Originally Posted by
Kommisar89
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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.
The long tubes welded to the seattube might result in an overly stiff rear triangle. Might result in a rough ride, possibly a dead-feeling frame. It's possible that after experimentation, that tying the head tube and the rear dropouts together was the best configuration.