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| The remarkable property of the Power Bounce dilatant
compound is that it is soft and pliable under slowly applied forces, but becomes stiff and elastic under rapidly applied
forces. This means that at relatively slow speeds, the dilatant-filled Power Bounce packet feels soft and cushy on the foot.
In this state, the shock of each heel strike is absorbed, resulting in a high level of comfort. As a runner’s speed
increases, the packet becomes increasingly elastic and bouncy, and more energy is returned as the heel lifts off, providing
an extra push, thereby increasing the runner’s speed and efficiency.
It is well known how dilatant
compounds such as Silly Putty flow when slowly squeezed in the hand, but bounce when dropped on the floor. Technically, this
sort of behavior is known as strain-rate sensitivity. As shown in the following figure, the material is soft and pliable under
slow application of load, or slow strain rate. At faster application of load, or high strain rate, the material becomes
increasingly elastic, as indicated by the steeper slope of the left-hand side of the fast-load. Moreover, the yield point,
i.e., the load at which the response changes from sloped (elastic) to horizontal (plastic) also increases at faster application
of load. Since the amount of elastic energy stored is equal to the area beneath the elastic portion of the curve, it is evident
that much more energy is stored during fast loading.
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Schematic Diagram Showing Strain-Rate Sensitive Elasticity . .
FREQUENTLY ASKED QUESTIONS
| Why does Power Bounce exhibit its unusual
behavior? .
The
Power Bounce dilatant compound is a boric-acid-modified silicone polymer. The silicone polymer consists of a spaghetti-like
tangle of long chain-like molecules. The polymer deforms by the chains sliding past each other. The boric acid tries to form
bonds, or cross links, between the chains which prevent them from moving relative to one another. However, the boric-acid
cross links are only temporary, and they are continually forming, breaking, and reforming. When the application of force is
slow compared to the time needed to break and reform the cross links, the chains are able to slide freely. But when the force
is applied faster than the time for the cross links to rearrange, the polymer chains are temporarily locked, and the compound
behaves elastically.
How does elasticity affect speed?
Elasticity affects speed in two important ways.
First, when the shoe behaves elastically, more energy is returned, and running becomes more efficient. Secondly, we know from
physics that the fundamental, or resonant, frequency (F) of simple harmonic oscillator (a mass connected to a spring) is given
by the expression,
F = A times square root(K/M)
where A is a constant, K is elasticity of the spring,
and M is the mass of the body. The amplitude of oscillation and energy efficiency is greatest at resonant frequency, and the
above equation shows that the resonant frequency increases with increasing elasticity, and with decreasing weight. A runner’s
resonant frequency also increases in a similar way, so that as the shoes become more elastic, the runner becomes more efficient
at a faster pace. (The above equation also explains why runners get faster when they lose weight!)
What
else are dilatant compounds used for?
Although the properties of dilatant compounds are fascinating and widely
known, the materials have remained mostly a scientific curiosity. Beyond toys like Silly Putty, limited applications have
been noted for rehabilitative hand exercise devices, and temporary sealing for leaky WW II fighter aircraft canopies.
How do shoes with Power Bounce inserts compare to other types of running shoes?
Other types of
running shoes, whether relying on foamed plastic, gel pads, air pads, or springs for cushioning, have a single, fixed response
to the impact of running. That response is designed to maximize either cushioning or bounce, but not both. The outstanding
feature of Power Bounce is that it is able to shift from maximum cushioning to maximum bounce, depending on a runner's
speed.
How durable are Power Bounce inserts?
Unlike other types of midsole cushioning material, Power
Bounce does not wear out, or lose its properties with prolonged use. In contrast, ethylene vinyl acetate (EVA) foam,
one of the oldest and most widely used insole materials, is a sponge-like plastic foam that becomes permanently compressed
with repeated usage. As it compresses, it becomes hard and loses its cushioning ability. Many running injuries can be directly
attributed to pounding the pavement on hard, compressed EVA. Gel-pad or air-bladder inserts are an improvement over EVA, but
even these tend to blowout with time, particularly in hard usage, such as downhill running. High-tech electronic and spring
devices placed in the midsole also have limited durability. |
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