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This is related to my previous question about seatposts and handlebars, but I’d like to generalize the question this time.

The majority of bicycle tubes are metric equivalents of imperial sizes. The point about engineers choosing the best size for the application was brought up in the previous question, but the fact that this is so common leads me to believe that it can’t simply be a coincidence. What makes bicycle manufacturers lean towards these unusual decimal sizes of tubing? Is it availability? Tradition? If anything, standard metric sizes would probably be cheaper to purchase tooling for.

Examples: seatposts, seatpost clamps, steerer tubes, and handlebars (except the pointless MTB 35mm standard).

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  • Nathan’s answer here is a good one, even though it’s aimed towards a different question: bicycles.stackexchange.com/a/50832/38536
    – MaplePanda
    Jan 7 at 19:55
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    The same trend exists also in other industrial sectors. For example in aviation, parts like rivets only exist in inches (except for Dassault, that uses custom metric rivets). The explanation there was that standardisation began in the US, and for European companies it was cheaper to use existing items than remanufacture the tooling. At the end the initial measurement does not matter, compared to the convenience of using well established and known components. For the 777, Boeing also switched to the metric system, but the rivets are still measured in inches.
    – Renaud
    Jan 7 at 21:16
  • Boeing "switched to a metric system" (but not the metric system) long before the 777. Their aircraft fuselages are designed in sections 2540 mm long. That's exactly 100 inches (or 8 feet 4 inches if you prefer).
    – alephzero
    Jan 8 at 9:18
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The decimal sizes such as 25.4 and 28.6mm are not "unusual" sizes. They are actually the standard sizes. You can count on engineers to do the easy and cheap thing, so they use the standard sizes. "Standard metric size" tubing (that would presumably be sizes in a round number of millimeters like 25mm or 30mm) is not really a thing in the world of chromoly tubing, and doesn't practically exist.

The reason the standard tubing sizes are not a "round" number of millimeters is, of course, historical.

Chromoly tubing is not just used for bicycles, and it wasn't invented just for bicycles. According to Wikipedia, it was invented in 1885 and commercialized around 1891. Chromoly tubing was used extensively to build airplanes (and still is for some small airplanes) because it was strong and light enough to displace wood construction, which standard steel was not. The standard tubing sizes were based on inch measurements, because it was commercialized in a country and an era where inch-based measurements were the standard. And these sizes are still the standard sizes today...they were simply re-named to metric by calling 1-inch tubes 25.4mm, 1 1/8" tubes 28.6mm, etc.

If you shop at a chromoly tubing supplier such as Aircraft Spruce or Wick's Aircraft Supply you will find that chromemoly tubing is available in sizes from 1/4" up to 2+ inches, typically in increments in 1/8-inch. This is plenty of choice of sizes for any engineering need. At the small end of the scale, there are also 5/16 and 7/16" tubes available just in case a 1/8-inch jump is too much. These are the only size tubes available. There are no commonly available tubes in "round" metric millimeters.

When countries began the switch from inches to millimeters, it would be possible to start using 25.0mm tubes instead of 25.4-inch tubes. But then you would have to change all of the tube-making factory machines, all the building fixtures for building things, and all the pre-existing airplane and bicycle designs in the world, and even make new lugs for bikes. Even today, there is no justification for doing that just so that we can all have the small aesthetic benefit of looking at "prettier" round number of 25.0 instead of 25.4. Before widespread metrification, there was even less incentive to do it because 1-inch was actually the more standard way of measuring things anyway.

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    There never was a metrification in Italy or German [both began metric (cgs)]. France first standardised measures were metric. China and Japan adopted MKS well before they became industrialised (1925). Inches were unusual sizes everywhere but Britain at the time Mannesmann invented rotary piercing to produce seamless tubing. I think it more likely that demand by British bicycle industry drove demand and the world produced to their specs. Bikes were often imported or license built English bikes.
    – gschenk
    Jan 7 at 21:51
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    Early bicycles were made of tubes that were brazed together with lugs. Precise metric dimensions were not that important because the lugs left some space to fit the brazing and to allow for some adjustments of angles. A frame can be built with a mix of metric and imperial sized tubes.
    – Carel
    Jan 8 at 19:05

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