Cane Creek Seatpost Shim Some frames have an odd seatpost size and standardized seatpost and dropper seatposts don't fit properly. With a seatpost shim, you can run a standard post or dropper post in a frame with an odd seattube size. It is best practice to used a seatpost or dropper post that is as close to the frame seattube size as possible. Seatpost Shim Sizes25. 4mm to 27. 2mm27. 2mm to 30. 9mm27. 2mm to 31. 6mm | Cane Creek | Seatpost Shim 25. 4 To 27. 2