So high-quality frames with 1 1/8″ seat tubes usually had 27.2 mm wide seatposts.Ī more modern trend, mostly on mountain bikes, where stronger seatposts are required (without too much extra weight) led to a new “oversized” standard of 27.4 mm (and wider). This meant that a wider seatpost diameter usually meant a higher quality (and lighter) frame. Higher quality frames are usually made with thinned down tube walls to reduce the weight of the frame. could be made in exact matching sizes (derailleur clamp inner diameter needs to match the seat tube’s outer diameter). Standard outer diameter dimensions were important so that derailleur clamps etc. Old US bicycles (mostly BMX and older bikes that used one-piece cranks) was 1″ (25.4 mm). The British and Italian standard diameter for seat tubes was 1 1/8″ (28.6 mm). The first bicycle frames were made mostly from steel, with steel tubes of a standard (outer) diameter. Picture 1 shows a seatpost (1), and a seat tube (2):
A separate article explains the maximum amount of seatpost extension from the frame (minimal insertion length). how much the saddle needs to be raised from the end of the seat tube.
The length of the seatpost depends on frame geometry (design) and size – i.e. Only diameter (width) is dealt with here. To help prevent mistakes when purchasing or changing seatposts, this article explains what kinds of seatpost diameters are most commonly used and how they are measured.
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