31.6 mm seatpost (31.6 mm)
Which frames use a 31.6 mm seatpost, and how it differs from the sizes either side of it.
Published dimensions
| Diameter | 31.6 mm |
|---|---|
| Designation | 31.6 mm |
| Typical frames | many mountain bikes and gravel frames, older oversized road frames, and the other common dropper-post diameter |
Used on: many mountain bikes and gravel frames, older oversized road frames, and the other common dropper-post diameter.
31.6 mm is the other mainstream dropper diameter and a common oversized road and MTB size. It gives the stiffest interface of the mainstream sizes. A 31.6 mm post will not fit a 30.9 mm frame, and needs a shim in a 34.9 mm frame.
Frames and related seatpost sizes
27.2 mm seatpost27.2 mm30.6 mm seatpost30.6 mm30.9 mm seatpost30.9 mm34.9 mm seatpost34.9 mm25.4 mm seatpost25.4 mmBSA bottom bracket shellthe frame's other threaded interface
SOURCES: BikeGremlin, Seatpost diameter sizes (standards) · BikeRadar, Bike seatposts explained: diameters, materials, lengths