How do I check seat tube diameter and insertion depth for a dropper post?
By Crankworthy · Published · Updated
Read the diameter off your current seatpost — it is printed near the bottom — and measure how far a post can go into the frame before it hits something. The dropper you choose must match the diameter and have a total length and travel that fit inside that insertion depth at your saddle height.
The three measurements
Diameter must match the seat tube. Common sizes are 30.9, 31.6 and 34.9mm; 27.2mm frames have fewer dropper options. A smaller post can be fitted with a shim, but a larger one cannot.
Insertion depth is how far a post can physically go into the frame — bends in the seat tube, the bottom bracket shell, bottle-cage bolts and suspension linkages can all stop it short. A long-travel dropper is also a long post, so it may not go deep enough.
Saddle height sets the other end. Measure from the top of the seat collar to the saddle rails at your riding height. The dropper's extended height above the collar has to fit within that, which caps the travel you can run.
How to measure
- Diameter
- Pull your current post and read the number printed near its base, for example "30.9".
- Insertion depth
- With the post out, slide a long straight rod or a tape measure down the seat tube until it stops. Measure from the top of the seat collar.
- Collar to rails
- With the saddle at your riding height, measure from the top of the seat collar to the centre of the saddle rails. Compare it with the dropper's published dimensions.