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Old 11-29-18, 09:31 AM
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Originally Posted by TiHabanero View Post
After drawing out 4 frames on top of each other in full size, I realized that seat angle is not quantified when fitting riders to a frame. In other words, we quantify reach, but going from the BB back to the saddle we do not have a measurement for it, and the seat angle does seem to affect the overall fit of the bike. Yes, the saddle can be adjusted fore aft, however when comparing frames, there should be a number for this measurement. Tell me where I have gone wrong here.
Measuring from the BB back to the saddle itself is an impossible number to give in this case, because it is not a frame measurement. It going to vary with the saddle used, and where it is set fore/aft (based on seatpost setback and adjustment on the seat rails). What you CAN quantify is the horizontal measurement from the BB to a line drawn along the seat tube. And you can get that distance by subtracting the "reach" from the "effective top tube length". This will give you the horizontal distance from the bb to the seat tube at a height that is level with the top of the head tube.

However, this really does not tell you much. Because what matters with fit is not where the seat tube is in relation to the bb, its where the saddle is in relation to the BB. And between the saddle rails and various setback seatposts, there is a massive amount of fore/aft adjustment available in that regard. From a fit (or handling) perspective, as long as the saddle in in the correct position relative to the BB, it does not matter whether you get there with a steep STA with a setback post and the saddle slid back on the rails, or a slack STA with a straight post and the saddle slid forward on the rails.

I guess it is possible that the STA could be so far off that you simply can't get the fore/aft position you need, but that would be rare. In any event, simply knowing the STA would let you know if this was the case.
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