Basics of Bike Fitting

Here is a brief summary of Ottawa Bike Fittings that we offer at the Ottawa Physiotherapy and Sport Clinics at our Westboro Physiotherapy location.

Fitting systems based on body measurements can provide a good starting position. To get a position really “dialed-in” to perfection, most riders will need to “tweak” the position – make small adjustments, up, forward, back, down, to accommodate their personal flexibility, range of motion, and other bio-mechanical variations.
Time-trial, triathlon, crit, downhill and other specialized riding events require different positions because the emphasis on  various criteria such as power, endurance, comfort and aerodynamics change. However, the basic principle, that the geometry of the position is a function of the rider’s anatomy remains the same. To my knowledge, these differences have not been published anywhere, but are embodied in many commercial fitting systems and fitting calculators.  
Cyclemetrics can be used both for measuring a rider’s anatomy, and position tweaking (a well-known pro dubbed it a “truing stand for your riding position”). 
The frame and seat height tables on the back of the use formulas originated by engineer, Wilfried Hüggi, and one of Greg LeMond’s cycling coaches, Cyrille Guimard. I first saw the formulas in Greg’s book, Greg LeMond’s Complete Book of Bicycling, published in 1987. For a thorough discussion of the formulas, and a broad overview of fit, Greg’s book is recommended. Further discussion of these formulas (and other generally sound fit advice) can be found on the Bike Fit Page of the Colorado Cyclist’s Web Site.

BIKE FRAME SIZE (in cm)  =  Inseam (cm) x .67 
This gives the recommended size for a road bike frame, measuring from the center of the bottom bracket to the top of the seat tube (also called center-to-top or c-t sizing)*. 

  • Larger riders (6’0″ and up) who want a frame that allows them to stretch out, may be better off selecting a frame 27-28cm less than their inseam length. In general, this will be a cm or two larger than the frames the formula recommends.
  • Mountain Bike frame sizes are generally 10 to 12 centimeters (4-5 inches) smaller than road frames.
  • On both mountain & road bikes, if it comes down to choosing between two sizes, choose the one that matches the length requirement most closely.

    I.E., bikes on either side of the suggested size allow the same saddle position, but tend to be shorter or longer than the suggested size. Choose the one which allows the rider to bend over to the desired degree. As a general rule, larger riders tend to be more comfortable with a slightly larger frame, while smaller riders often do better with a slightly smaller frame.

* The original formula used a factor of .65 times inseam to give a size measured as center-to-center (center of the bottom bracket to center of the top tube). These days many bikes use odd-sized tubing, sloping top-tubes and other non-traditional geometries, so center-to-center sizing is less meaningful. Accordingly, the factor was adjusted slightly to yield the center-to-top. Remember: if the length of the top-tube is right, you can usually adjust the seat height and setback to obtain a good fit. 

SEAT HEIGHT (cm) = Inseam (cm)  x .883

This formula assumes that at the bottom of the pedal stroke, the knees should be slightly bent, about 15 degrees. Riders who pedal slightly toe-down will find the resulting number a bit short. Riders with reduced flexibility or other special considerations want the saddle slightly lower to start (This tends to change as the rider gains experience).
In general, the resulting number will be within a cm or two of “correct”. So, use this as a starting point. Many riders may never need to alter this setting. For the rest, make small adjustments as needed. Use the FitStik to track changes so you can return to a previous setting if need be.
While promoted as Natural Law by some, and Hype by others, the consensus seems to be that your knee should be more or less over the pedal spindle when the cranks are level. Truth is, the “rule” evolved because most riders find that this setting works for them. If it doesn’t work for a particular rider, so be it. As always, if the rider doesn’t like it, it’s wrong…
Note: Most fitting systems first set the saddle height, then position the knee by sliding the saddle fore and aft. Catch is, when you slide the saddle back and forth, it also moves up and down because the rails are angled in relation to the saddle’s top. But when you move the saddle up and down, it also moves back and forth because the seat tube is angled. In other words, it takes some trial and error to get the saddle in exactly the right spot. 
The FitStik can also be used for torso and arm length measurements.  Many well-known formulas convert torso and arm length measurements to total reach (length of top tube + stem). One such common formula, for a  road bike sport/racing position is:

     ( (Torso Length + Arm Length) / 2) + 4  = ( Top Tube + Stem)


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