If you ride with your seat too low, you lose power. This is because the amount of power your quadriceps can develop increases proportionally to leg extension (Your leg can push progressively harder as it becomes straighter). A seat position which is too low may cause excessive knee joint stress, and therefore, long term wear on your precious knee joints.

Years ago, The United States Cycling Federation (now USA Cycling) did some testing to find optimal seat height. The researchers found that power output kept increasing as the seat height was raised in small increments, but at a certain height, oxygen consumption increased abruptly. It was determined that the higher oxygen need negated the benefits of potential power increase from more leg extension above the optimal height. This seat height was at the point where the pedaling technique became sloppy, perhaps with rocking hips, or feet angled too far downward in order to make the stretch to the pedal. The exact height where energy consumption becomes disproportional to energy output takes place just above the proper seat height I described above.

Careful when changing shoes or pedal systems. Different brands of shoes or cleat/pedal designs can make as much as a 1.5 centimeter height difference. Changing bike saddles can cause a substantial height change as well. I always recommend that you take careful measurements before and after changing any parts an your bike to limit the chance of unexpected or unwanted changes.

There's an ongoing debate about how to best utilize the well-developed musculature of runners for multisport competition. The "forward" seat positioning, favored by some athletes, is an attempt to pedal in a position closer the motion of running on the bicycle. We know that running is a much more hamstring-powered movement than cycling which is traditionally a quadriceps/glutes powered activity. Most of the runners' power happens behind the runners center of gravity as the leg moves back and pushes off. It's believed, that during the cycling portion of a multisport event, if you're using similar muscle movements to running, the difficult transition from bike to run will be easier. The thought on seat positioning seems to be shifting away from this forward positioning though. The potential force lost from the quadriceps muscle group outweighs the intrinsic gains of the seat forward position As the seat position moves forward, the hamstring muscle must deliver proportionally more of our pedaling force. But, the hamstring muscles are significantly weaker than the quadriceps, which work in unison with the gluteus-maximus on the down stroke. The "quad" muscles which favor pushing from farther behind the bottom bracket, also have the additional advantage of their own weight working with gravity's downward pull. The hamstrings pull back with no gravity assists advantage, and then upwards, fighting gravity. Also, as a riders position moves progressively farther forward, the down force initiating motion of pushing forward over the top of the pedal stroke becomes progressively more difficult to apply, because of the lower legs’ angle relative to the crank arm. With the seat forward-position it also becomes increasingly difficult to climb out of the saddle effectively since the nose of the saddle hits your inner thighs, interfering with a natural rocking motion.

The foundation for fore-aft seat positioning is your frame's seat tube angle, which can either tip you forward or backward as measured in degrees from horizontal. There are many seat-tube angle choices to consider when buying a new bike (assuming a new bike is an option). The typical range is from about 73 to 78 degrees. The former is a traditional "laid back" road bike configuration, while the latter is a "radical" forward angle, allegedly suitable for multisport athletes. In my opinion a mid-range choice is best because it allows an optimal quadriceps power stroke with no off-the-saddle climbing restrictions. This range of about 74.5 degrees for the tallest riders, to about 76 degrees for the shortest riders is used by many elite cyclists for their time trial bikes.

The fore aft seat position affects bike positioning in another important way that should be mentioned. As your body moves forward in relation to the bike's bottom bracket, the angle between your hip and thigh area and torso "opens up." When this angle opens up, your lower back is spared some of the stress required in making the tight radius of an aerodynamic position. For riders with a history of tightness through the lower back and/or hamstrings, or with chronic low back injury, the forward position can afford some relief.

If your bike’s frame does not have the seat tube angle you think best for your body and riding style you can still make small changes. A saddle has rails clamped by the seat post which allow it to slide forward and back several centimeters. Special seat posts are available with a forward angle that can shift your position forward. On a medium size frame, each centimeter of fore or aft seat movement equals roughly 1 degree of seat-tube angle. Of course, if you make this change of your seat position, you must alter the stem length accordingly. Often keeping the stem length within a reasonable range can be the limiting factor on your frame. One caveat regarding making these changes on an already existing frame is that you can radically change the overall balance of the bike for the worse by shifting too much weight to the front of the bike. This can make the handling very twitchy and unpredictable.

The next step after choosing a seat height and fore aft position is to find the optimum stem length and height. And stem positioning, in turn, must be determined by how well you can flatten your back while bringing it as close as possible to horizontal. As with most adjustments on your bicycle, the position of your upper body is a compromise between speed and comfort. The ultimate position is the one that offers the least drag coefficient through the wind.

Figure 2. Effects of hip rotation on lower back and aerodynamic positioning.

Figure 2 shows three slightly different versions of bike positioning based on various angles of "hip rotation." Rider (A) shows a typical position for a rider who's not comfortable with rotating his or her hips forward on the saddle. This is easily recognizable when a rider shows a significant hump in their back. An overly long stretch of the arms in making the reach to the handlebars, on a bike that is in fact the proper size, is another visual clue. This type of positioning problem can also be caused by an overly tight lower back and hamstring combination.

A tight lower back and/or hamstrings, can be relieved by stretching, and perhaps learning to relax more effectively on the bike. As you rotate your hips forward, the pressure on your saddle moves from where the pelvic bones contact at the wider rearward part of the saddle, to fleshy areas which rest on the front of your saddle. Tipping the saddle downward a few degrees to reduce this pressure is totally acceptable. Notice the angle of the darkened hip bone area in the three variations of Figure 2. As the hips tip forward, the lower back does not have to make as tight of a radius to accommodate the horizontal aerodynamic position. The area between the arrows is the span of the spine where muscular stress originates, potentially causing problems. Rider (B) has an acceptable but not great position. Rider (C) is as low and aerodynamic on the bike as anyone can be, with a flat back and very low drag coefficient (not much frontal area to catch wind). If you're not sure you have achieved this type of forward hip rotation with a flat back, try this: Sit on the bike with your hands on the drops or aerobars. While either motionless or riding (don’t crash into each other), have a friend push down on your lower back. Try to accommodate this pressure by flattening this area of your back. It’s almost impossible to do without tipping your hips forward. So this is what you must do to be like Rider (C), It’s not a natural body position, or easy to perfect, but well worth the extra speed you’ll gain. A few riders have little or no flexibility through the lumbar area and have no choice but to hold this position.

The next step is to find the best stem length. With regular drop style handlebars, I can give a general rule for sizing. While sitting on the bike, preferably riding relaxed, look down at your front hub. When your hands are at the drop position, you should see the hub just ahead of the handlebar where its held by the stem. With your hands on the brake hoods, the view of the front hub should be blocked out by the handlebar. This is a very general guideline for stem length. With bull horn or time trial style bars, there's no easy rule for fit. With the same top tube length as we used with the drop style bar, the stem must be approximately 2 centimeters shorter. And, for the rider's position to be aerodynamic, the stem must be significantly lower. Because of this, a slightly undersize frame with relatively short head tube is required. Riders under 5'4" may need a 650c wheel bike to get front end low enough. If it's not possible to move the bars low enough in relation to the seat, it will be difficult to set up a proper aero bar position.

Consider, Chris Boardman’s position, one of the all-time greats for timed events. As you can see Boardman's position goes to the extreme, but he shows us what's possible. Chris’ shoulders are actually lower than his hips in this image. I've found that the vast majority of even moderately flexible riders, can maintain a horizontal back for the duration of an international distance event. And there’s nothing wrong with raising your stem a centimeter or two for any event over international distance, and on up to an Ironman event.

Aerodynamics is everything if you're attempting to maximize cycling speed. The weight of your bike is really of little consequence unless you're accelerating, climbing,—or having a hard time lifting it to your roof rack! Mechanical resistance is also remarkably insignificant in comparison to aerodynamic losses. But the compromise between speed potential and comfort is pivotal.

From aviation aerodynamics and the hydrodynamics of water craft, we know that increasing length without increasing frontal area improves air/water flow characteristics. So, this would lead us to believe that the more stretched out we are on our aero bars the better. But, another consideration is our power output and stress to our lower back muscles. I find that a rider given a trial and error session with a variety of aero bar adjustments, can always find a good compromise between comfort and speed. Wind tunnel trials have found evidence that the aerobar position with least drag is when arms are tipped-up about 30 degrees. Only one elite cyclist I can think of uses this position. Having your forearms level just feels right, and that's what we all gravitate to eventually. If you've have found a proper handlebar height, then aero bar height positioning must originate from that. With regular drop style bars, the aero bar should be mounted as low as possible on the top of the drop bar with no additional spacing for height. On bullhorn style time trial bars, the arm rest area needs spacing to increase height. This height adjustment must be modified by experimentation and observation of one's position while actually riding the bike. The spacing of aero bar armrest width should be set for comfort in proportion to shoulder width. The benefits of a very narrow arm position are minimal as it might reduce steering control, and perhaps in extreme narrow settings, lung expansion. Looking from the side, the aero bar length I recommend should give you an angle of 100-110 degrees with forearms level. For non-adjustable aerobars I've found that this arm position is achieved one size shorter than most manufacturers recommend.

Many of these bike fit considerations are best set be an experienced coach.