Place a screwdriver through a hole at the top of the rod, just to make sure it doesn’t drop all the way in if the sizing rod is too small. The rod is simply placed in the seat tube and the matching diameter is the one just above the end of the seat tube (the first visible number on the rod still sticking out of the tube). These are rods with an increasing diameter from one end to the other, with a scale noting the diameter in standard sizes. Method 3: using special seat post sizing rods. Measuring the inner diameter of the seat tube. Method 2: using calipers, as shown in picture 3. Method 1: if the frame already has a mounted matching seatpost – one that attaches firmly enough so it doesn’t slide, while being narrow enough to allow easy mounting and dismounting – then it is sufficient to measure its outer diameter, as was explained above in the section – “How to measure the seatpost diameter?”. How should one do that? Three methods will be explained here, but one can always be creative. Measuring a seat tube diameter is often necessary before purchasing, or changing the seatpost. This means the seatpost will probably not fit firmly enough to stay in place inside a seat tube meant for 31.6 mm wide seatposts. It can be seen from picture 2 that the seatpost is marked as 31.6 mm wide (and 410 mm long), while it really measures only 31.42 mm. Measuring seatpost diameter using calipers. How to measure the seatpost diameter? The easiest and most accurate method is to use calipers (Vernier, or digital), as shown in picture 2. “ One (precise) measurement is often better than a thousand expert opinions” – author’s expert opinion. 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):
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