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The formulas discussed here yield starting
positions for "normal" road and mountain bike sport / performance riding.
They (or slight variations) are the foundation of virtually every fitting system
in use today.
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' FitStik 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").
For info on position tweaking, click
here.
FRAME SIZE AND SEAT HEIGHT - The "LeMond
Formulas"
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.
KNEE OVER PEDAL SPINDLE
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.
TOP TUBE AND STEM LENGTH
The FitStik can also be used for torso and arm length measurements.
For more info, click
here.
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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