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I want to convert my old raleigh, which has 3 chainrings on the front, into a single-speed. Do I have to replace the bottom bracket? I keep hearing conflicting things. Thankyou.

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4 Answers 4

An eccentric bottom bracket is one way of getting the chain tension correct, but it's not the only one.

I can't see any other reason why you'd need to change it, unless you can't get a useable chainline otherwise.

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Not sure if you are talking bottom brackets or cranksets. But the answer for both is "no".

If you have a singlespeed specific hub, this mainly depends on the chainstays of the bike and the size chainwheel you want to run. For a conversion I imagine you have a standard cassette hub though - this gives you some flexibility in positioning the rear sprocket. You should be able to put your single chainwheel in the inside or middle and adjust position of rear sprocket if needed.

If you have vertical dropouts, some people might recommend an eccentric bottom bracket to tension the chain. But you can also just use your rear derailleur, or a bolt-on tensioner like this one for this.

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Why does that tensioner not recommend you use it with a fixed gear bike? Does that affect this plan ion any way? –  Brad Sep 26 '12 at 16:45
    
I suspect that you're more likely to drop a chain (which can be more dangerous on a fixie) running that kind of setup than you would with track-style dropouts or an eccentric BB. –  AlexCuse Sep 26 '12 at 16:51
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Derailler style hanging Chain tensioners do not work when you reverse pedal with a fixed drivetrain, it will probably be ripped off or damaged any time you backpedal. –  Benzo Sep 26 '12 at 17:08
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On a fixed gear bike, locking up the rear wheel will tear off a chain tensioner from the force. Single speed setups with freewheels can use a chain tensioner, but fixies require horizontal/track dropouts or an eccentric hub/BB (or lots of fiddling with rings, cogs, and half-links). –  WTHarper Sep 26 '12 at 17:08
    
Thanks for clearing it up guys, only ever used a freewheeling SS here :) –  AlexCuse Sep 26 '12 at 17:23

You need one of four things:

  1. An eccentric bottom bracket. Like these
  2. An eccentric hub. Like these
  3. Horizontal dropouts. Like these
  4. A chain tensioner. There are a number of different types.

You only need one of the four, but these are the only ways to get the chain tension correct on a single speed. (I should say "almost only." Blind luck works every so often.) Additionally, as others have mentioned, a chain tensioner will not work with a fixie.

It's also possible that your spindle is too long or too short to get your chainline correct. If that's the case, you'll have to get a new bottom bracket with the correct spindle length even if you already have an eccentric hub or horizontal dropouts. It's usually possible to position the chain rings on the inside or the outside of the crank's spider and get it correct, but not always.

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You left out 5: Sliding dropouts. The Voodoo Dambala frame uses them, there might be others. –  Mike Baranczak Sep 26 '12 at 22:24
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6: Magic gear ratios with or without half links is often considered the blind luck method. –  whatsisname Sep 26 '12 at 22:43
    
You're right, I totally forgot about sliding dropouts. Although with an older frame I doubt the OP has them. And yeah, "magic gear" is another term for blind luck. But to the best of my knowledge, there's not a straight-forward to calculate that. Either of you should feel free to edit those into the answer though. –  jimirings Sep 28 '12 at 19:05

It's almost never necessary, I don't know where you got this idea. The only possible reason I can imagine for changing the BB during a single-speed conversion is if the chainline is way off, and you can't fix it by respacing the chainring.

You might be thinking of an eccentric bottom bracket. This is indeed one way to adjust the chain tension on a single-speed; but an eccentric requires a special BB shell, so if your frame doesn't already have an eccentric, there's no way to install it.

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