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What is your explanation?
 peeds
are traditionally measured as a unit of length per unit of time...
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miles per hour or kilometers per hour
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feet per second or meters per second
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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 8-1/3feet
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 couldn't tell very much from wheel size. Measuring sprockets
was unhandy and not meaningful -- particularly to long-time riders of the
traditional, highwheeled 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 modern 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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