Seatpost diameter sizes (standards) | BikeGremlin

To help prevent mistakes when buying or changing seatposts, this article explains what kinds of seatpost diameters are most normally used and how they are measured. only diameter (width) is dealt with here. The length of the seatpost depends on frame geometry ( design ) and size – i.e. how much the saddleback needs to be raised from the end of the seat tube. A separate article explains the maximum amount of seatpost extension from the frame (minimal insertion length). For seatpost height in terms of bicycle fitting, see : Setting up comfortable riding position .

Table of Contents

0. Terms

It is important to understand two terms :

  • seat tube” – the tube in the frame that holds the seatpost.
  • seatpost” – the post the saddle is mounted on.

painting 1 shows a seatpost ( 1 ), and a buttocks tube ( 2 ) :

Seatpost (1), and seat tube (2)
Pictures 1a, and 1b

1. A bit of history

The first bicycle frames were made largely from steel, with steel tubes of a standard ( outer ) diameter. The british and italian standard diameter for buttocks tubes was 1 1/8″ ( 28.6 millimeter ). Older french bicycles used 28 millimeter tube. Old US bicycles ( largely BMX and older bikes that used one-piece cranks ) was 1″ ( 25.4 millimeter ).

Standard out diameter dimensions were significant so that derailleur clamps etc. could be made in claim equal sizes ( derailleur clamp inner diameter needs to match the seat tube ’ south out diameter ). Higher choice frames are normally made with thinned down tube walls to reduce the system of weights of the frame. This mean that a wide seatpost diameter normally meant a higher timbre ( and lighter ) human body. then high-quality frames with 1 1/8″ seat tubes normally had 27.2 mm wide-eyed seatposts. A more advanced course, by and large on batch bikes, where stronger seatposts are required ( without besides a lot extra weight ) led to a new “ oversized ” standard of 27.4 millimeter ( and broad ) .

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2. Measuring required seatpost diameter

“ One ( accurate ) measurement is often better than a thousand expert opinions ” – generator ’ s expert opinion. ? How to measure the seatpost diameter? The easiest and most accurate method acting is to use calipers ( Vernier, or digital ), as shown in painting 2 .

Measuring seatpost diameter using Vernier calipers. Source: Picture 2Measuring seatpost diameter using calipers.
Picture 2 It can be seen from picture 2 that the seatpost is marked as 31.6 mm wide ( and 410 mm long ), while it truly measures alone 31.42 millimeter. This means the seatpost will probably not fit securely enough to stay in place inside a seat tube meant for 31.6 mm wide seatposts. Always measure ! ? Measuring a seat tube diameter is often necessary before buy, or changing the seatpost. How should one do that ? Three methods will be explained here, but one can constantly be creative. ?

Method 1: if the frame of reference already has a mounted equal seatpost – one that attaches hard enough so it doesn ’ deoxythymidine monophosphate slide, while being narrow adequate to allow easily mount and dismounting – then it is sufficient to measure its out diameter, as was explained above in the section – “ How to measure the seatpost diameter ? ”. Method 2: using calipers, as shown in picture 3 .Measuring inner diameter of the seat tube. Source: Picture 3Measuring the inner diameter of the seat tube.
Picture 3 Method 3: using special seat post sizing rods. These are rods with an increasing diameter from one end to the other, with a scale noting the diameter in criterion sizes. The rod is plainly placed in the induct tube and the match diameter is the one fair above the end of the seat tube ( the first visible number on the rod even sticking out of the pipe ). Place a screwdriver through a hole at the top of the perch, merely to make sure it doesn ’ triiodothyronine drop all the direction in if the size gat is excessively small. ?Seat tube sizing rods. Source: Picture 4Seat tube sizing rods.
Picture 4 Since measuring frequently can ’ triiodothyronine be done to a 1/100 of a millimeter in accuracy, the future section gives a tilt of the most normally used standard seatpost diameters. If a mensural diameter differs, it can be assumed that the correct diameter is the criterion one that most closely matches the measured diameter .

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3. Standard seatpost diameters

table 1 gives a list ( in millimeter ) of standard diameter sizes :Standard seatpost diameters Table 1Standard seatpost diameters
Table 1 Seatpost diameters are normally a multiple of 0.2 millimeter ( but not constantly, as tables 1 and 2 read ). If a measured value differs and most close matches a measure that is not a multiple of 0.2, there is a high probability that the closest multiple of 0.2 millimeter size will fit. For case, a measured 25.5 will most credibly fit a 25.4 millimeter wide ( nominative and quantify width ) seatpost. A dominion of thumb is that the widest post that slides in without being forced ( other than pushing or twisting by hand ) is the right one. If it drops in, with play, before the pinch thunderbolt is tightened, the seatpost is credibly excessively pin down.

A few notes, good in case :

  • If a seatpost wobbles or rocks left-right (before tightening the clamp), it is probably too narrow.
  • Don’t punch it in. If it can’t be eased in by hand, it’s probably too wide. Forcing it in makes it very, very difficult to move or take out later. It might even damage some frames.

For easier wield, postpone 2 gives an overview of seatpost diameters most normally used on advanced bicycles ( from the end of the twentieth hundred to now ) .Seatpost diameters most commonly used on "modern" bicycles. Table 2Seatpost diameters most commonly used on “modern” bicycles.
Table 2 Related post – How to choose a comfortable saddle (the first in a series of 5 posts explains saddle materials):Bicycle saddle materialsBicycle saddle materials

source :
Category : Cycling

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