Will a 30.9mm dropper post fit my 31.6mm seat tube?
By Crankworthy · Published · Updated
Yes
Yes, with a 30.9-to-31.6mm shim — a thin sleeve that takes up the gap. It is a standard, safe fix. The reverse does not work: a post wider than the seat tube cannot be made to fit.
Why
Seatposts come in several diameters, and the post must match the inside of the seat tube closely to clamp securely. A shim is a precisely sized sleeve that makes a smaller post fit a larger tube.
Shimming is widely used and safe for small steps like 30.9mm to 31.6mm. Very large shims, bridging several millimetres, are best avoided on a dropper that sees heavy use.
There is no way to go the other way. A post that is wider than the seat tube will not go in.
How to find your seat tube size
- Read your current post
- The diameter is almost always printed near the bottom of the seatpost, for example "30.9".