Some of the lowers for the Rockshox SID 32mm chassis forks have a manufacturing flaw that results in the front wheel not sitting straight in the fork. In other words, one of the dropouts is longer than the other, causing the wheel to sit in off-centre.
There are a number of reports of Rockshox warrantying this problem and replacing the lowers. People have also reported that the replacement lowers have the same problem.
It is, however, fairly easy to fix using a little patience and a round file.
Here's a diagram and formula that should help you to estimate how much material you need to remove from one dropout to center the wheel:
To put some real world numbers in, if you're wheel is 6mm offset at the rim, your rim radius is 280mm (about that for 26" MTB rims) and your front hub is 100mm wide then 6mm/2 * 100mm/2 / 280mm = 0.53mm.
- Material you file off your dropouts you can't put back. Don't try to file off all the material all at once. File off a little, then check with your front wheel. Repeat.
- Make sure when you measure the rim offset that the rim on your wheel is actually centered properly. Put the front wheel in both ways to make sure the offset is the same. If it is the same then the fork dropouts are the problem. If it isn't the same, then your wheel's rim is not centered properly.