Originally Posted by
Schwinnsta
My approach to seatpost slippage is to add a band clamp on the seatpost and that transfers load to the top of the seatpost tube. This works well and avoids over tightening. I would in the future keep away from shims. They lead to cracks from over tightening.
Originally Posted by
Fentuz
I don't think over tightening is at fault here.
I believe there is a design flaw in the frame's seat tube. Dahon placed the clamping relief notch at the front of the tube and added an oval hole at the bottom of the notch to prevent vertical cracking. However, as the rider uses the bike, the weight/load on the saddle creates a significant moment toward the rear of the bike. This moment is significant due to the length of the seatpost.
Since the clamp holds the top of the seatpost tightly within the seat tube, this moment is transferred to the seat tube, leading to repeated fatigue cycles at the oval hole of the notch—primarily in tension. Over time, this results in horizontal cracking of the seat tube.
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.