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During WW II, GE researcher James Wright
was looking for new types of synthetic rubber. By heating a mixture of silicone oil and boric acid, he inadvertantly
created a compound that we now know as Silly Putty (a registered trademark of Crayola, LLC). Materials
of this type are known as dilatant compounds. They are remarkable because they exhibit strain-rate-sensitive elastic-plastic properties. When a load
is applied slowly, they flow like a liquid. When a load is applied quickly, they behave as an elastic solid.
These unusual properties led to its popularity as a novelty toy.
Otherwise, there has been little practical use for it.
As a materials research scientist, Herb Townsend often thought about the relationships
between the composition, structure, and properties of various materials. Like many others, he was familiar with the strain-rate
sensitivity of dilatant compounds, and wondered how this unusual property could be put to practical use. Townsend also happens
to be a long-distance runner, who has won his age group in over 1000 races and is a veteran of over 60 marathons worldwide.
One day while training, it occurred to him that the strain-rate sensitivity of a dilatant would make it an ideal
material for the midsoles of running shoes.
It wasn’t until several years later when Townsend retired that
he had the time to pursue his insight. After several unsuccessful attempts to find a way to keep the dilatant compound in
place within a shoe, he developed a urethane-jacketed packet of dilatant that could be placed in the midsole beneath
the heel. Packets were placed in several old, worn-out pairs of running shoes. It was quickly apparent that the old shoes
were restored to life, actually becoming more comfortable than when they were new.
While running in the
restored shoes to test the durability of the dilatant compound's effects, Townsend further noted that workouts
seemed easier, and that his speed work times for a given distance were reduced. At
this point, it was clear that a significant breakthrough in running shoe technology had been acheived. The dilatant-compound
inserts were named Power Bounce, and patents were filed accordingly.
To better define the speed-enhancing
effects, a series of carefully controlled speed tests were conducted in which runners were given two pairs of shoes which
were identical, except that one pair had Power Bounce inserts. The runners were then timed with as they ran fixed
distances, alternating between the two types of shoe. In some of these tests, neither the runners nor the timers were told
which shoes contained the Power Bounce inserts. In most cases, the runners were significantly faster when wearing
the Power Bounce shoes. Typically, per-mile times of the test runners were reduced by 5 to 10 seconds!
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In 2004, several hundred pairs
of prototype shoes were produced in China and shipped to the US. These shoes were then fitted with Power Bounce midsole
inserts and test marketed under the name Power Bounce. An enthusiastic customer response to these prototypes
clearly demonstrated the demand for shoes with the speed-dependant elasticity provided by Power Bounce inserts.
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| Herb Townsend at Age 51 Finishing First Overall in the 1990 Great Valley Marathon in 2:51:39. |
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