Bike Fit Evaluation: Can It Help Diagnose And Prevent Cycling Injuries?
- Volume 19 - Issue 12 - December 2006
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Record the distance from the top edge of the book to the floor in centimeters as the inseam height. When you multiply the inseam height by 0.67, the resulting number is the frame size in terms of the seat tube height (Bike frame size (cm) = inseam height (cm) x 0.67). Accordingly, if the inseam is 86 cm, then the patient will probably fit a 58 cm road bike (86 x 0.67 = 58 cm). For fitting a mountain bike, subtract 10 cm and convert to inches to give an estimate of mountain bike frame size (58 - 10 = 48 cm or 19 inches). If there is any question about appropriate frame size, then obtain more detailed measurements that take specific bike frame geometry into consideration.
What To Consider When Evaluating The Bike Seat
Seat angle. The seat ideally should be positioned horizontal or slightly nose-up (up to 3 degrees). This promotes a stable base for the cyclist. Confirm this by placing a level on the seat. Nose-up positions can cause pressure points in the crotch area. A common misconception is to point the nose of the seat down to relieve excessive crotch pressure. This downward position transfers too much weight to the handlebars, affecting bicycle handling and increasing upper extremity fatigue and discomfort.
Seat height. This remains a controversial topic of bicycle fit. This is a critical factor because improper seat height can commonly lead to knee problems. People have commonly used various percentages of inseam height in order to determine appropriate seat height but these measurements can be confusing.
We recommend the following simple technique in determining seat height, which establishes a great starting point. After the cyclist assumes the riding position, place the heel of the bike shoe on each pedal and then have the rider pedal backwards. Adjust the seat height so the cyclist’s knees are fully extended at the bottom of pedal stroke (with the pedal crank in line with the seat tube and the heel on the pedals positioned parallel to ground). The cyclist then clips into clipless pedals with the forefoot on the pedal and again positions the foot at the bottom of pedal stroke in an identical manner as mentioned above. In this position, the knee should be flexed about 15 to 20 degrees. Check this position with a goniometer.
When evaluating the cyclist from a rearview while he or she rides on a trainer, the rider’s pelvis should be level. If the pelvis is rocking side to side in the frontal plane when pedaling backwards or forward, the seat is too high and should be adjusted. Once one has established the seat height, mark the seat tube with an indelible marker for easy future repositioning.
Seat fore-aft adjustment. To determine this, have the cyclist in the usual riding position with the crank arm in a 3 o’clock position parallel to the floor. Drop a plumb line (a thread with a nut on the end works fine) from the inferior patella and adjust the seat fore and aft on the seat rails until the plumb line bisects the pedal axle. This is referred to as the KLOPS (knee line over pedal spindle) position. To ensure this position is correct, we recommend the balance technique. From the KLOPS position, one can adjust the seat forward or backward to the point where riders can lift their hands off the handlebar and maintain the torso position without strain.
Cyclists should not feel like they are falling forward when they lift their hands off the handlebar. If they are falling forward, move the seat back until you have reached a balanced point. It is important to know that saddle rails are sloped. As the saddle moves forward, the seat height gets higher and as the seat moves backward, the seat height gets lower. Always recheck saddle height after any fore-aft seat adjustment.