Ok. Here are some pics with my seatpost details. See what you think:


Preliminary bike - wheels not the final ones. Bar ends not the final ones.


Detail of the seatpost (Ritchey Pro alloy 31.6mm x 400mm) with shim. It shows the shim extends well past the joint in the 2 frame parts.


Shim construction: Made from an identical Xootr Swift seat post (taken from wife's Dahon Yeah, she got the Swift post which has a better seat clamp), sawn off at the bottom and the inside widened on a lathe to match the Ritchey seat post. Instead of making a conventional shim which usually have just a slot, this one is like a mini seat tube with short slot and stress relief hole. The location of these match those on the frame. I made a ring for the top to prevent the shim from sliding right into the seat tube.


Here you see the stampings on the original seatpost.


The Ritchey post fits very tightly into the shim - in fact it can't go much deeper, it becomes too tight. So it is a tighter fit than what you would normally find.

With this construction, the bottom QR can be left fastened, and just the top one needs to be loosened to adjust seat height. For folding, undo both QRs to extract post plus shim. Additionally, because this shim has no slot where the bottom QR clamp is located, the shim won't collapse when the QR is fastened.

The only thing I am uncertain about is the shear stress exerted on the shim by the loaded bike. The Ritchey seatpost does not extend past the frame joint so the shim is the only part providing strength here. The shim wall is 1.2mm thick.