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.
