Many handlebars have a 31.8mm clamp section, and a common MTB seatpost size is 31.6mm.

I know these are both around 1 1/4 inches. Why the 0.2mm difference though?

  • because not everything is designed in inches or adheres to large fractions Aug 9 '20 at 20:03
  • @ArgentiApparatus A lot of common diameters are based off of imperial: 28.6mm as used by headsets is 1 1/8, 27.2 seatpost is 1 5/64, 30.9 is 1 7/32, the newer 34.9 seat tube is 1 3/8....
    – MaplePanda
    Aug 9 '20 at 20:30
  • 4
    Totally a guess, but bars have to be bent in the course of manufacture, whereas seatposts have to be straight. So there are different material requirements, and no reason for them to be the same. That they are so close is happenstance. I can't imagine anyone using a seatpost as handlebars, and the levers wouldn't fit anyway because 31.8mm is only the center clamp area.
    – Criggie
    Aug 9 '20 at 22:18
  • Because seatposts are available in 0.2mm size increments, so you can also get 31.8mm seatposts if you want to clamp handlebars to them (front seat of a tandem, for example). Or possibly the other way round: seatposts are sold that way because there are so many options.
    – Móż
    Aug 10 '20 at 1:32
  • 1
    As an aside, you will sometimes see 31.7 and 31.8 mm handlebars. These are really the same size—the difference is a rounding error in English to metric conversion.
    – Adam Rice
    Aug 10 '20 at 15:11

Frame tubing is generally standardized by external diameter to imperial dimensions in increments of 1/8 inch, which makes it a lot easier to standardize things like front derailleur clamps. Seat tubes therefore tend to have external diameters of 1 1/8” (28.6 mm), 1 1/4” (31.8) or 1 3/8” (34.9 mm).

Handlebars are similar. Though there has been more variation historically (eg 26.0 and 26.4 mm in addition to the imperial sizes), modern bikes tend to use handlebars with imperial sizes for the clamped diameter.

The problem, however, is that the internal diameter is not standardized, because it depends on the thickness of the tubing wall, which varies quite a bit. High quality lightweight frames can have very thin-walled tubing, while lower quality frames might need thicker walled tubing to get the same strength. So two seat tubes that are both 28.6 outer diameter could have different inner diameters.

Since the seatpost diameter has to closely match the inside diameter of the seat tube, variation in inner diameter of seat tubes has resulted in a large number of different seatpost diameters. Makers of more expensive bikes have largely converged on 27.2 and 31.6, but there are still many others, especially on lower end bikes.

  • "1 1/8” (28.6 mm), 1 1/2” (31.8) or 1 3/8” (34.9 mm)" How am I supposed to read those imperial measures ? Is it (1 + 1/8) inches, (1 + 1/2) inches and (1 + 3/8) inches ? If so, it doesn't make sense that 1 1/2'' is between 1 1/8 and 1 3/8. Did you meant 1 1/4'' ?
    – Puck
    Aug 10 '20 at 7:18
  • Yes that should have been 1 1/4. Sorry about the confusion
    – Andrew
    Aug 10 '20 at 10:53

The dimensional differences have to do with purpose of the individual part. The seat post outside diameter has to match the seat-tube inside diameter.If you are asking why they didn't make it .2mm thicker it was a design decision. Based solely on an engineers experience or opinion. It may have been because they wanted a thicker wall on the seat tube or they wanted to lighten the seatpost by a few grams either way the only one that knows for sure was the designer. Why they do it only the manufacturer can say for sure, it may be solely to make the part not interchangeable requiring you to buy parts from a single supplier.

  • 1
    I suspect interchaning handlebars and seatposts is sufficiently rare that that is not a design consideration.
    – Móż
    Aug 10 '20 at 1:33

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