Originally Posted by
RonH
My 2011 Specialized Tarmac is a size 56.5. The "effective" top tube length is 565 mm. Specialized says the "TT is measured horizontally from center of HT to center of ST". So I guess the effective top tube length is the new frame size measurement.
Effective top tube is an important measurement. It can be treated the same way you would the actual top tube length of a horizontal top tube frame. Nominal frame sizing is still generally based on seat tube length. Many frames in the 56-58cm range have top tube and seat tube close to the same measurement. Of course, with the shorter seat tubes of compact frames, both seat tube and top tube may be virtual measurements rather than actual. Confused yet?