On Fri, 10 Jun 2005 13:50:53 +0000 (UTC), Donovan Rebbechi <[email hidden]>
wrote a response to [email hidden], which included in part:
Quoted message said:. . . Whatever shoe you get, you'll
want something with some durable cushioning. Take a look at roadrunnersports.com,
and they show a number of shoes that are suitable for heavier runners. These
tend to have higher durometer rubber, which means it's stiffer to compensate . . .
Please pardon me jumping in with some science trivia. From the Parker
Hannifin Corporation's Parker Seal Group, O-Ring Division "O-Ring Handbook"
c.1991, page A3-7:
"Hardness. Throughout the seal industry, the Type A durometer,
manufactured by the Shore Instrument Company, is the standard instrument
used to measure the harness of most rubber compounds. (The Type D
durometer is recommended where the Type A reading is over 90.)
"The durometer has a calibrated spring which forces an indentor point into
the test specimen against the resistance of the rubber. There is an
indicating scale on which the hardness is read directly. It is calibrated
to read 100 if there is no penetration, as on a flat glass or steel
surface.
" . . . [S]ofter materials, those with lower hardness readings, will flow
more easily . . . Conversely, the harder materials offer greater
resistance to flow."
(end of quote)
Having personally tested rubber samples with a Shore durometer unit I will
also note as being of interest to runners that the hardness of most
compounds changes significantly with temperature. A 70(A-scale) "neoprene"
type rubber at 75°F can measure 90A at 35°F, and 60A at 120°F
(subjectively, using the dent-it-with-your-thumbnail technique, it moves
from normal/room-temp to hard/cold to soft/hot.
There is usually high durometer thermosetting (vulcanized rubber) material
only at the bottom of a shoe for wear resistance (some ad copy calls this
"carbon rubber"😉. The high durometer material does not flow well, and
hence does not cushion. Softer elastomer or polymer material in the sole
may "flow" (pillow out to the side under load) for cushioning. Aerated
elastomer or polymer (foam) contains compressible air pockets that provide
cushioning. Plus, there seem to be as many ways to (a.) add
air/cushioning, and (b.) reintroduce rigidity to a floppy shoe sole, as
there have been enterprising/imaginative designer/engineers in the shoe
industry ( "gel", "shox", "grid" etc. ).
--
Daniel
[email hidden]