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I guess this may have already been covered so apologies if it has. I am looking at stack/reach figures of various bikes but wondered if anybody had a calculation of how much extra the horizontal top tube works out to be with every extra mm on the reach. Obviously this depends on the seat tube angle but just wondered if anybody had a rough guide for general TT and Road bikes?
Thanks in advance
Reach is a horizontal measure. So is effective top tube. So 1mm of extra reach equals 1mm of extra effective top tube. For effective top tube to be different on bikes with the same reach either the stack or the seat tube angle has to differ. In either situation the difference is wholly behind the centre of the bottom bracket.
If the seat tube angle is difference you're just going to adjust your saddle setback by moving the saddle on the rails to get the same setback measure.
If the stack is different then you need to correct for that. There is a rule of thumb that every 10mm increase in stack results in a 3mm decrease in reach.
Personally I just size the bike on stack and reach and disregard ETT.
So I ask, why do you need to know this difference?