I do not know of a formula per se, but with all other things being equal, including fork length, a 10mm change in rake will either raise or lower the front end of your bike about 3mm.. ie. A 365 mm fork with an offset of 28 will raise your bars about 3mm.....a 365 fork with an offset of 48 will lower them about 3mm. This is not significant, or anything to worry about. Your top tube angle would change only .2 deg...something that would not be noticeable. However, as HillRider notes, trail will be affected significantly. For example with 700c x 23 tires and a 73 deg head tube angle, switching from a fork with 38mm of offset to one with 28mm will move the trail from 62mm to 73mm, which will definitely affect handling.

Dave

FYI...a formula for calculating trail can be found here: http://en.wikipedia.org/wiki/Bicycle...cycle_geometry