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By Coach Steve
With so many tri-bike options available it can be a challenge to make an informed decision about which one to buy. There are steel, aluminum, and composite frames, traditional designs and beam bikes, shallow to steep seat tube angles, even effective seat tube angles for bikes that don't have a seat tube! Picking the right bike can be as tough as choosing an energy bar…or a coach! I'll try to help by explaining some of the variables in bike positioning, specifically how your seat positioning affects your comfort, efficiency, and most important, performance.
The seat tube angle of your bike's frame is a critical factor for positioning and performance. It's important because the angle determines your positioning relative to the bike's bottom bracket (the area that contains the bearings that your cranks rotate on). This positioning profoundly influences the dynamic of your muscle movement while pedaling. When you switch to a frame with a different seat tube angle, a change of only 3 degrees is analogous to completely changing your swim stroke or your run's stride; it will take time to adapt to, and can have either a positive, or a negative influence on your performance.
Production bike frames available offer a range from 73 degrees for a traditional 'road bike,' to 78 degrees for a full-blown 'tri-bike.' This range can make a difference in seat position of as much as 3 inches forward or back (depending on your leg length) relative to the bike's bottom bracket.
So what effect does your positioning in relation to the bike's bottom bracket have on your pedal stroke and power? Seat tube angles that are shallow (73 to 74 degrees) place you further behind the bottom bracket and favor quadriceps strength over hamstring strength. They benefit athletes who've spent significant time 'in the saddle' (bike racers) getting their bodies used to flexibility and power demands on their quads and lower back. The shallow seat tube angles also create a better position for climbing—power at lower cadences. Steep seat tube angles (75.5 to 78 degrees) put you further forward and benefit athletes who have well-developed hamstring muscles (runners). Hamstring muscles along with our gluteus (butt muscles) are the dominant muscles driving us forward when we run. When you watch an experienced cyclist at speed notice they often move forward at high effort/cadence on the flats. Because of that steep seat tubes are more appropriate for flat courses. The steeper seat tube angles also open up (increase) the angle between our torso and legs as we maintain our optimal aerodynamic position. We're able to rotate our hips forward making less of a bend through our lower backs, thus reducing the stress and fatigue to that area.
A steep seat tube angle (forward position) simulates the motion of running progressively more the further forward we place our seat. A forward position can ease the transition between bike and run, which is especially helpful in sprint distance events. But there's a downside to moving forward, a point of diminishing returns if you will...
The quadriceps is our strongest muscle, and it can store much energy in the form of glycogen (carbohydrate in storage form). Our quads apply force during the down stroke of the pedaling motion—working with gravity through our body weight. The hamstring muscle group has less power potential, and in addition must work against gravity while pulling back and up to generate power. Favoring hamstring powered pedaling tends to deplete and fatigue the same muscles during both the bike and run, so we may be able to get away with this in short events, but it will probably catch up with us to some degree during longer events.
Another consideration is variation of power requirements on different parts of a race course. While riding fast on flats, it's natural to slide forward on the saddle as we increase our momentum with greater leg speed (cadence). As we climb a hill or fight headwinds, our cadence slows and power becomes critical. This is when we need to slide back on the saddle to develop more torque. Riders with ultra-steep 78-degree seat tube angles often complain that they can't climb comfortably, especially when out-of-the-saddle. Clearly both "forward" and "traditional" shallow seat tube angles have merit and and every frame is a compromise.
I favor a seat tube angle of 75 to 76 degrees and feel it gives me a balance between top-end speed on the flats and power on climbs. In general this is the position used by bike racers for their time trial bikes, and triathlon is nothing but one long time trial unless you race junior or pro ITU draft legal races. Taller riders can move back one-half to one-degree and smaller riders should move forward proportionally. This seat tube angle adjustment accommodates body size and femur (thighbone) lengths. Most frame manufacturers do adjust seat angles for height on their stock frame sizes.
You may be saying to yourself, "I haven't seen many frames with 75-76 degree seat tubes," and you're correct. There are two ways to compensate for a seat tube angle that you feel isn't optimal. The first adjustment option is at the clamping area of your seat post to saddle rails. The clamp that holds the seat's rails allows a range of about one-and-a-half to two inches of forward or backward adjustment. The second option is a special post that's curved or 'bent.' Either of these options can effectively shift you to the position you want with a variety of seat tube angles. Of course moving your seat forward or back also affects the distance to your handlebars and aero bars, but discussing that would go beyond the scope of this article. Suffice to say, keep all of your bike's proportions (especially top tube length) in mind when purchasing a new frame or making changes to your current bike.
The majority of multisport athletes seem to be moving away from the radical forward positions of 5 to 10 years ago. I encourage experimentation in the off-season to determine what works best for you. In setting a new seat position keep this in mind: You will perform most efficiently with the positioning your body has adapted to over time. Changes force your body into a new adaptive 'learning curve' and will not be advantageous in the short-term unless your position was very inefficient prior to the change. | 
