 
What is Common
While cranks are
available in lengths from 140 mm to 185 mm, most production bicycles are
equipped with 165 or 170 mm cranks. While 165 – 170 mm cranks are
appropriate for the average rider of 5’ 8” to 5’ 10” in height, they
may not be the best choices for riders above or especially below this range.
A Starting Point
Given that the average
adult male is 70 inches tall and has a femur length of 43.2 centimeters, a 170
millimeter crank is 39.5% of the femur length. I selected this as a starting
point for calculations. 39% and 40% yield 168.5 mm and 172.8 mm crank lengths
respectively, which would appear to be an acceptable range. 38% and 41% yield
crank lengths of 164 mm and 177 mm respectively and these lengths may still be
acceptable for some riders depending on their riding style and cadence.
Why proportion crank length to femur length?

The leg and crank arm
forms a lever system. In this system, the crank arm rotates and the femur
rotates through a limited angular range. In affect, the knee is moving
mostly up and down with a small for and aft displacement. The lower leg
and foot form a linkage between the pedal axis and knee and has little
bearing on the transmission of power.


In a standing
position, the hip joint range of motion is 98 degrees or less. That is to
say, that the femur can be raised just slightly above the horizontal
position when the knee is lifted. In a normal riding position, the pelvis
and lower back are rotated forward 20 – 30 degrees relative to the legs
to compensate for the seat tube angle. The remaining range of motion
precludes raising the femur to the horizontal position without
experiencing binding at the hip joint that forces the pelvis to be lifted
from the saddle.

So why do we care about all this?

Longer cranks provide
more leverage, so you can push larger gears. These longer cranks cause a
greater angular change at the hip joint and it is this angular change that
limits the cadence that can be produced. In other words expect to spin
lower RPM’s with longer cranks


If the crank length
is excessive or outside of the acceptable range, the binding discussed
above will take place and limit your riding efficiency.


Shorter cranks while
providing less leverage, will allow you to spin higher RPM’s. This will
in most cases reduce the stress on the knee joint.


Longer cranks will
require that you use a shorter top tube length or a higher handlebar
position while shorter cranks will allow longer top tube lengths to be
used and a lower handlebar position.

Lets put this in perspective!
The shortest commonly
available cranks produced today are 165 mm. The 50th percentile female is 63.6
inches tall and has a femur length of about 38.9 cm. The 165 mm crank is about
42.5% of the 38.9 cm femur. This is like putting a 70 inch rider on 184 mm
cranks. While this crank length might be preferred by some small percentage of
riders in the 70 inch height range, it would undoubtedly be primarily for off
road use. Lets
take a look at this another way. The 50 percentile male is 68.8 inches tall,
and the 97.5 percentile female is 68.5 inches tall. The average femur length
for both groups is 42.3 cm, and a median crank length (39.5%) of 167 mm. Given
this data, I think it is fair to assume that based on averages, 165 mm cranks
are appropriate for men of 68 inches (~49 percentile, and women of about the
same height (~97 percentile) or about 27% of the adult (US) population. If we
assume that a crank length of as high as 41% of the femur length will provide
acceptable performance, those 165 mm cranks can be used by as many as 75% of
the males and 25% of the females, or about 50% of the adult (US)
population.
Bottom
Bracket Height
