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Consider two twins, a brother and a sister.

They are of identical height. They have precisely the same arm length. Their legs (both femur and tibia) are exactly the same lengths. But, as would be expected, the woman has a larger distance between her sit bones.

They purchase one bike for both of them. The man gets it adjusted to fit him with expert advice. He rides it and is delighted with the bike fit.

When the woman takes the bike, she determines the distance between her sit bones and her brother's sit bones. She calculates, given the particular curves on the saddle they have, the exact distance she needs to move the saddle forward to keep all other distances constant (see figure), in particular the leg stroke and the arm reach.

adjustment needed to bike saddle to accommodate distinct sit bones distances between otherwise identical twins

Does the thought experiment above make sense? Is this how bike fit, sit bone distance, and the curves on the saddle connect?

Context: I've been trying to convince friends and acquaintances who are newly jumping on the pandemic bike craze to skip buying a "comfort" bike with a "comfort" saddle, telling them that such a saddle is alright for 30 minutes of riding, but that the pressure on soft tissue will get worse on longer rides, and elaborating that the dominant shape of the bike saddle has these unusual curves to accommodate arbitrary distances between the sit bones of different cyclists. Rather than continue spreading misinformation, I thought I'd confirm the accuracy of my tales in this forum.

(If this is all true, who first introduced the particular curves on the bike saddle as we know it? Clearly this continues to evolve. The curves are different on different saddles. The relatively minor invention of padding the bike saddle appears to be due to Arthur L. Garford of Cleveland. It's interesting to know, in the pantheon of bike inventions, who contributed the different-sit-bone-widths-are-handled-by-just-one-bike-saddle invention.)

Secondary question: Crankarms that suit one of the two twins should suit the other, but should, ideally, the woman not also change the distance between the two crankarms (if the difference in sit bones widths is large enoug to warrant it)? Is this parameter accounted for on bikes labeled specifically a man's or a woman's bike?

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    If you look at men's vs. women's saddles, they are different not just mounted forward or back. I don't believe that your idea here is valid because there is more to being comfortable on a saddle than just the width.
    – jwh20
    Oct 7 '21 at 22:23
  • @jwh20 Makes sense, but what about unisex bikes? Are they meant to be customized at the shop with one kind of saddle or the other?
    – Sam
    Oct 7 '21 at 22:52
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    A case of theory and practice colliding in a puff of antilogic. In practice variation in peoples physical size and shape, flexibly and desire for performance vs comfort make finding ideal bike fit an exercise in Yak Shaving. You very quickly get to 'good enough' for most riders. A few tweaks over a few rides gets most of the rest, leaving a tiny fraction actually benefiting from super fine tuning bike fit. We are not all Tour riders....
    – mattnz
    Oct 7 '21 at 23:55
  • As far as seat choice of your friends and acquaintances, 'forcing' them to buy a 'proper' saddle is wrong. They know what is comfortable for them, you have no idea how a seat feels for them. Experienced cyclists often evangelize the benefits of a 'proper' saddle, and forget that until you are conditioned to them they hurt like hell. Forcing someone off a comfort saddle will as likely have them stop riding as thank you for your wisdom. By all means discuss the pros and cons, so when the comfort saddle starts to chaff and hurt too much they know that saddles come in different styles.
    – mattnz
    Oct 8 '21 at 0:07
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    I offer an alternative explanation why the curves are what they are: legs are round and the saddle is shaped so that they have room to extend downwards.
    – ojs
    Oct 8 '21 at 6:15
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This isn't how you use a bicycle saddle.

The sit bones are quite far away at the rear of the saddle. The saddle in your picture is one that is ideal for women. If a man moves the saddle backwards to have the correct width at the correct distance from handlebars and cranks, the man soon finds that the nose of the saddle is too short and the saddle feels strange. Also a saddle isn't a triangle. The shape of the contour is different at different points. So a saddle should be used only in such a manner that the sit bones are where they are intended to be.

Also this would affect the suspension of the saddle. The location where the sit bones are meant to be has full suspension, being at the midpoint between the saddle rail attachment points. If a man moves the saddle backwards, then the man sits very close to the front saddle rail attachment points. Sitting in such a manner on the saddle means there's very little suspension, so the ride would be very harsh.

The man and the woman need different width saddles. The saddle shouldn't be moved forwards / backwards.

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With the saddle being important aspect of the bike fit, I would not recommend to compromise on it. Usually bike and rider are mapped 1:1. In this sharing scenario, It is good to get an additional saddle, saddle post and a quick release lever.

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    The idea of swapping two saddle & seatpost combos with the correct fore-aft position dialled-in and correct height marked is probably the best solution.
    – Carel
    Oct 8 '21 at 9:06

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