Dale said:
You do have to be very careful to set it correctly so the frame remains symmetric. That is the same amount of change on each side.
A Campy straight edge tool is very useful here. ...
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A string can also be used, tied to each rear fork end and looped around the head tube. Make sure the string leave the fork ends symetrically. Then measure the gap between the string and seat tube on each side. I use a caliper to measure. It is surprisingly accurate. It is also shocking how poor the alignment is on many frames.
These two methods have slightly different datums for alignment. The straight edge method aligns to the BB shell and should optimize the chainline. The string method aligns to the head and set tube and is an estimate(about three steps removed) of alignment to the plane formed by the centers of those tubes. If both methods don't agree you've got some bigger problems. Some of the most cronically baffling shifting problems turn out to be BB shells that are askew.
Joe Bender-Zanoni
Great Notch, NJ