Limb-length Ratios and Bike Fit
Basic sizing for a new bike purchase involves measuring inseam, torso, and arm length. If this was sufficient, I’d be broke. Limb-ratio variability is an important consideration for buying a bike, adjusting the bike you have, and understanding why basic measurements are not sufficient to ensure a good fit.
Look at the people around you. Compare the upper and lower aspects of arms, legs, and bodies. Not at all the same, right? Now find a full-length mirror, and do the same. Document your findings and use the following (basic) guidelines for current adjustments and future bike considerations.
Basic Guidelines for Limb Ratio Variance
(Remember: These are only guidelines, contingent on too many variables to follow with absolute certainty.)
Long Upper Leg (Femur) = Lower Saddle Height and Increased Nose Distance Behind Bottom Bracket.
The femur is more horizontal than vertical in the pedal stroke, allowing for a longer top-tube, cranks, and saddle further back on the rails. The longer top tube scenario can be difficult when paired with a short torso or lower arm (hence the dilemma with “women-specific sizing”). Having the saddle back on its rails can help with femur length but extends effective reach to the handlebars, requiring a shorter, higher stem. Many professionals suggest using a plumb from in front of the knee cap to the front of the pedal spindle (or tibial tuberosity to middle of the spindle) to indicate saddle positioning, but while it is a good place to start, Keith Bontrager published an article, ”Dispelling the myth of KOPS” (knee over pedal spindle), which does just that. I believe that the longitudinal saddle position should be adjusted relative to the femur length so that the hands (with the proper length and rise stem) are not showing “white”, aching, or going numb. “Actual” femoral LLD’s are more difficult to correct, though there are a number of Internet articles and a specific crank design available to help.
Long Lower Leg (Tibia/Fibula) = Higher Saddle and Decreased Nose Distance Behind Bottom Bracket
Tibia length is more vertical than horizontal in the pedal stroke, allowing for shorter top-tube, cranks, and more forward saddle position. “Actual” tibia/fibula LLD is relatively easy to correct with spacers under the same-side foot. Be careful to check vertebral rotation and pelvic angle to calculate the proper height spacer.
Longer Upper Body (Torso) = A long torso usually indicates a Longer Top-Tube, contingent on arm and leg ratios.
Longer Upper Arm (Humerus) = Lower Stem or Greater Differential
The humerus is more vertical in the pursuit and triathlon positions. Adjusting for imbalances becomes more a vertical consideration – extra arm-pad, etc. – than horizontal for these riders.
Longer Lower Arm (Ulna/Radius) = Longer Stem or Longer Top-Tube.
These bones are more horizontal than vertical, like the Femur. Limb length inequality is corrected by adjusting brake hoods, double tape, grip modification, etc.