All cyclists are biomechanical gyrators. Fixed gear bicycles
operate under a scleronomous holonomic constraint. Free gear
bicycles operate under a rheonomous holonomic constraint.
Gibberish, yes, but here are two real examples:
Watch Lance climb out of the saddle: he jackhammers the
bottom of the stroke just as every tyro does; this is because
human anatomy of itself cannot rotate the cranks at constant
angular velocity. The presence of a free wheel in the drive
train allows this by decoupling the forward rotation of the
rear wheel from the forward rotation of the cranks. The
effect is not so apparent when the rider is seated, but
riding a freebie, seated or standing, a roadie’s--or anyone
else’s--legs cannot catch up to the rear wheel until the
leading crank is about at two o’clock.
Then watch world class trackies, say the Uruguayan Olympic
team at Alpenrose Velodrome in 2004: their smooth loping
cadence is perfectly even, seated or standing, because their
cranks always are synchronized with the rear wheel; there is
no dead spot at the top and bottom of their stroke. This is a very big deal!
It means that the fix rider can apply force to the crank
wherever she or he chooses, and the human system of optimal
adaptive control soon will figure out how best to do so. The
fix rider can get into the power stroke at 12 o’clock, or a
little before, and so utilize the entire downward stroke for
propulsion. Way strong!
The free rider can apply downward force beginning only at two
o’clock, and so has wasted fully one-third of the downward
stroke. This is not good!
Looked at the other way, the fixer has a 50% longer power
stroke than the freebie, which equates to 50% more torque
and therefore 50% more power at a given cadence. I’m all steamed up.
There is another effect working here.
A cyclist’s legs operate like the two opposed pistons of a
steam locomotive: both generate greater forces at lower
speeds, and maximum force at “stall,” or zero revs.
As a fixed gear rider begins to climb he or she necessarily
slows, and so can apply more force to the cranks. But at
some speed the knee joints protest, and then the rider rises
out of the saddle; this is less efficient than being seated,
but more forceful. Given the 50% torque advantage of riding
fixed, one can climb grades with surprising ease and
quickness this way. Of course a lower gear allows one to
climb a steeper grade either seated or standing; this is a
matter of terrain and ability for every rider.
But a free gear rider has a necessary decision upon beginning
to climb that we fixers do not: way down in torque, she or he
wants to shift to a lower gear, usually several times; the
tendency is to chase lower and lower gears, because the lower
gearing is more than offset by the decreasing torques at the
higher cadence after each shift. Therefore she or he applies
progressively less force to the road and continues to slow.
This process stops only when the rider’s cadence has slowed
enough to regain useful torque in some lower gear; depending
upon the grade, that gear can be far removed from the initial
gear.
(Trained road-racers know about this trap: when beginning an
ascent they shift up a cog or two and stand, staying in the
big ring; when reaching terminal speed they sit, shift to the
small ring and climb efficiently--but not so smoothly and
easily as a fixer in a considerably higher gear.)
Loss of a third of one’s power stroke is a big price to pay
