What advantages does a purpose built tri frame with a steep seat tube angle have over a frame using standard road bike geometry, but using a zero offset or ‘fast forward’ seat post to provide a steeper seat tube angle? Is there any advantage if I can duplicate the positions?
Short answer: you can usually replicate a tri position on a road frame, but the handling of the bike will be very squirrelly because the wheels are in the wrong placxe relative to your center of gravity.
Longer answer:
Your body position on a bike is controlled by three points in space: your butt, your hands on the bar or elbows in the pads, and your feet. Ignore the bike for a second and just think about your body: if you’re more flexible you can be more “folded up” than the next person, and you are said to have a tighter hip angle. If you are less flexible the angle between your torso and thigh has to be wider.
Your bike should be set up to accomodate your body by giving you the correct hip angle. If it’s not, you’ll be uncomfortable and lose power and have lower back problems and all kinds of other bad things will happen. When in doubt, a more obtuse angle is usually easier to handle, and you can always make your position more “aggressive” by bringing your torso closer to your thigh.
But you can have the same hip angle with your back vertical to the ground (as you might in a recumbant bike) or with your back horizontal to the ground (as you might on a very aggressive tri/tt setup). The general effect of being rotated further forward is that you support more of your weight on your elbows, so your upper body muscles can relax. Additionally, and more important to faster racers, you’re generally more aerodynamic because you present a smaller front profile and you’re more elongated.
A “steep” seat tube allows you to rotate your whole body forward and down, without forcing you into a very tight hip angle.
To get a position that’s rotated far foward, you need the saddle to be higher than the elbow pads and closer to the handlebar. One difference between a tri frame and a road frame is the geometry makes this arrangement easier. As you point out It’s possible to accomplish this on a road bike by using a longer seatpost with a zero or negative setback, combined with a shorter steering tube and flat stem.
However, picture the difference between a road position and a tri position. In road position, your butt is on the saddle, behind the bottom bracket, your torso is behind the head tube, and your hands are the only thing over the front wheel. On a road bike, your center of gravity is pretty far back, so you need a longer chainstay to divide your weight appropriately between the wheels. There are also steering issues that are dealt with by controlling the rake of the fork and the length of the stem.
In an aggressive tri position, your butt is almost right above the bottom bracket, your torso is spread out over the top tube, your head and elbows are over the front wheel and your hands are in front of the front wheel (well, in front of the axle.) On a road frame, that puts a LOT of your weight on the front wheel, which is one of the reasons your handling is tricky and descending is scary on a road bike with aerobars. On a tri bike, the rear wheel is brought in much closer underneath the rider. That’s why many tri specific bikes have the rear wheel faired under the seat tube. The P3 is an extreme example. The position of the front wheel, fork rake, and (ideally) stem length are also different to accomodate the way you steer when your elbows are resting on the pads.
So: yes, you can replicate the same body position on a road bike but the handling will suffer. A tri bike will get you rotated further forward without sacrificing handling and power transfer.
Lee Silverman
JackRabbit Sports
Park Slope, Brooklyn
