My first local fixed gear street ride on FGG 2219 was a
familiar route, in a 66 inch gear. Climb like that in 66
inches? Must be some mistake! So I re-rode the route in my
nice 27 speed Campy equipped road machine...yes, 44 inch gear
for the same climbing ability as the fixer!
Now Dick DiGennaro, Mr. Solo Velo, who had built the frame
and persuaded me to try riding fixed on street and road, is a
very nice guy. Besides being a master frame builder he works
every day with crazy physicists at Lawrence Berkeley
Laboratory, which is why we get along well. To return some
part of the great favor he had shown me I decided to work out
the peculiar and perplexing physics of fixing. It took two
years of meditating on the subtleties of classical mechanics
to ascertain a set of simple truths:
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.