I have been riding my road bike for about 2.5 months and I just recently raced it in my last half ironman. I noticed that my normal power ranges which I can hold on my TT bike were really tough to hit on my road bike -not only in training, but also in this last race.
I finally got a TT bike and I was anxious to see how my power measured up. Since May 21st, my weekday repeats have been along the same exact route that I repeat for each interval. I have three days worth of repeats (6 repeats total) of intervals on my road bike. The last interval set (2 total) on June 7th was completed on my TT bike. Oh, the power meter I use is a hub based Power Tap so I was using the same power meter for all these efforts. Here is what the results showed.
Observations – I had a lot less body movement while maintaining this power. The steeper seat angle allows me to open up my hip angle and stay low with much more ease.
Position Changes – On my road bike I ride at a 75 degree seat angle. Sitting in this angle and maintaining an aero position closes off the hip angle (torso and femur), resulting in a less than ideal position for maintaining continuous power. On my TT postion, the geometry of my frame allows my to ride closer to an 81 degree seat angle, resulting in a much more open hip angle and an even lower front profile. I have tight hips and hip flexors so the opening up of the angle really allows me to produce power more efficiently.