Terry,
     I'm no rocket scientist, or materials scientist, but my guess is the
plys could be a different weave or density (like thicker plys) or there could
be more rubber involved.  It's like Kevlar -- you can make fishing line or
bullet-proof vests, depending on how you apply it.  I wish I did know more
about tire manufacturing to make even more informed decisions.  Maybe someone
on the list who worked for a tire co can fill us in.  As it stands now, the
tires I get from Big O Tires have a sidewall that feels stiff even when it's
off the rim.  These are tall skinny tires, too.  The guy at the shop says
they sell them mostly to pickup truck owners with 'dualies' on the back.  The
handling and wind-sway resistance is incredibly different than the little car
tires that came with the van.

PS: I actually did take some rocket science classes in college :)

Bill Marshall
85 GL Tiico