Originally Posted by
PromptCritical
I'm curious. The rule of thumb is that the knee should be directly above the pedal spindle when the knee is bent 90º and the cranks are parallel to the ground and seat is properly set fore and aft. This is true on my road bike that has been professionally fit. Our tandem has more relaxed seat post angles which puts the seats further back and our knees are bent about 70º when the cranks are parallel to the ground.
Both bikes fit well and we have no pain or discomfort.
Is this theory only applicable to some bikes?
The rule of thumb is for fast road positions. 90º isn't part of it - the location comes from having the correct saddle height and then moving in an arc around the BB until the knee happens to be above the spindle with the cranks level.
This position is really just a forward limit - it is the furthest forward you can put your saddle before you start to fall forward onto the handlebars. That's why it is favored by racers - because it gives the position with the flattest back and lowest handlebars.
Otherwise, anything further back also works, but a similar waist bend means the bars are going to have to be higher, which is less aero. And that's how many tourists sit on their bikes.
As far as seat tube angles, they should be ignored. You locate the seat where it works for you. But it does make sense that the touring bike may have a more relaxed STA than a race bike.