Thread: Frame
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Old 07-31-17, 08:31 AM
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Location: Upstate NY
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Bikes: Bianchi San Mateo and a few others

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There is no single, uniform standard.

Traditionally, on road bikes with horizontal top tubes, frame size was measured along the seat tube. Everyone pretty much agreed that the low point was the center of the bottom bracket shell, but the top end of the measurement could be the center of the top tube, the top of the top tube, or the top of the seat tube. And because of geometry differences between makes and models, two bikes that are listed as the same size might fit differently. Their top tubes could be different lengths, for example.

On more modern road bikes with sloping or curved top tubes, numeric sizes generally aren't measurements, but equivalents to bikes with horizontal top tubes as rhenning mentioned above. My 2006 Bianchi has a sloping top tube, so we can use it as an example. It's a 53 cm frame, but its seat tube is only 50 cm long. If I was to draw an imaginary horizontal line from the junction of the top tube and the head tube back to the seatpost, that line would probably be about 53 cm from the center of the bottom bracket.

Some manufacturers don't even bother with 'virtual' or 'effective' measurements and just label them small/medium/large, etc.

...and if we're talking about bikes other than road bikes, different sizing conventions may apply. For example, road bike frame sizes haven't been listed in inches for a few decades now, but it's still the norm for mountain bikes.
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