Originally Posted by
GV27
I was just thinking back to the discussion a couple months back about compact geometry. People kept crowing about "smaller triangles" being orders of magnitude stiffer. So here's my question. When you have roadies constantly in search of ultimate stiffness and carbon construction bringing the weight of a bike down well under UCI regs - why are modern race bikes not all mixtes? Not only are the triangles WAY smaller than even a compact frame, there's an extra one......
Because standard mixtes, with the twin tubes are not stiffer, and really are not even as stiff as a diamond frame. The top tube on a diamond frame, together wth teh down tube, resists the bb twisting with pedal force. The stiffness is proportional to the fourth power of diameter. Each twin tube is about half the diameter of a top tube and hence 1/16 the stiffness. Two of them are 1/8 the stiffness. But you still don't get even that reduced contribution to stiffness because the twin tubes are not TIGged or lugged to the seat post, they're just located with some little links. The "modern" mixte like the Schwinn that was shown has a standard diameter tube attached to the seat tube with either TIG or lugs. That gives you back the stiffness relative to the diamond. And, that dropped down tube gives the front "triangle" a shape that is very similar to MBs and compact road frames. So teh "modern" mixte should be about as good as a compact frame, but the true mixte should not.