Seat post

A bicycle seatpost,seatpin,saddlepole,saddle pillar, or saddle pin is a tube that extends upwards from the bicycle frame to the saddle. 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). Seatposts can be made of steel, aluminum, titanium, carbon fiber, or aluminum wrapped in carbon fiber.

Seatposts generally clamp directly onto saddle rails with which they must be compatible, while old or inexpensive seatposts slide into a separate clamp that then clamps the saddle rails.

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 seatpost is held in place by squeezing the top of the seat tube with a tightening ring (temporarily reducing its diameter; a vertical slit cut into the tube allows this to happen without crumpling) until the tube firmly hugs the post where it leaves the frame. A hole for a pinch bolt (also known as a "binder bolt") may be built into the frame for this purpose, or a "seat post clamp" may be purchased separately (but must be sized to closely fit the diameter of the seat tube). Whether integrated or separate, the seat post bolt can have a simple nut, can be an Allen bolt, or can include a quick-release mechanism, with a handle that releases the clamp without tools. A quick-release allows easy height adjustment of the seat, though increases the risk of seat theft unless it is also used to detach the seat when parking.

The size of the seatpost is dependent upon the internal dimensions of the seat tube of the bicycle frame. They come in various diameters, lengths and offsets. Offset is the distance between the centerline of the seatpost tube, and the centerline of the clamp area. Shims are often available to adapt a too-small seatpost to a too-large seat tube.

Diameters range from 21.15 mm to 35 mm. 27.2 is a common size for road or mountain bicycles. However, with modern mountain bikes having thicker diameter alloy or carbon tubing, manufacturers are now using more larger diameter seatposts. 25.4 mm (1 in) is a common size for BMX bikes. Documented sizes are almost always a multiple of 0.2 mm and include: 22.0, 22.2 (common size at top if tapered, ⅞ in), 23.4, 23.8, 24.0, 25.0, 25.4 (1 in), 25.8, 26.0, 26.2, 26.4, 26.6, 26.8, 27.0, 27.2 (most common, 1.07 in), 27.4, 27.8, 28.0, 28.6, 29.0, 29.2, 29.4, 29.6, 29.8, 30.0, 30.2, 30.4, 30.8, 30.9 (common on mountain bikes), 31.4, 31.6, 31.8, 32, 34.9.

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