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Bicycle
tires are composite materials with a rubber matrix in which fabric and tire
cords are present to define and maintain a shape. The rubber can stretch when
it is placed in tension or compression, and the diagram shows the result
of a tensile cycle in which the material is stretched and then allowed to relax.
The blue lines on the graph represent the shape of the rubber macromolecules
at two stages of stretch. In the relaxed state the molecules are "tangled,"
but as the material is strained the molecules tend to straighten.
This conformation change is responsible for the elastic response of the
rubber.
An important feature of the curve is that the path followed in
stretching is not the same as the path followed in unloading (see the small
arrows on the graph). The area inside the curve (yellow) measures the work done
on a unit volume of the material as it is taken through the stress/strain
cycle. As a wheel rotates, each section of the tire that contacts the ground
is deformed in a similar manner and the work of deformation has to be provided
by the bike rider and becomes a component of the rolling resistance. To
minimize the tire deflection and reduce this energy loss, high tire pressures
should be used. |
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