|POSITIONING SEAT HEIGHT
MAXIMIZE POWER, AND AVOID INJURY.
Seat height is critical for proper positioning. Coaching experts have different ways of measuring and checking seat height. Surprisingly, most methods agree within 1/3 inch or about 1 percent. By taking leg measurements and then converting these numbers to a distance from the top of the seat to the crank center will get you close. The final adjustments are done by rider feel and trial and error.
Why not use a method that gets every cyclist to the same leg angle and then start the fine tuning? Once this seat height is found the final adjustments can be focused on which injuries are likely and how the rider uses their ankles especially at the bottom of the pedal stroke.
The method described gives guidelines for positioning seat height within 1/16 of an inch and then suggest ways to adjust your seat height to maximize comfort and minimize injury.
To evaluate seat height, measure the leg angle between the center of the hip, the pivot point of the knee and the ankle bone when the crank arms are vertical and the lower foot is in its natural pedaling position. This measurement is best done on a stationary bike trainer. This method is direct and precise; it takes into account foot length, the way the rider sits on the saddle, the way the rider moves their ankle at the bottom of the stroke, and the type of shoe and pedal system in use. This method is used in the John Howard school of champions and John recommends a 27-34 degree angle. This can be measured with a goniometer or other angle measuring devices.
When taking measurements make sure that the pivot points on the body are in line with the angle measuring device. In addition, the ankle should be in its natural pedaling position. For example, if the seat height is too high the rider can pedal with the correct leg angle by keeping the heel high at the bottom of the pedal stroke. Ideally the foot should be parallel to the ground when the cranks are vertical. However, ankle position can vary and can be determined by rider preference. Isolated leg training or one legged pedaling encourages riders to adopt proper ankle movement and will help riders who have inflexible ankles or who keep there heels high at the bottom of the pedal stroke. As ankle position changes, adjustments can be made to seat height to keep the leg angle constant.
CRANK ARM LENGTH
Determining the correct crank size is simple once the physics of the bike and biomechanics of the body are understood. For every body height there is an approximate leg length and for every leg length there is an appropriate crank size. However, riding conditions and leg muscle type can affect crank choice but not by more than one crank size. The approach to take is to get an individual to a nominal crank size and then either move up or down one size if appropriate. The sizing chart in the appendix uses a math model that assumes that the area carved by the pedal circle is proportional to leg length. This model matches the crank size of hundreds of elite athletes from 5' 2" to 6' 4".
A longer crank length allows the rider to increase the distance on the push down part of the pedal stroke. One might think this should increase power but other parts of the pedal stroke could suffer negating the gain from the power part of the stroke. For example, the bottom and top parts of the pedal stroke would suffer due to your leg being extended and compressed more. In fact, a 2.5 mm increase in crank length will cause your leg to bend an extra 3-4 degrees when the leg goes from top to bottom and bottom to top. This extra extension and compression will be an added stress on your knees similar to that if your seat is either too high or too low.
A shorter crank arm allows you to accelerate quicker due to a more even distribution of force around the pedal circle. In addition, you can spin efficiently at a higher cadence which avoids shifting. This can be useful during criteriums were acceleration and sprinting are important.
For courses with lots of hills a longer crank arm is useful. When standing out of the saddle the angle your leg bends is not nearly as sharp causing your knees to be under less stress. Also much more emphasis is placed on the push down part of the pedal stroke. Another factor often overlooked is that with a longer crank you tend to spin at a lower cadence. Since hill climbing out of the saddle is usually done at about 70 rpm the transition from sitting to standing is done smoothly and without much loss of speed.
With a longer crank you tend to push a larger gear at a slower cadence. However just because you like to push big gears and ride at a slow cadence doesn't mean a longer crank is advisable. This relationship is a cause and effect relationship rather than a rule of thumb. To increase speed, you must consider how much power you gain from the push down part of the pedal stroke versus how much you lose during other parts of the pedal stroke. Whether one spins or pushes big gears does not necessarily affect crank size because one can always shift to increase cadence and thus decrease torque. In general, a fitter cyclist or one that has large muscle mass will be faster at a slower cadence. For example, the same cyclist will spin more during the off season and push bigger gears during race season.
Once the seat height and crank length are determined. The final adjustments to seat height can be focused on reducing riding stress. The knees are subject to a continual flexing of about 75 degrees. The knees are sensitive when the leg is fully extended (6 O'clock) and fully compressed (12 O'clock). Different muscles and tendons are stressed depending whether the leg is extended or compressed. A cyclist can then adjust their leg extension/seat height depending on which injuries are most likely to occur. A small change in seat height can have a large affect in preventing injury without any loss in power. Some injuries are caused due to first flexing a muscle and then requiring it to supply force to the pedals. These types of injuries are the most difficult to troubleshoot. Generally, a small change in seat position or a shorter crank arm in addition to pusher smaller gears will help. Ankle movement during pedaling will improve blood flow and may reduce cramping.
SEAT POSITION (forward to back)
Your body has an ideal seat height and seat position which determines your power output. However your body is not a perfect motor and can become fatigued. Luckily moving your seat forward or back up to 4 cm does not greatly affect power and you can be efficient in different positions. The standard way to determine seat position is to check to see if the forward knee is directly over the pivot point of the forward knee when the cranks are horizontal.
However, when descending, it is wise to be back on the saddle for more control. When climbing it is natural to be back in the saddle for many reasons: 1) Comfort -gravity tends to push you back. 2) Better transition to standing -Being back in the saddle encourages a slower cadence which allows little loss in speed and cadence as you stand. In addition being back in the saddle uses different muscles than standing while being forward uses similar muscle groups to standing. 3) Having your saddle back will prevent you from hitting it when standing.
APPENDIX (making an angle gauge)
A 30 or 150 degree angle can be created by marking a 10.0" x 5.77" right angle on a sheet of 8.5" x 11" sheet of paper. Once this right angle is formed connect the endpoints and extend the 10" line on another sheet of paper taped to the first. You now have a calibrated template which you can use to check a 36 inch bent piece of aluminum welding rod or equivalent to make sure it is precisely 30 degrees. Use the aluminum rod to precisely match all pivot points of the ankle, knee and hip. Be sure to accurately position the ankle angle because this can affect your knee angle. This is the parameter that requires the most thought because incorrect seat height can cause improper ankle movement.