another ring. If it does, but you've turned the cog any real distance, the problem is a loose lockring. Turn the sprocket counterclockwise until it is tight, then turn that ring clockwise (remember, it's a left handed thread) until it's snug against the sprocket. That should fix it

If you can't get it snug, it means you're either missing a spacer or the thread is damaged. If you need a spacer, it should go *inside* of the sprocket, but only very old hubs need spacers. If the thread is damaged, it might be possible to clean it up. Ideally, you'd have a die to do so with, but most shops won't have the correct die, the chances of you having one are very low. You can, if you are very careful, clean the threads up with a needle file. Practice on something you don't care about first.

However, that's the least likely result. My guess is that you'll spin the sprocket right off. This means the lockring is gone. Here's what a fixie hub without lockring looks like. This is dangerous, do *not* ride the bike without that lockring.

If you are missing the lockring, you may be forced to replace the hub. You can try your local fixie shops, but I don't know if there's a standard size and threading for these lockrings. If there is, getting one will be easy, if there isn't, you may be forced to replace the hub -- which is darn near the same as "replacing the wheel" to get one.
posted by eriko at 5:14 AM on August 23 [1 favorite]

Seems like you stripped your hub. If so there is no fix: it's time for a new one. Perhaps it was cross threaded during installation, perhaps it was a cheap one to begin with. Bummer.
posted by stachemaster at 2:41 PM on August 23

those are your brakes man. dont fuck around, get it to someone who knows what they're doing ASAP.
posted by swbarrett at 4:48 PM on August 23