Originally Posted by ieatrats
Now there's something I've never seen mentioned. Is there any way to calculate what size to use, or do you just stick a sprocket in there and check your tension?
I'm surprised, I thought it was, more or less, common knowledge among FG riders, since it is the only viable way to tension the chain of a FG.
Well, you know, there's always a way to calculate things
I just wouldn't know how to, in this case. What I do is, I stick a worn sprocket in there, of size based on how slack the chain is, and then push it back towards the rear sprocket as much as it will go. If the teetch of the sprockets touch, then the size is too small. Sometimes the chain is so slack, you need a middle chainring.
Careful with your fingers, while you do this, expecially while you spin the pedals/rear wheel to see if the drivetrain is OK. If they get caught between chain and chainring, chances are they will be cut off, expecially with brand new machined chainrings (was goofing around with a Raceface alloy chainring once - that's the closest I came to have a finger severed, so far).