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The Perfect Bike Fit

You can’t do anything to change the dimensions of your body, but there's a lot you can do to custom fit your bike to you. Not everyone can afford to have a bike custom built, but there are probably more ways than you realize to modify the bike you already have. With a few minor adjustments, and perhaps a new part or two, your "ride" can fit like that bike your buddy just paid four-thousand bucks for. Here's a list of possible adjustments to your bike: Seat height, seat front-to-back adjustment, seat tilt, stem height, handlebar angle, brake lever positioning, and last but certainly not least, the cleat position on your cycling shoes. Here are some items that can be replaced to make major changes to your bike's fit: a different seat, longer seat post, longer or shorter stem, stem that angles up or down different type or width of handlebar, different type or size of aero bar, and longer or shorter cranks.

Setting a proper seat height is the first step. The correct height is quite easy to define since there's a widely accepted rule that works well for everyone. Generally, the optimal position is to set your seat height at the highest possible level, while still maintaining a smooth spin with absolutely no rocking of the hips, or feet reaching excessively to make full extension of the pedal stroke. There are all sorts of measurements and numerical computations to try, but by far the easiest and most sensible way to determine seat height is as follows: Get dressed in your biking outfit and sit on the bike with bike shoes on and clicked into the pedals. Then, extend your leg to the farthest reach of the crank on either side (crank arm about 5 degrees forward of vertical, in-line with your seatube). Lock your knee back, putting your leg at full extension and check the angle of your foot relative to the ground. Your foot should be parallel to the ground, with both your heel and ball of foot at the same level. It's as easy as that. You can do this by yourself while coasting past a reflecting plate-glass window or you can lean against something solid and have someone else take a look. Check and double check on both sides, and make sure you aren’t rocking your hips while over stretching to the bottom. Your foot angle from the side should look like the ideal seat height/foot position in Figure 1. When you examine Figure 1, this simple technique of seat height determination makes perfect sense. We know that as you pedal and your leg is at the fullest extension of the stroke, we want just a very slight downward angle of the foot. This slight pointing down of the foot shortens your effective leg length just enough to give a few degrees of knee bend at the bottom of the pedal stroke. When compared to the foot flat, knee locked, leg fully extended position, which we used as our reference to set the seat height, the foot tipped slightly down with the knee slightly bent relationship makes sense. Also, 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.

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). This was thoroughly discussed in the energy transfer/pedaling force article (check it out). Also, some experts contend that 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 did some in depth sports science research on optimal seat height. The researchers found that power output kept increasing as the seat height was raised in small increments, but at a certain critical height, oxygen consumption increased abruptly. It was determined that the higher oxygen need negated the benefits of potential power increase from anymore 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.

Determining Seat Height at Full Leg Extension

Figure 1. Proper angle of foot with leg fully extended (knee joint locked back), at lowest point of pedal stroke.

There is an ongoing debate about how to best utilize the well developed musculature of runners for multisport competition. The origin of the "forward" seat positioning, as accepted by many multisport athletes, is an attempt to reproduce 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 powered activity. Much of the runners movement is "behind" the runners center of gravity as the leg moves back Also 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. Personally, I believe the latter to be correct, especially in "sprint" distance events. Over the last five years though, the thought on seat positioning seems to be shifting away from this forward positioning. The latest thinking is that the great amount of potential force lost from the quadriceps muscle group outweighs the intrinsic gains of the seat forward position. You see, we do know, through basic physiology, that 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 downstroke. 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 backwards with neutral gravity, 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 virtually impossible to supply, because of the lower legs’ angle relative to the crank arm. With the seat forward-position, it becomes increasingly difficult to climb out of the saddle effectively since the nose of the saddle interferes with the bike's natural rocking motion while climbing, as it hits your inner thighs.

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 72.5 to 78 degrees. The former is a traditional "laid back" road bike configuration for a tall rider, and the latter is a "radical" forward angle, suitable for multisport athletes only. 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 typically used by elite cyclists for time trial efforts.

The fore aft seat position has another strong effect on bike positioning that should be mentioned. This is your hip/lower back angle. You see, 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 making the tight radius of an aerodynamic position. For riders with a history of tightness through the lower back, 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, there am ways to make small changes. A typical saddle has rails clamped down by the seat post which allow it to slide forward at back (depending where it started, of course). Special seat posts are available with a forward angle that can shift your position as much as 4 centimeters. 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 reasonable can be the limiting factor on your original 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. Especially risky is 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 fine balance speed and comfort. The ultimate position is the one that offers the least drag coefficient through the wind. Consider, Chris Boardman’s, (former World Pursuit Champion and current Olympic Individual Time Trial bronze medallist) position. Chris’ shoulders are actually lower than his hips. Perhaps Chris should wear a sign that says: "Don't try this at home, it could be hazardous to your health." Boardman's position goes to the extreme, but he shows us what's possible. 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. 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.

Effect of Forward Hip Rotation on  Positioning

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 only 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 more 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 is totally acceptable, and may relieve some of the pressure caused by this forward hip rotation. 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 fantastic 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.

The next step is to zero-in on the proper 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 upper part of the bars, the view of the front hub should be blocked out by the handlebar. This is a very general guideline for stem length, but it should give a measurement "in the ballpark." 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 is often required. 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 areo bar position.

Aerodynamics is everything if you're attempting to maximize cycling speed. The weight of your bike is really of no consequence unless you are accelerating, climbing, or having a hard time lifting it to your roof rack! In multisport racing, which is predominantly flat steady speed time trialing, we don’t do much sprinting or climbing. Mechanical resistance is also remarkably insignificant in comparison to aerodynamic losses or gains. Aerobar position is another subjective positioning consideration that's difficult to define. From aviation aerodynamics and the hydrodynamics of water craft, we know that to increase length without increasing girth (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. An example of this is wind tunnel testing results, versus the consensus among aero bar users. The wind tunnel trials found unequivocal evidence that the aero bar position with least drag is when they are tipped-up about 30 degrees. Not a single elite cyclist or triathlete uses this position though. 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 come 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 time trial style ban, the area bars do need some sort of spacing to increase height. This height adjustment must be modified by experimentation and observation of one's position while riding the bike. The spacing width of the aero bar armrests should be set for comfort in proportion to shoulder width. I feel that the benefits of a very narrow arm position are over-rated, it reduces steering control, and perhaps in extreme narrow settings, lung expansion. The aero bar length I recommend should give you a upper arm position similar to that which appears in either Figure 1 or 2 (upper arm angled forward 10 to 15 degrees). I've found that this arm position is achieved with one size shorter aero bar then the manufacturers recommend.

This leaves cleat position, and this can literally make or break - your knees. There are so many options for the foot/pedal relationship that there are rarely any reasons for unwanted stress here. Several pedal types allow for lateral rotation and that's the key to healthy knees. In the old days, we had to nail cleats to leather shoes. What a pain. Once those cleats were nailed we were committed! But, now for the front to back adjustment you should just find a spot somewhere in the middle of the range and make sure both shoes are the same. Make sure there is minimal yet adequate clearance between crank arm and your shoe. Too far away is awkward, and if the shoe touches the crank it will eventually wear a grove there, and the crank may break at that spot. If your pedal system has no "floating" capability then you must take some time to get then initial cleat setting just right. Analyze the natural positioning of your feet. Do they toe in or out? Set your cleats appropriately. Some types of pedal systems are capable of accepting an increase or reduction of "height," which is invaluable for people with leg length discrepancies. Take advantage of this opportunity if you need it. It works well for me!