Getting into position: what's the difference between bike sizing and bike fitting, and when is close-enough close-enough?
As cycling is a motion activity and a sport of repetition, should we put more emphasis on dynamic method (using movement capture hardware) to fit a bike? Or, otherwise for most riders, with the static method (using goniometer, tape, plumb line) applied judiciously, will we get within of 1 cm or less on either size of this fictional perfect bike fit?
For the most riders, will the static method be a very good starting point?
Answer from Kit:
I loved your question. It is a good one. “…will we get within 1 cm on either size of this fictional perfect bike fit?”
This question is a direct reference to “bike/frame sizing” which is different than “bike fitting.” This is a common misnomer and needs to be clarified.
Bike sizing is, exactly as you stated, finding the correct bike frame size for the cyclist. Therefore, the answer here is, “Yes, you can get within 1 cm of the correct size. ” However, the accuracy of the bike size for the cyclist is directly dependent on the SKILL and ACCURACY of the person taking the measurements.
Bike sizing , of course, also varies between brands. Frame size and geometry is dependent upon the various anthropometric measurements of the cyclist, including overall height, inseam, trunk length and arm length. Frame size/geometry is also affected by the cyclist’s ratio between arm/length/trunk length and the various limitations (including but not limited to neck, hip, etc) that can sometimes be measured by the goniometer. Therefore, bike sizing is more dependent upon static measurements but definitely can affect the dynamic motion of the cyclist on the bike.
Bike fitting, on the other hand, is the specific adaptation of the bike and its connection points (pedals/cleats, handlebars and seat) to the specific needs of the cyclist. In this case, frame size has already been decided and the bike fit now begins in earnest, especially regarding the dynamic pedaling. The best bike fits are performed with both static measurements (including the degree of forefoot varus, knee/shoulder/hip angles, laser line for the fore-aft ) AND the dynamic assessment with simple video or movement capture hardware.
You need both static measurements AND dynamic assessment from the side, front (with lasers on the knees) and posterior views. That being said, motion capture information tells the fitter WHAT the cyclist is doing on the bike (good or bad) but the fitter still has to use his/her skills to assess this motion and decide WHAT needs to change on the bike. No software gives the bike fitter instructions. Again, the quality of the bike fit is dependent on the skills of the fitter … not the type of motion capture he/she uses.
To answer the second question: “For the most riders, will the static method be a very good starting point?” Yes, the static method is a good STARTING point but the a good fitter always looks at the dynamic motion, especially the efficiency of the legs. That is where the magic of the fit happens because the legs/lower extremities (which includes the pelvis and lumbar spine) create the power that makes the bike move.
Ultimately, two things reign supreme by the end of the bike fit. 1) How does the cyclist feel on the bike? 2) Dynamically, is the cyclist more efficient on the bike? Adjust accordingly to the satisfaction of the cyclist because he/she is also your consumer.
Interesting point to think about: Does a wide stance affect the SIZE of a frame or does it affect the FIT of a bike?
Have a question for the panel? Send it to Coachespanel@competitorgroup.com
Katrina Z. “Kit” Vogel was described as one of the “rock stars of cycling science” in VeloNews in 2007. She earned her Doctorate in Physical Therapy at USC and MS in Biomechanics/Human Movement & Performance at WWU. She is the Director of Education for Bike Fit Systems, teaches clinically-based bike fitting classes and guest lectures in Biomechanics for the Univ of Wash PT Department. She is a Cat 2 track cyclist.
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