Originally Posted by philbob57

I don't get the 90 degree shoulder-torso angle recommendation, given the wide variation in bone sizes, conditioning, flexibility, and desires from person to person.

The position of the hands on the 'bars comes from the combination of, at least, top tube length, torso size, humerus size, forearm size, shoulder width, riding style (bent back vs. bending at the pelvis), degree of elbow bend, etc., etc., etc. Add to that the way a rider wants to ride (tops, ramps, hoods, drops, upright vs aero) and the handlebars themselves (width, reach, and drop). Then there's the question of stem height, especially for those of us with quill stems which can vary from essentially slammed to Technomic 9" stems. Then, those of us who use biking to get into shape (whatever shape one wants to get into) are likely to get more flexible as we cycle more, and that can affect stem length.

I know there's at least one study that shows maximum power output when a rider's shoulders form a 90 degree angle with his body, but IIRC (can't find it now) the sample size was minuscule and averaged. Meanwhile, one of the 1st photos a web search turned up was of a woman in cycling kit, 90 degree angle at shoulders ... and straight elbows, a sure recipe for pain on roads around here.

I'd say choosing a stem that obscures the front hub when on the drops (which was the recommendation when I set my bike up 39 years ago) can work - except that I understand the current standard reco is obscuring the hub when on the hoods, and that probably has some basis in evidence.

My bet is that there's no formula for stem size that doesn't cause pain for a LOT of people.

*****

My bet is that stem lengths are chosen by manufacturers based on looks, based on the proposition that smaller stems look better on smaller bikes and bigger stems look better on bigger bikes.