John Retchford said:You miss two of Carl’s points. The first has to do with the nature of
the materials. In the railway wheel analogy the steel is behaving
elastically at the contact patch – it is being loaded and unloaded along
the same (linear) path in the load versus elongation plot. As work is
force times the distance the force moves, no net work is being done.
In the case of the bicycle tyre, you are dealing with an anelastic
material. Because its force vs elongation characteristic exhibits
hysteresis, work must be done on it during loading and unloading. The
rate of doing work is power and this power comes from the rider’s legs.
The area of the contact patch of a tyre is almost solely a function of
force on the tyre (the weight) and its pressure. This just comes from
the definition of pressure (force/area). The force supported by the
sidewalls is negligible for a feasible bicycle tyre. Try pressing a
fully deflated tyre with your thumb. So the thick walled tyre will have
essentially the same contact patch area as its thin walled counterpart
and will absorb more of the rider’s power in hysteresis loss as its
walls deform as it rolls along. Just like Carl said.
John Retchford
Alright, then I must be still missing the two points you say, because
after a review I still don't understand it. If a tire's contact is
alsmost "solely a function of force on the tyre (the weight) and its
pressure" like you say, then what difference does TPI have to do with
it? That's what we've all been discussing to date.
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