Bike Fit Evaluation: Can It Help Diagnose And Prevent Cycling Injuries?

By Richard T. Bouché, DPM, Peter M. Vincent, DPM, and Katrina Sullivan, DPM

How To Assess Frame Size

    There are several key factors to consider in doing a cursory bike fit evaluation. By considering each of these factors and making appropriate adjustments as necessary, one can help optimize patient comfort and efficiency on the bike.     Frame size. One should consider this factor, especially if it is obvious during the initial observation that the bike frame is either too large or too small. Two methods of determining proper frame size include crotch clearance and use of the inseam measurement to determine seat-tube height. Crotch clearance applies to most road bikes with a horizontal top tube. (This method will not work for bikes with angled top tubes.)     While patients straddle their bike in bare feet, there should be at least 3/8-inch to 1 inch of clearance between the top tube and the cyclist’s crotch. On most road bikes, one can measure frame size in terms of seat-tube height from the center of bottom bracket to the top of seat tube. This is called center-to-top or c-t sizing. To determine seat-tube height, measure the inseam height. Have the cyclist stand barefoot with his or her back against a wall and his or her feet 6 inches apart. Place a book between the legs and push it firmly up into the crotch area to simulate saddle pressure while riding.     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.

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