Originally Posted by
DaveSSS
Totally wrong. Effective top tube adds nothing that isn't covered by reach. Top tube is affected by the seat tube angle. Reach is not.
The only other dimension to consider for fit is rhe seat tube angle. I know what range I need and how much seatpost setback I need to go with each STA. Frames in my size nearly always have a 74-75 degree STA. I've never had a 75, but if I did, it would need a 32mm setback post. For 74-74.5 degree I want a 25mm setback, but some frames only offer a 20mm setback on a proprietary post that can't be changed.
Why the vehement disagreement!?
Yes, the effective top tube is affected by the seat tube angle (and vice versa). Therefore, if one knows reach and stack, and also the effective top tube, one can determine the seat tube angle. Similarly, if one knows reach and stack, and also the seat tube angle, one can determine the effective top tube. In other words, the difference (in length) between reach and effective top tube for a given stack is determined by the seat tube angle. It is just trigonometry. Your two quoted statements that I have bolded above contradict each other.
I would even venture that the effective top tube is easier for most people to visualize and evaluate whether they would be too stretched out or cramped.