Xem Studiomeasured geometryLibrary › Everyday mounts › Bicycle ·
Bicycle seatpost diameters: 25.4, 26.8, 27.2, 28.6, 30.9, 31.6 and 34.9 mm
What diameter is a bike seatpost? · Standard part · basis: Sizes from Sheldon Brown's seatpost list (27.2 the de facto size for 1 1/8 in seat tubes) and BikeRadar's dropper-post guide (30.9 and 31.6 most common, 34.9 and 27.2 also made)
Exact dimensions. Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part. Mating features
- sizes
- 7 in table
Notes
- A seatpost's diameter is set by the frame's seat tube; there are dozens of sizes in 0.2 mm steps, and the right one is usually stamped on the post near the minimum-insertion mark.
- Most common today: 27.2 (road, gravel, older mountain bikes), 30.9 and 31.6 (most dropper posts), and 34.9 (large droppers). 25.4, 26.8 and 28.6 cover many older frames.
- Files: each size as a Ø × 100 mm section, axis on X, plus a DXF of the circle: the outside of the post only (walls differ and are not drawn). A saddle-bag, light or bottle mount that clamps the exposed post uses this Ø.
- Seat-tube clamps (the collar on the frame) are sized by the frame's outside diameter, not the post: that is a different number.
Sizes
| post Ø (mm) | where it is used | from |
|---|---|---|
| Ø25.4 | 1 in posts on older and lower-cost frames | Sheldon Brown |
| Ø26.8 | common on older frames | Sheldon Brown |
| Ø27.2 | the de facto size for frames with a 1 1/8 in seat tube; still the usual road and gravel post | Sheldon Brown, BikeRadar |
| Ø28.6 | 1 1/8 in | Sheldon Brown |
| Ø30.9 | one of the two most common dropper-post sizes | BikeRadar |
| Ø31.6 | one of the two most common dropper-post sizes; common on mountain bikes | BikeRadar, Sheldon Brown |
| Ø34.9 | large dropper posts | BikeRadar |
Mating features
Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.
| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| post outside | Ø25.4 / 26.8 / 27.2 / 28.6 / 30.9 / 31.6 / 34.9 | frame seat tube of the same Ø; mount clamps on the exposed post | slide | seatpost-* |
Where the files are
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| Sheldon Brown (Harris Cyclery) | Seatpost sizes by make and model (27.2 the de facto size for 1 1/8 in seat tubes) | web | reference verify |
| BikeRadar | The anatomy of a dropper post: 30.9 and 31.6 most common, 34.9 and 27.2 also made | web | reference verify |
Free files
★ Free files 7 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| seatpost-25.4-L100 | Seatpost section Ø25.4 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø25.4 circle | STEP · STL · DXF |
| seatpost-26.8-L100 | Seatpost section Ø26.8 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø26.8 circle | STEP · STL · DXF |
| seatpost-27.2-L100 | Seatpost section Ø27.2 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø27.2 circle | STEP · STL · DXF |
| seatpost-28.6-L100 | Seatpost section Ø28.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø28.6 circle | STEP · STL · DXF |
| seatpost-30.9-L100 | Seatpost section Ø30.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø30.9 circle | STEP · STL · DXF |
| seatpost-31.6-L100 | Seatpost section Ø31.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø31.6 circle | STEP · STL · DXF |
| seatpost-34.9-L100 | Seatpost section Ø34.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø34.9 circle | STEP · STL · DXF |
Plain text: sheet.txt · JSON: /api/geometry?object=bicycle-seatpost-diameters
How it fits
Parts that fit it
Specialized Power saddleon the seatpost clampAlternatives
Bicycle handlebar diametersbar sections for mounts on the handlebarAlso needed
Socket head cap screws, M2–M8M4 / M5 screws for a clamp on the postYou might also need
Questions
What diameter is a bike seatpost?
Most often 27.2 mm on road and gravel bikes and 30.9 or 31.6 mm on mountain bikes with dropper posts; 34.9 mm on some large droppers. The size is marked on the post.
What are the most common dropper post sizes?
30.9 and 31.6 mm; 34.9 and 27.2 mm droppers are also made.
Is a 31.6 post the same as 31.8?
No. 31.6 is a seatpost size; 31.8 is a handlebar clamp size. Do not mix them.