Originally Posted by
eddiearniwhatev
This thread actually answers the question pretty well:
http://www.bikeforums.net/touring/52...ng-frames.html
in particular,
"Different styles of riding require different positions... if you're a triathlete as opposed to a pure cyclist, you will ideally want a position that takes advantage of your strongest muscle groups (e.g. hamstrings iirc). As a result, TT frames have very steep seat tube angles (e.g. 78º).
Touring bikes have slacker angles primarily for positional comfort. Also, the more upright position gives you a better view, albeit at the expense of worse aerodynamics. The design also adds a little bit of flex (and therefore comfort), since the seat tube will be longer. IIRC slacker angles also uses the quads more, which is better for seated climbing."
Because the mtb I'm using as a touring bike has a very slack seat tube, and a very long wheelbase it is both comfortable and slower, because I'm using primarily my quadriceps. So I'm powering up hills very efficiently but when on the flats I'm a lot slower even if I'm exerting myself the same.
So the answer to my original question is pretty much just that.
For those interested, and as this is the bike fitting forum I think this is relevant, this article about frame geometry is of some interest as well:
http://www.cyclingabout.com/understanding-bicycle-frame-geometry/
thanks for all the advice, appreciated!
For TT and tri bikes the steep seat tube angle is to provide a position that opens the hip angle as much as possible with the back flat and level. Hence both TT and tri bikes use similar seat tube angles, even though there's no running necessary after a TT. Even with the steep seat tube angle, you still see riders "on the rivet" on TT events because the UCI limits saddle position w/r to the BB.
Your first quote also has the muscle group thing wrong: it doesn't make any difference, road or tri bike. Muscle usage is the same.
different muscles used on tri vs. road bike: newbie question
If your hip angle is open, you can use any muscle groups you want, no matter your seat tube angle or saddle setback. When my quads go out I can still pedal almost entirely with my hams and glutes and vice versa. Your pedaling technique should allow you to trade back and forth.
Your MTB is likely slower because the frame is more flexible, little or nothing to do with frame geometry. Stiffer frames are faster. MTB frames are notably flexible. Road frames are stiff. That's also the reason you find it comfortable. And of course a modern road frame will also climb much faster than any MTB.