This is about the final position of the rider on the saddle. See below:
-
Note the Backward Shift of 25mm for the Saddle vs the Forward Shift of 25mm for the Saddle.
- It is an extreme 50mm difference fore-aft.
-
Note that the Saddle Rail Height and Seat Tube Angle are identical for both sides.
-
And assuming the rider places Sit Bones on the same point on the saddle (top point about 1/3 from the end), the Effective Saddle Height for the rider is longer on the Backward Shift when compared to the Forward Shift.
-
This happens anytime you make a pure fore-aft shift change on the saddle. Again, assuming you follow the saddle with your position on it, the effective saddle height is altered every time.
-
The difference varies with the precise seat tube angle, but it is there in all cases since we pretty much never have a 90* seat tube angle (and even then, the Eff Saddle height would possible change, unless you made a perfectly even shift ahead or behind the seat tube angle, and also had dead level saddle rails… again pretty much never the case).
-