In the article Get Fit! I outlined my four crucial parameters for bike fit: saddle height, saddle set back, handlebar reach, and handlebar drop. In this article I want to take that one step further and talk about how to use those same guidelines to dial in your time trial position, either on your road bike or on a dedicated time trial bike.
When Boone Lennon first pioneered clip-on or "tri-bars" in the late '80's, his initial model and inspiration was downhill ski racers: head low, back flat, arms together and pointed up in front. He almost had it right; it was certainly an improvement, and evidenced most convincingly by Greg Lemond's barnstorming win in the final time trial of the '89 tour, but it was only the first step towards a real understanding of the ideal aerodynamic position on the bicycle. What worked for skiers was close, but didn't translate perfectly to what would work for cyclists. Over the next decade and many, many hours of wind tunnel and wattage testing later, the "perfect" aero position is still evolving, but has changed considerably, most notably in the position of the arms.
How you approach your time trial position will depend on a number of things: do you have a dedicated TT bike, or will you be modifying your normal road bike? Will you be able to train in your TT position regularly? How many TT's will you do? Is it a focus, or is it something you're simply trying to make the best of? Whichever your approach, the way to visualize changing your normal road position to a TT position is to view it pivoting around your crank arms as the center point of a circle. If you're set up well in your road position, imagine rotating that exact position forward with your feet in the same place: your saddle comes up and forward, your back becomes more horizontal, and your head and arms go forward and down. The question is just how far forward can you rotate without compromising your pedaling power or setting up a position that goes beyond the UCI rules, if that's a concern for you.
There are two extremes you can take as your starting point. Let's assume you despise time trialing, and only do it when you have to as part of a stage race. You don't train for it, you don't care about it, and you certainly don't have a separate bike set up to focus on it. But you also hate losing all that time in the stage races every time you have to do it. At the very least, you'd like to limit your loses or do the best you can with the equipment you have. What can you do to maximize your position with these parameters?
From this perspective, I'd expect to simply modify someone's position on their road bike, and would be looking to keep them in that position as much as possible. That means there would be a minimum of forward rotation. If for whatever reason you can't or won't be able to train in the TT position, it's never a benefit to go for some extreme forward and low set up just for the day of the time trial. The increased discomfort and inefficient pedaling will counteract any increase in aerodynamic advantage.
At this extreme, I wouldn't change a riders saddle height or set back at all. I would leave the rear of the position in place, but then see how much the front of the position can be made ideal with addition of aero bars. With your road bars in their normal place and the addition of clip-on bars, see how comfortable you are. Your goal is to get the front end as narrow and low as you comfortably can, without changing the way you sit on the saddle. Gradually lower your handlebars to the limit of what your bike will allow. If you find your breathing compromised or that your thighs begin to make contact with your abdomen, then you're at the limit of what you can do.
You might stop there, or you can start to change the rear of the position to accommodate a lower front end. If you can train in this position and have time to adapt, you can start slide the saddle forward and up to allow yourself to go as low as your road bike will allow you up front. One low-cost quick fix is to have a second seat and seatpost that you use just for time trials. This way you can experiment with your TT position without worrying about not being able to get back to your ideal road set up, and you can switch back and forth quickly when you want to do a training session or local TT. By the same token you could have a second stem that has some drop to it and allows you to get lower, or is longer or shorter to change your reach, without recabling your bike.
If you're at the other end of the spectrum and time trialling is a focus, then your approach will work the opposite way. Rather than basing things around the rear of the bike and working forward, you can start with the front and work back. How low does your TT bike let you get up front? Start there, and see how high and how forward you have to set your saddle position to let you ride comfortably in the bars. That doesn't mean you'll end up in this position; you might find that there's too much weight on your front wheel and is unsafe, or that you loose too much power and have removed your hamstrings from the pedaling equation completely. The UCI rule for saddle set back is that the nose of the saddle must be at least 5 cm behind the vertical plane of the bottom bracket spindle. If you're going to do any major races, then you know that's the limit of how far forward you can go. If not, you might go so extreme as to have the nose in the same plane as the bottom bracket. That's really the limit of what you should push. The key is to find a balance between what's aerodynamic and what's biomechanically optimal.
Study photos of the top riders in their TT positions. Many riderset up by aero expert John Cobb sit very upright with their arms ever so slightly up, whereas others able to sit very far forward and rotated down, so that their heads are almost lower than their hips, and their arms are level or almost slightly pointing down. If you can sit on a trainer in front of a mirror, you can tweak things and actually look at how the changes affect your profile, then see if it's a position you can ride in comfortably or adapt to. If you have a way to measure power, then you can experiment with different positions and see how it effects your wattage at a set heart rate.
Remember that no amount of riding around easy in your TT position will give you a feel for how it will be in a racing situation. The only true test is to get out and ride it at the same effort you'll make in the race. Your goal here is to look for the balance between increased aerodynamics as you can best estimate without a wind tunnel, versus a minimal loss of pedaling power. For someone who rarely rides a TT, that might mean staying in their basic road set up. For someone focusing on the race against the clock, it might mean a dedicated bike rotated 5 or more centimeters forward from their road position, and weekly training sessions in that position.