Originally Posted by
Kontact
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.
That makes all kinds of sense. Thanks!
Of course, that raises all sorts of geometry and weight distribution questions and how they change handling. I guess what works on my Colnago road bike is going to be very different on our tandem. Interestingly, the Colnago feels much more stable while steering quicker than the Santana tandem.