Quote:
Originally Posted by Mark McM
From your example, you could have two frames with the same Reach, but one has a 72 degree STA and the other has a 74 degree STA. These frames might have a difference in top tube length of 2cm. After adjusting the saddle setback to the same position (which might require different seatposts), the bikes will fit and handle the same. But what if you had 2 frames with the same STA but one had a top tube 2cm longer than the other? You could use different length stems to achieve the same fit, but the two frames would never handle the same, due to differences in the lengths of the stem and front center.
So, the primary dimensions for proper frame fit are Stack and Reach, and STA is just a secondary consideration.
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The front center may or may not change, depending on HTA and rake. True, those would also effect handling. But even in the first example, whether the two bikes handled similarly or not would depend on HTA and fork rake, and consequently, wheel flop, trail, and front center (not to mention chainstay length, though maybe not discernable). It's true that in the first example if the front end geo were the same on the two bikes you would have the same or similar handling, where in the second example either front end geo or front center would be have to be different. But only in the fringe cases could you have some likelihood of knowing what the effects on handling might be. Aside from that, from the perspective of fit alone, adjusting via stem length is a wash with adjusting via saddle/seatpost setback. You cant make honest comparisons in handling without additional assumptions.