Originally Posted by
Duragrouch
You're both right.
Schwinnsta: A locking collar above, would have allowed reduced clamping loads with the plastic bushing. However, with the aluminum bushing, and most especially with my added shim so the slot doesn't get compressed, high clamping loads should have no effect on the seat tube.
Fentuz: Your analysis is right on the money. I wrote up similar and sent to Dahon when the first frame cracked and they blew me off, said I was too heavy and too tall, neither of which was true. The front side of the tube is loaded in tension, so susceptible to fatigue, whereas the back is loaded in compression, much less of an issue, which is why 99% of bikes put the clamp slot in back, for eons. Notably, Dahon moved the slot location to back, and changed the bushing to aluminum. I recommended they preemptively send out metal bushings to Dahon owners with plastic bushings, to perhaps prevent or delay cracking, and they had no interest in doing that.
By over tightening, you stretch out the clamp, this in turn allows more stress on the seat tube.
I don't see or understand Fentuz analysis. I agree that aluminum bushing, actually seat tube clamps are better than plastic. Dahon could have gone to stainless steel clamps and that it would have been better still. Either way using an additional clamp on the seatpost, pre-cracking, solves the problem