This has resulted in a de facto standard of 27.2mm for high-quality bicycles that have 1 1/8" O.D. High-performance bicycles are made of thinner-walled tubing, so the hole will be larger if the outside diameter is kept the same.Īs a result, a larger seatpost size is often a sign of a higher-quality bicycle. Standard outside diameter accommodates standard lugs and clamp-on parts such as front derailers, pulleys and other cable guides, pump pegs, etc. Old American style, commonly used on BMX bicycles and other bicycles that use one-piece-cranks, uses a 1" seat tube.
Frames made to British or Italian standards will typically have 1 1/8" (28.6 mm) seat tubes. Most steel bicycle frames have tubing of standard outside diameter. On a related topic, we have an article (by Sheldon Brown, updated by John Allen) on Stuck Seatposts which may be of use to you. It will be near the minimum-insertion mark, not visible when the seatpost is installed on a bicycle. The size is usually stamped on the seatpost near the bottom.
So this article also describes how to take measurements, even if you don't have the "right" tools. We recommend measuring, both because it is more reliable and because we can't possibly have information on every make and model of bicycle.
John Allen and Harriet Fell have received more reader-provided information but we will no longer update this file, except to correct errors. Sheldon Brown started compiling a list, and realizing that it was too large a project for one person, he solicited information from readers. Tweet Follow page describes how to measure bicycle seat tubes and seatposts, and lists various bicycles, and the seatpost diameters that fit them.