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I'm converting my old favorit bicycle to a fixie, but the hub is obviously not made for a fixed gear cog. I've removed the freewheel (not sure if it's called that, the thing that spins backwards on the hub..) but I'm left with a "screw on hub" (the cassette with the freewheel was screwed on to the hub). How can I install a fixed gear cog on that? Is there even a way? please help..rear hub, with freewheel and cassette removed

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This looks like it might be built up to the left a touch, you might have to change the spacers and redish the wheel to get a nice chainline. –  alex Jan 18 at 14:26
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I've not done this, but I beleive the threads are compatible with a standard fixed gear cog. If so, just buy a cog and install it. Then put a lockring from an adjustable bottom bracket on as a lockring (it has the same threads as the hub).

Note that this will not works as well as a proper fixie hub, most obviously because it's missing the left hand threaded section that normally holds the fixie lockring in place. I strongly suggest using locktite, or possibly even superglue, to hold the cog in place. Otherwise when you back pedal to brake there's a real danger that the cog will undo and you'll lose drive.

There's a detailed explanation here that covers what you've done plus the next steps.

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Don't do this if you're going brakeless, get a real track hub. Also, get brakes even if you're going for a real hub. –  alex Jan 18 at 14:24
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The threads should be fine. You'll need to redish the wheel after you figure out the spacers though. The lock ring isn't strictly necessary, but if you don't put it, make sure you have a front and rear brake in case something goes wrong. Also, note that you need horizontal drop outs since you can't use any chain tensioners with a fixie (sheldonbrown.com/fixed-conversion.html has an alternative for vertical dropouts). –  Batman Jan 18 at 15:11
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@Batman How is a lockring not strictly necessary? What else would keep the cog from unthreading? –  WTHarper Jan 18 at 17:39
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I can confirm that the threads are the same as a fixed ring cog as I've made the reverse conversion, freewheel on a fixed hub. Lots of people use a bottom bracket lockring as you suggest, but I personally wouldn't be comfortale doing it. I'd just get a wheel that was designed for fixed gear. They're not that expensive and worth it for the extra safety. –  jimirings Jan 18 at 17:45
    
@WTHarper - a lot of people run that way with some locite (for the record, I do recommend running a lock ring). It could unthread if you locked up the rear often, for example, but there are a lot of people doing it without adverse effects. –  Batman Jan 18 at 18:47
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