how do you size a seatpost clamp
 

Hi; I have a trek 5000 frame from 1996. According to Trek docs it has a 35.5mm seatpost tube, and a 27.2mm seatpost. Can you even find a 35.5mm clamp? Seems like it would need a shim to be able to grab onto the seatpost itself. The current setup does have a shim.

Appreciate any insight!

That is not a common size. I googled the size and found this.



https://cimg5.ibsrv.net/gimg/bikefor...fc8607955d.jpg

The clamp shouldn't touch the seat post, just the seat tube. 35.5 mm is an uncommon size, but 35.8 is common and easily fitted with 0.006" shim stock.

the clamp "grabs onto" the Frame, not the post itself. Many clamps have a shoulder that helps locate the Clamp on the frame without a hassle every time it's reclamped... that shoulder needs to be a size that's a bit larger than the post's O.D.

the clamp imaged above will fit great... they're on Ebay for about $20
Search: "35.5mm Seat post clamp"
and yes, that's an odd size!
Campagnolo is in a world of it's own.

Rather than go off of listed specs, just measure the frame, where the clamp sits, with an accurate caliper.