My question is very specific towards bikes which have a curved top-tube. How is it decided that a particularA Cannondale Dutch Bike

height of the top tube above the ground for a set of people in a certain height range is good enough to allow easy step through? Is there any biomechanical aspect to it or is it simply decided based on a survey conducted among a large number of people in that height range?

Or is it decided purely on aesthetic reasons?

I am trying to come up with such a concept, but I am simply unable to find any strong basis for determining this particular parameter.

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    I don't think a specific height is an input design parameter. Clearly cannot be lower than the top of the chain ring. From there you take a design and see how much room you can make taking structure, aesthetics, and manufacturing cost into account. From there test the design on a target audience. Start with a mock up of a singe common size. If it works then mock up other sizes. Just use some soft tubing you can bend for the mock up(s). Clearly you could also just do some CAD models with typical human dimensions. But I would still mock and test before building a working frame. – paparazzo Sep 24 '15 at 17:42
  • To add to Frisbee’s comment: A lot of it probably depends on how important stability/stiffness and weight are. A frame with pretty much only a downtube will require strong (heavy) tubing and will still be very floppy. That’s why you’ll never see a bicycle intended for serious bicycling with such a frame (which, by the way, has a step-through height lower than the bottom bracket and chainring). – Michael Sep 24 '15 at 19:04
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    @Michael: A lot of bike share bikes have only a downtube, and while they are quite heavy, they aren't floppy. You just have to have a larger tube to compensate. – whatsisname Sep 25 '15 at 15:21
  • Heavy is in the mind of the beholder as well, unless you struggle to deal with 7.3kg. If Bram's mum can do it, so can you! – Móż Sep 27 '15 at 4:23
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    Generally the an exceptionally curved top tube is done for aesthetics, since a straighter tube would provide the same strength for less weight. The old traditional "girls bike" with the tubes parallel until near the bottom (the top tube bending back to be roughly horizontal at that point) is probably about the best compromise for weight and strength using conventional materials. (Fiber composites, of course, would be different.) – Daniel R Hicks Sep 27 '15 at 11:57

the step-through height must be no higher than the top of the pedal stroke, since the rider will need to have their foot that high to ride the bike.

The exception, obviously, is bikes for people who can't lift that high but can accept forced motion that high. But those are generally built for rehabilitation rather than primarily for transport, like the one below. Those do tend to be somewhat more flexible than is ideal, and often have quite low rider weight limits.

Sun Streamway ultra low step-through (from CyclistChic)

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    The very low step through frames, like the one in the picture, are getting more and more popular in Europe. I have seen a fleet of rental bikes in Germany which was all those very low step through frames. And in the Netherlands I am getting used to see them on a daily basis, and not just one but maybe up to 1% of the bikes around. (Numbers of bikes here are high, so specials show up more often as well.) When new they are often bought by people with special needs, second hand everybody uses them. – Willeke Sep 27 '15 at 8:53
  • Don't you really mean that there's no point in having the step-through height be lower than the top of the pedal stroke? Any reduction in top-tube height can assist someone who has difficulty mounting a regular diamond frame (or, in the old tradition, whose skirt would be "indecently" positioned by the top tube). The classical mixte frame has the top tube above the top of the stroke. – Daniel R Hicks Sep 27 '15 at 11:53
  • No, the idea is that the top of the stroke defines the highest point someone has to lift their foot. Anything higher is not a "low step through" frame. I don't think a mixte is a step through frame, it's more like "can lift your foot over if you're fit". – Móż Sep 27 '15 at 21:47

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