Seatpost

The amount that it extends out of the frame can usually be adjusted, and there is usually a mark that indicates the minimum insertion (or maximum extension).

[1] To attach it to the bicycle's main frame, the seatpost is inserted into the seat tube, which must be of a very slightly larger diameter (or a cylindrical shim can be used).

The size of the seatpost is dependent upon the internal dimensions of the seat tube of the bicycle frame.

In some modern bikes with thicker alloy or carbon tubing, larger diameters such as 30.9 mm are used.

Sheldon Brown collected and published a list of seatpost diameters on his website, evidence of the common problem of finding compatible replacements.

Some high end road and track bicycle frames are made from one piece of molded carbon fiber with an integrated seatpost that is cut to length depending upon the rider, also known as a seat mast.

[11] In the case of aero seat tubes, there are a variety of clamping mechanisms for such seatposts that include pinch bolts and wedges.

Aero seatposts are typically proprietary designs for a specific frame model and thus cannot be mounted on bikes of other manufacturers.

[12] Suspension seatposts allow the saddle to move up and down with either a telescoping or parallelogram mechanism and incorporate a spring, an elastomer, or compressed air and possibly a damper to insulate against bumps.

Some bikes, such as Trek Madones, provide saddle height adjustment with a seatmast and cap arrangement.

[15] Matej Mohorič used a dropper seatpost on his road bike to help him win the 2022 Milan–San Remo one-day race.

Care should be taken not to overtighten the bolt or quick-release lever which clamps the post in the frame, especially where this acts on two brazed lugs rather than a separate clamp-on collar.