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Seatpost
The tubular support that holds the saddle. The seatpost telescopes into the seat tube of the frame, providing the adjustment for saddle height. It is usually secured by a pinch bolt at the top of the seat tube.
Older seatposts, and those on cheap bicycles, are basically pieces of pipe, perhaps with a different diameter (usually 7/8" / 22.2 mm) at the top end. A separate clamp attaches the saddle to this type of seatpost.
Modern, high-quality seatposts have the saddle clamp mechanism built into the top of the post. This type is also commonly referred to as "microadjusting" because it permits a finer degree of adjustment of the saddle angle.
Seat posts come in a wide range of diameters, from 21.15 mm to 31.8 mm. Low end department-store bicycles are typically 21.15 mm (13/16"). Most bicycles with one-piece cranks, including most BMX machines use this size. Bicycles with standard-size (1 1/8" / 28.6 mm o.d.) seat tubes usually have seatposts between 25.4 (1") and 27.2 (1.07") in diameter. Seatposts typically come in sizes with even-numbered tenths of millimeters (26.0, 26.2, 26.4...).
For standard-sized seat tubes, the larger the seat post the thinner the tube. Thus, a larger seatpost size is often an indicator of a lighter, fancier frame.
I have a Database of Seat Post Sizes for various bicycles available on this site.
More information on seatposts is available in my article on Saddles.
