Bicycle Mechanics - Single-speed / fixed chainline question

I'm thinking of building up a fixie / single-speed bike as my next project. Just how "exact" does the chainline have to be on fixies / single-speeds? i.e., +/1 mm, +/- 2mm? What problems arise if the chainline is less then perfect? Does the chain have a tendency to derail, or is the issue more to do with accelerated chain wear?

Thanks!

i would say no more than 2 mm. if your chainline is off, your chain can/will derail, causing you to fall and break all your bones. chain wear will be accellerated in a weird pattern on your chain as well.

Hi, I've been riding fixed gears on road and track for over thirty years. When you set up the chainline, it has to be <dead on>. This is absolutely critical. If it is not, as you ride on the road, the chain will "stretch." Then one day, as you are spinning at high speed down a hill at about 140 rpm's, the chain will fall off the chainring. If you don't have a lock ring, the chain will jam around the bb and the rear tire, the cog will unthread, and you will coast/brake to a stop. If you use a lockring, the rear wheel will lock, the bike will fishtail, and you may crash. So on a road fixie, keep the brakes and toss the lockring.

On a track bike, you want the chainring dead on because then you can run a looser chain. Much faster. When the chain is set up properly, it's difficult to make it derail. This is also why you want to use a 1/8" track chain. They are made to run straight, not to easily derail in order to facilitate smooth shifting.

- L.

I am interested in this question too, but in regards to single speeds with a rear derailluer used to take up chain slack.