Originally Posted by
Road Fan
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.
This same debate comes into play whenever a thread mentions the stiffness of the classic Mixte frame (twin top tubes). I’m intelligent enough to understand the theories of torsion load and stiffness of tubing in relation to thickness and diameter, but what I don’t like about comparing diamond frame and Mixte frames, is the theory applies best only if the two frames are constructed exactly alike; and they are not.
It doesn’t take into account that Mixte frames are not only braced between the top tubes, but also braced to the seat tube, but most importantly the top tubes go all the way to the drop outs, where they are tied together by the rear axle.
But to try and infer that Mixte frames are significantly weaker and inherently overly flexible is in my opinion a fallacy. My own riding experience tells me that the Mixte frame I’m riding is no more, or less flexible than the dozens of diamond frames I’ve ridden in my life. Actually the stiffest frame I’ve ridden in some time was a U-frame aluminum bike, with oblong tubing. There was no discernable flex in that frame at all. I wonder why all the new super bikes haven’t adopted that style frame?