Longer or shorter cranks can change
the geometry and range of motion of muscles. How does this affect
power?
Say a rider is contemplating changing crank length.
What does the Pedaling Model say about different crank lengths and power?
For example, say a rider contemplates that shorter cranks would allow a
quicker cadence. A rider can make assumptions regarding how shorter
cranks change the Strength Functions and test the effect on Power using
the Pedaling Model, all without having to learn to pedal differently just
to see if power improves.
Thigh and Shin Strength Functions depend on cadence and
range of motion. A change in crank length changes the geometry and hence
changes this range of motion. If it is assumed that, while geometry changes,
the magnitude of the Strength Functions stay the same over the new range
(a reasonable assumption for relatively small changes in range of motion),
how does this affect Power output? The following table shows the power for
different Crank Lengths.
It may be that a different range of motion does change
the magnitude of the Thigh and Shin Strength Functions. Questions such as
this can be tested here. Of course, there are other considerations
as well such as appropriateness of the range of motion considering a riders
size and what may or may not be comfortable.
Figure 3. A "Sprint" Crank Length
In a sprint context force to the pedals tends to be applied
as "impulses." That is to say that the Thigh and Shin Strength
Functions are "narrow and pointed," much as shown in Figure 3.
What happens if a rider changes the Crank Length while using such Thigh
and Shin Strength Functions?
Using "narrower and high" Fit Points (Thigh Extensor
Fit Points of {0.,20.,60,300.,1000, 300.,60,20.,0.} and Shin Extensor Fit
points of {0.,2.8,8.4,42.,140.,42.,8.4,2.8,0.} and default values for other
Fit Points) gives the
results in the following table:
Other considerations may affect choice of crank lengths.
Cranks lengths in track tend to be short due to limits imposed by track
banking and minimum speeds.
The numbers in the above examples were selected for illustrative
purposes; your values could be very different.
Sample Output:
The table below shows examples of range motion for different
crank lengths (refer to Figure 1 for nomenclature):
