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Tension not Compression
It's easy to think of the spokes as columns supporting
the wheel and helping it retain its shape. But, the "support"
that the wheel receives is created by pulling the spokes towards the center
of the wheel (tension) rather than pushing out from the center (compression).
If you've had the occasion to hold a spoke that was removed from a wheel,
you've probably noticed how flimsy it is. You could bend one in half without
too much effort. However, if you tried to pull one apart you would not be
able to. The "pulling" of the spokes toward the center of the
hub is what gives the bicycle wheel its strength.
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Bicycle maker Paolo Salvagione
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So just how strong are bicycle wheels? "Wheels, from
what I remember, can hold about 400 times their own weight on a regular
basis and they won't collapse until roughly 700 times their own weight,
which makes them one of the strongest man-made structures on the planet,"
explained bicycle maker Paolo Salvagione.
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Pneumatic Tires
The pneumatic (or air-filled) rubber tire is something
we take for granted today. Almost every type of bicycle wheel has a pneumatic
tire on its rim. The development of the pneumatic tire was an important
landmark in the development of the modern bicycle. Prior to its invention
in 1888 by John Boyd Dunlop, bicycling was a bumpy and somewhat uncomfortable
experience. Tires were made out of leather (and later solid rubber) attached
to a wood or metal rim. The air-filled tire brought with it a smooth, comfortable,
and stable ride. It's no surprise that it also helped make bicycling more
popular.
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Are your tires fat or thin?
Depending on the type, your bicycle has either fat tires
or thin tires. Most road bikes and touring bikes have thinner tires, while
mountain bikes have big fat tires. Each type of tire has been adapted for
the surfaces they ride on.The road tires are inflated to 100 or even 120
PSI (pounds per square inch). A firm thin tire on the asphalt surface won't
flatten much. The less the tire flattens out on the bottom, the less surface
area is in contact with the road. Less contact in this case means less friction, and
more speed. This is why keeping tires properly inflated is so important.
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Wide and fat mountain bike tires flatten out more on a hard
asphalt surface. However, on a dirt trail, a mountain bike tire "floats"
on top of the rough surface. A thinner road tire would cut deep into the
dirt, forcing the cyclist to pedal her way out of a hole.
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It is easy to imagine a pneumatic tire "flattening out"
on the bottom as it rotates. But surprisingly, steel train wheels on a steel
rail experience the same effect. The temporary flattening-out of the wheel,
as well as sinkage on the contacting surface, is what leads to "rolling
resistance." This term is used to describe how much energy is "lost
to the road" as a wheel moves forward. Tires with low pressure traveling
on soft ground tend to have higher rolling resistance. This one of the major
reasons why road racing is a faster sport then mountain biking.
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Although you can't tell from this image of a locomotive, a steel wheel will
"flatten out" on a steel rail. All wheels must overcome this "rolling
resistance."
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This mountain bike tire is made of natural rubber, which some believe grips
the trail better than synthetics.
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Tire treads
The treads of mountain bike tires can affect performance.
Rough or "knobby" treads grip dirt trails better, but create greater
friction on smooth roads. Smooth tires grip smooth roads better, with less
resistance, but slip on dirt trails. Mountain bike tires manufacturers produce
a variety of different patterned nobby treads. While cyclists have different
preferences, there has been little scientific support for one tread performing
better than another.
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The Wheel
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Exploratorium
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