I have been riding for a while now on long, proportional length cranks.
My performance level has increased since the switch from 175 to 195. I
measured my performance on a climb, and also have felt stronger, and
have received unsolicited comments from 3 strong riders in our club who
noticed I have "kicked it up a notch." There are certainly many other
factors involved, but I still believe that the switch was a major
contributor to my improvement. Whether it is a direct result of
improved efficiency, a result of a more comfortable position on the
bike, or just from chemicals in my head, I don't know. But I have
though about it quite a bit, and here is my conclusion.
Crank-length is much like stem length. It is a variable parameter that
has both a comfort component, and a speed component. While both
parameters need to be considered in choosing a stem or crank length,
one does not necessarily come at the expense of another.
Stem length is used to get a comfortable position, and to help put the
rider into an aerodynamic position to make best use of available power.
There are many factors in determining stem length, and there is
probably no such thing as an optimal stem length for any given rider.
In general shorter riders use shorter stems, and taller riders use
longer stems, but there are many exceptions to this which are not
deemed strange. Also, most people pay no attention to the stem length
and do just fine on the one that came with their bike.
Crank-length I believe falls into the same category. Most people are
comfortable in the standard range, and as a general observation, larger
people can make use of longer cranks. And as with stems, most people
pay no attention to crank-length and do just fine.
Crank-length is both a comfort issue, and a power production issue. And
as with stem length these two do not necessarily detract from each
other.
In an earlier thread on the same subject, Ron Ruff did an interesting
experiment trying different cadences on a hill. His results showed
pretty much what has been shown elsewhere that power isn't really
affected that much by cadence, cadence, gearing, and most likely
crank-length. But he also noted that certain cadences didn't "feel"
right. That got me thinking. And after much thought, I think that
cadence, crank-length, pedal speed, and gearing are of no interest on
their own. They are only of interest as secondary indicators. The only
factors that are actually of interest are force applied to the pedals,
and muscle contraction speed. Gearing, cadence, and crank-length are
just factors juggled to get a chosen power output from a comfortable
combination of applied force and muscle contraction speed.
When bikes with gears are ridden, riders use the gears to regulate
their cadence. Cadence remains more or less constant, while force
applied to the pedals varied greatly dependent upon speed. People are
apparenlty much more comfortable changing force than cadence. While a
given cadence may feel too hard or light, I beleive we are more likely
to perceive a cadence as too fast or too slow. We are much more
flexible when it comes to force than cadence.
Cadence can also be thought of as muscle contraction speed. How muscle
contraction speed is measured, I don't know but it the context of
cycling, I guess we could say that the speed of angular change of the
knee during a power stroke would work. Say (to keep it simple) a
rider's knee bends 90 deg from top of the stroke to bottom of the
stroke. At a cadence of 90 this is a "speed" of 270DEG/second. That is
a strange unit, but we can at least compare it to another situation.
Say the rider only bends their knee 85 degrees in a power stroke (due
to short cranks). This means at 90 cadence they have a "speed" of
255DEG/sec. About a 5% difference. Suppose this speed is outside of
what is comfortable. The rider must increase the cadence to get the
angular speed to something comfortable. But they then must adjust
gearing to compensate for the increased pedal speed, and the gear feels
too light so maybe they just up the power and ride faster instead, but
it doesn't really matter. This is fine until the riders reaches max
power and is not able to find a comfortable balance between angular
speed (and thus pedal speed), and force to the pedals which gives a
balanced equation with available max power.
Riders with long (possibly heavy) legs that are riding relatively short
cranks at the same cadence as a shorter rider, are using their muscles
more slowly than the shorter rider. To get the same muscle speed they
would need to increase their cadence which would feel odd, and perhaps
become inefficient just from moving the mass around with no other
benefit other than canceling an otherwise sub optimal situation. Crank
length can be used to effect changes in muscle contractrion speed while
keeping cadence within a narrow band. This may be of interest to riders
of all sizes, but I suppose in general it means (much as with stems)
taller riders will want longer ones, and shorter riders will want
shorter ones. But it is more tha height. It is a preference of the
balnce of the involved factors, which perhaps transcend something as
simple as height, and perhaps have to do with body or muscle type, etc.
In short, I think my 195's allow me to use my muscles at the same rate
as shorter riders without having to adjust my cadence. Maintaining the
comfortable balance between muscle contraction speed and thus pedal
speed (as a result of cadence) and pedal force allows me to use a
higher gear than otherwise which results in higher speed. In general I
am comfortable applying more force to a pedal than a shorter rider of
similar condition, but I am not comfortable applying that force at a
different rate. Using a sub-optimal rate wastes my ability to otherwise
apply more force by forcing me to compensate by increasing my cadence
which has a very narrow band of comfort (and I presume efficiency).
Achieving an optimal rate without otherwise resorting to sub-optimal
cadences allows me to take advantage of my ability to generate more
force.
This rant reminds me of a quote from "12 Monkeys":
"Science isn't an exact science with these clowns!"
Joseph