Originally Posted by
Kommisar89
Interesting discussion thus far. One thing I started thinking about as I was making some adjustments to my wife's mixte is why the designers decided to run the twin diagonal tubes back to the rear dropout rather than attaching them to the seat tube or just using a single sloping downtube. According to what many are saying here, a frame should be stiffer by positioning the top tube lower on the seat tube. But in the mixte design (at least the Peugeot that I'm looking at) the twin diagonals go from the head tube to the rear drop outs and aren't connected in any way to the seat tube. So those designers apparently felt that it is not necessary to brace the seat tube at all as it's only attached at the bottom bracket and the thin seat stays. Now granted mixte frames aren't likely to be put through the same level of stress as a racing frame but still, it does make one wonder about the argument that the top tube is there to somehow reduce flexing of the seat tube. Apparently its primary function is to support the loads on the head tube.
Kommi, go back to the mixte thread started by Veloria earlier this summer, we went through all of that. My reference for believing the top tube is involved is basically my knowledge and physical insight as an engineer, but it's shared by a physicist/frame builder/author, Tony Oliver, and explained very well in his book, "Touring Bicycles; a practical guide." I provided a complete reference to the book in that thread. Sorry to pass you around, but I have made this case a number of times on BF.
The fact that Peugeot and others did not choose the solid top tube design does not mean it is not beneficial, or better with respect to torsional stiffness than the (let's call it) floating twin lateral design. I still don't fully understand the purpose of the conventional mixte frame design.
Maybe it doesn't have a strong enough effect for the expected mixte riders. Maybe ... it is just style, a nod to the space frame concept - aircraft, Bucky Fuller, the Birdcage Maserati, Raymond Leowy, I don't know. But engineering-wise a large (say 25 mm or more) top tube in line with the rear axle and the top of the head tube or all the way up to hirizontal will make a frame that better resists twisting around its longitudinal axis, as might be created by pedaling and handlebar reaction forces.