for a single-speed conversion, is it always necesary to change bottom bracket?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.