Phil_gretz

The rear takes a little more math. There are several ways to do this, but the most straightforward is to take the distance from the outside of the non-drive side locknut to the outside of the drive side locknut. It's best to make this measurement with digital calipers set to the metric scale, but you can use a mm-scale ruler if you are careful. With calipers, you can make the measure with the wheel off. With a ruler, do it with the wheel mounted in the rear dropouts.

Measure the distance (C). This is the outer locknut distance of your rear hub, which will be in the neighborhood of 120mm-130mm, depending on the wheel that you're using. Now measure from the drive-side outside of the locknut to the center of the rear cog teeth. Similarly to the chainring, make this measurement a few times around the cog and average the result. This distance is the in-board centerline of the cog (D).

Take the (C) measurement and divide by two = C/2. Next take the inboard cog distance D and subtract it from C/2. That is the rear chainline measure, or C/2-D=rear chainline. Compare this to the front chainline measure, and you'll know how far off you are and which of the two you need to change to get better alignment. Some folks will accept 5mm or so error. I'd suggest that <2mm would be better.

Hope that this helps you.

PG