This is an old post from Tony Peet (tpvw@hotmail.com) but it
is a pretty good answer to why reinforced sidewalls are
important:

Reinforced sidewalls are extremely important for Vanagons
because their high center of gravity and narrow track puts
excessive forces on the sidewalls! (Stand up and lean to one
side and then imagine what would happen if your
bones/ligaments etc were "mushy"!)

Some list members reported 'cracks in the sidewalls' -- that
is why it happens. The folks at Michelin have had more than a
few of the wrong tires on Vanagons returned to them with
cracked sidewalls, which can (and I don't want to be alarmist
here, but just inform) lead to what they call something
like 'bead failure' (the inability of the bead to keep the
tire on the rim I think) and resulting 'catastrophic
blowout.' Not a pretty picture, not likely to happen, but a
possibility in cases of extreme wear and something the list
needs to be more aware of in my opinion.

This is why Michelin corporate was so adamant about the
Agilis on the phone with me. One tech guy was not quite, but
nearly, hysterical.

I hope this helps, and please, let's be more conscientious
about informing people - especially new list members/owners --
 about the sidewall considerations in tire
shopping/purchasing -- irrespective of what brand they chose.

Thanks,
Chris

---- Original message ----
>Date: Mon, 25 Feb 2002 10:15:53 -0600
>From: "Terry K." <CTONLINE@WEBTV.NET>
>Subject: Re: Any air cooled owners out there running
oversized tires?
>To: vanagon@GERRY.VANAGON.COM
>
>I just have to know this----
>
>Why in the heck is this reinforced truck tire thing so
important?
>
>I had 2 ( 63,& 66) Chevy vans---car tires---
>I had a brand new 73 Ford E-250 Van---car tires---
>And had zero problems with any of them---and I know they all
weighed
>more than a Vanagon, or Westy---especially with my bike's in
the back of
>them----
>
>I have currently have 4 Cooper 205 x 70 R14's on my 85 GL,
and they are
>rated at 1540 psi., at 36 lbs.--and they are car tires----no
>problems---and the set of 4 cost only $136.00, and I've been
running
>them for 1.5 years--good wear characteritic's, good traction
in the snow
>and rain----
>
>What's the big deal?
>
>
>Later,
>
>            ______________
>           |[  ] [  ] [ ]\
>           |    |    |    |
>     ~~~ ~||-(())----(())-|
>
>Terry--
>
>       "Zoom, Zoom, Zoom"
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~