If you're going to be using a fixed gear rear wheel, even some of the time, it's fairly important to get the chainline matched between the crank and the sprocket. The first thing to do is find out what the chainline is for the wheel. It's typically 42mm if the wheel was designed for fixed gear riding, but Paul hubs are different and even have a different chainline between their fixed and free sides. If you're using a freewheel in back (one that doesn't need to spin if you aren't pedaling), you have a little more flexibility (about 2mm off either way is OK) but it's still preferable to get things lined up.

Once you know the chainline of the sprocket, you need to figure out how to get your crank to match that. If you're using a square taper bottom bracket, you can generally adjust the chainline by getting a different length bottom bracket. Start by measuring what you have and then figure out if and what you need to change. Because newer bottom brackets are symmetric you have to double the change you want. For instance, to move the chainline in 2mm you need a bottom bracket that is 4mm shorter than what you have.

You can make a big change by deciding whether to put the chainring inside or outside the tabs on the crank. This usually is a 5mm move. You can move it out a millimeter or two with spacers.

This is probably more information than you can process right now, but I'm spilling it all here so you can refer back when you're ready for it. cny-bikeman gave you a link to the standard reference from Sheldon.


Also, be sure to leave a little slack in your chain. The more your chainline is off, the more important the slack is.