Join Date: Dec 2004
Location: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
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I'd aim for about 65 degrees from the line described by the power side of the chain. That's not a hard and fast number, there's wiggle room in either direction. Within reason, a shallower angle will let you shift a greater range of chainrings, and a steeper angle... won't.
If in doubt, measure the angle on some uprights.