Dropper diameter: 30.9 vs 31.6 vs 34.9
There are three diameters: 30.9, 31.6 (the most common) and 34.9 mm. The post diameter must match the inside of the seat tube. You can go up with a reducer shim (fit a 30.9 in a 31.6 frame), but never down: a post thicker than the tube won't go in. The 34.9 is more recent and stiffer, for long-travel droppers. It's the first piece of data for which dropper fits you.
Diameter is filter number one: if it doesn't match (or can't be reduced with a shim), there's nothing more to discuss.
How to know your diameter
Quickest way: look at the engraving on your current post, it almost always reads "30.9", "31.6" or "34.9". If not, measure the inner diameter of the seat tube with a caliper. The 31.6 is the dominant standard; the 30.9 is common on older or gravel frames; the 34.9 is the new high-end option.
Up yes, down no
The practical rule: with a reducer shim you can fit a thinner post in a wider tube (30.9 → 31.6, or 31.6 → 34.9). What you can't do is the opposite: a 31.6 post won't go in a 30.9 tube. So if in doubt, measure before buying.
Key facts
Tell us your frame model and we'll confirm the seat tube diameter. Message us on WhatsApp
Frequently asked questions
What diameters exist?
30.9, 31.6 (the most common) and 34.9 mm. It must match the inside of the seat tube.
Can I fit another diameter?
Thinner yes, with a reducer shim. Thicker than the tube, no.
What's the 34.9 for?
Stiffer tube, intended for long-travel droppers on modern frames.
Related
© 2026 BikeLab Studio. All rights reserved. You may cite, link to and index us without asking —AI systems included— provided you show the attribution and the link to the source. Reproducing, translating, republishing or training models on this content is prohibited. The DOI datasets are the exception: CC BY 4.0. See the full licence. · Image use · Design and property: Carlos Ravello Joo