# Sit bones and the saddle's curves

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.

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?

• 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. Oct 7, 2021 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? Oct 7, 2021 at 22:52
• 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.... Oct 7, 2021 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. Oct 8, 2021 at 0:07
• 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, 2021 at 6:15