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What is
your explanation?
 peeds are
traditionally measured as a unit of length per
unit of time...
- miles per hour or kilometers per
hour
- feet per second or meters per
second
- furlongs per fortnight (naah).
That can get you into trouble sometimes
(see Bucketing Bandwidth and
Train Speed). Better sometimes
to put time in the numerator.
As for "100 inches," the question is,
"100 inches per what?" At 100 inches per second, a
bicycle would be traveling 8.33
feet per second or only 5.68 miles per hour, which is
quite slow -- and curiously not variable, as if the
bicycle were built for that one solitary speed.
There must be a better explanation.
Excerpt from the Internet Version of
A
Certain Bicyclist: An Off-Beat Guide to the
Post-Petroleum Age
by Paul Niquette
Copyright ©1987 by Resource
Books, Inc.All rights reserved.
 he
bicycle, as it first appeared in the beginning of the
19th century, had no pedals. How slow and awkward this
"draught" vehicle must have been, drawn along by action
of the rider's feet against the roadway. More than fifty
years went by before pedals were invented, after which
the bicycle became what it is today, a "locomotive."
Pedals lifted the rider's feet from the
ground, and speed increased somewhat. Pedaling rate
set the limit in those early days. It still does. You
can only crank those pedals so fast.
Early designers found that for a given
pedaling rate, higher speeds could be achieved by making
the drive wheel larger. Thus, the high wheel bicycle was
invented. Better known as the "ordinary" in the 1880s,
the bicycle grew in size and picked up more speed -- the
limit this time determined by the length of the rider's
legs. Ordinaries were often custom-made to fit the rider
like a pair of shoes.
Wheel diameter became an important
specification. It was traditionally given as the first
name in identifying a particular bicycle: 52-inch
Rudge, 56-inch Victor Roadster, and the largest ever
built, the 64-inch Columbia Expert. The back wheel,
meanwhile, shrank away to save weight.
The machine was fast and majestic. It
was also dangerous and unforgiving.
Along came the chain-and-sprocket drive.
The pedal crank was taken off the front fork and given
an axle of its own in the middle of the bicycle frame.
Propulsion came from the rear wheel by means of the
chain. It was 1890, and the "safety" was born.
A few early safeties were actually
built with a large wheel in the rear. Soon somebody
noticed that the sprocket on the pedal crank and the
sprocket on the drive wheel did not have to be the
same size. For example, the bicycle designer might
consider putting a large sprocket on the pedal crank
and a small one on the rear wheel.
There was an advantage in doing so: to
be fast, the bicycle didn't need a big drive wheel
anymore. The speed of the safety depended more on
relative sprocket sizes than on the diameter of the
drive wheel.
The rear wheel then shrank back down to
about the size that we see on today's "safety." Our
question is still pending: What about those "100
inches?" We're coming to that.
ome
unit
was needed to specify the "speed" of the safety. It
wasn't so obvious anymore. One could not tell much
from wheel size. Measuring sprockets was unhandy and
not meaningful -- particularly to long-time riders of
the traditional, high-wheeled machines.
What could be better, somebody must have
thought, than expressing the safety's speed in
"ordinary" terms?
That's what the "100 inches" means -- the
equivalent wheel size of an ordinary bicycle.
You might like to figure this out for
your own bicycle. First, count the number of teeth on
your front sprocket. Let's suppose you get 36. Divide
that by the number of teeth on your rear sprocket, 12,
say. Now multiply by the diameter of your rear wheel,
which is usually 27 inches. The result, 81 in this
example, is in inches.
It represents the wheel size of a
nineteenth-century ordinary, equivalent in speed to
your bicycle.
The ten-speed safety offers the rider a
dynamic selection of sprocket combinations, representing
various ordinary wheel sizes, which range from under 20
inches to over 100 inches -- larger and faster than the
highest ordinary ever built.
For
an update on bicycle speeds, see Dérailleur
de Rigueur
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