Saw the On-One in chainreactioncycles. Was comparing the frame size to IRO Mark V.

I tried to be really scientific, but most likely failed miserably. So my question to you is how compatible are medium On-One Track and a Mark V in size 56?

The most obvious difference is the head tube length (100mm vs. 125mm) and the top tube (54.5cm vs. 55.5cm), but I am at a loss when trying to figure out the seat tube length. The On-One has 53cm c-t and I believe the IRO has 56cm, aboutish, but On-One's angle is tighter at 74.5 when compared to Mark V's 73. From what Sheldon has told me each degree of angle adds 1cm to the length of the tube. So at this point -- and if the above does stand -- the difference in seat tube lengths would be narrowed down to 1.5cm . Next up is the issue of bottom bracket drop, which I believe does affect the equation, too. The On-One frame's bottom bracket drop is 38mm and the IRO has 50mm. So the difference is 12mm. Would this affect the effective seat tube length, push the final difference to 0.3cm? And if so, how is it possible the head tubes are so differently sized with the same angle of 73?

This is just for me to figure out which size I would get if it came to that.

dont get obsessed with seat tube length. it really doesnt matter too much in terms of fit. The extra degree and a half of st angle is only going to affect fore and aft positioning of the saddle on the post. Focus on TT length + stem when trying to figure out what bike to buy.

Thanks, onetwentyeight -- will keep that in mind.