Crank Length, Leg Length and Power
The biggest misconception about short cranks is that you will lose power.
But for smaller riders, it's long cranks that slow you down.
A good rule of thumb for shorter, non-triathletes, with no knee, hip or other issues is 21.3-21.6% of inseam.
Muscles have a sweet spot, typically at about 120% of their resting
length, where they can apply maximum tension. 170-175mm cranks are the
standard because they work well for average height male racers. For a
smaller rider on these lengths, the hip and knee angles in the top part
of the pedal circle are much more acute than for larger riders. So your
glutes and quads are stretched too long, and your hip flexors &
hamstrings are too short, to efficiently apply force, through much of
the top half of pedal circle.
(The 5'3" rider looks larger in this photo, due to her small, 650c wheels.)
Power is basically F x L x C
F = AVERAGE force applied to the pedal at 90 degrees to the crankarm, throughout the circle.
L = Leverage (Crank length)
C = Cadence (RPM)
The key here is AVERAGE force.
Everyone seems hung up on leverage. But if cranks are
too long, for you, they will cause you to be ineffective
through much of the top part of the pedal circle.
Your peak pull on the chain will be higher due to more
leverage. But the average will be lower due to a
longer, deader "Dead Spot".
With cranks that allow you to put power out through more or all of
the circle, the increase in "F" will more than make up for a
slight reduction in "L". And since cranks that are too long inhibit the
rider’s ability to spin, there is usually an increase in "C", cadence,
As one 5'-2” (157.5cm) woman wrote after getting
150s "So THIS is how a correct pedal stroke is supposed to
feel!" I felt like I had power for the entire stroke instead
of just the bottom half. Riding the Texas hill country was
interesting, too. While others were mashing, I was able
to spin up with a fairly high cadence. Now on training rides I
compete for hill sprints instead of struggling to hang onto the back of
Hills and Headwinds
With shorties your PEAK chain tensions are lower but the length and
intensity of the "Dead Spot" is far less. Much less if your old cranks
were too long.
This has special implications for high resistance situations such as climbing, headwinds, sand or snow.
At each dead spot you slow down a little (or a lot if your cranks
are too long) and at each peak you speed up a little. These
accelerations use up energy that is wasted because it
contributes nothing to your average speed. This is masked by
inertia on the level but very noticeable climbing. I've had several
triathletes report setting personal bests, getting their first
podium finish and even winning a world championship qualifier in their
first event with shorties. And nearly every time they say "There
were headwinds and the course was hillier than usual".
The most common comment I get, from geriatrics on adult trikes to triathletes and MTB'ers is improved climbing.