RE: Speed ratings

["Lin, Gary" <Gary.Lin@wang.com>]

You both are right and wrong.  A tire's greatest enemy is speed and heat
combined, with more of it coming from top speed in a straight line than from
executing turns.  You can only go so fast (80, 90?) while turning and are
not doing this all the time.  Even on a large (300') skidpad max speed would
be middle double digits or so, and the heat of friction would rise and then
reach a certain point.  You shouldn't be any farther over on the sidewall
than the edge of the tread 1/2" down the side.

But highway top speed generates more heat due to the quick flexing of the
carcass as it rolls along.  Plus centripetal forces, the major factor, come
into play above 88 mph (non-rated tires limit).  Tremendous stress is acting
upon the carcass and belts trying to make everything fly apart the faster
the speed.  Notice how a dragster tire enlarges on burnout or those truck
retreads scattered along the road (examples, but related)?  Almost all tires
above S (H, V, Z, W) have cap plies like nylon which shrink when heated,
thus tightening around the carcass and holding the belts together.

Tire speed ratings are determined by putting the tire on a dyno drum with a
simulated load and then increasing the speed until the tire self destructs
as explained above.  Coming off the highway at a constant cruise of 85-95
mph, my Yoke Ints. are pretty warm.  If you guys are going above 110, you're
cutting the safety margin on a H-rated pretty close.  Get at least a V.
Even a cold H can blow at or above 130 due to the speed, esp. if the tire is
slightly out of spec or previously damaged (hard pothole).  Plus a V will
generally have better handling at all speeds over an H.

Intro:  Resubbed after a period of too much mail.  My uncle has a '95
GTI-VR6, but I only a BMW (but it is German;-) and like VW's and am on the
GTI list.  Hope to contribute to anything technical.

Gary Lin     Burke, VA     National Capital Ch.
Metro Washington Council of Sports Car Clubs <CSP>
'88 325is lachssilber   Conforti chip, Supersprint, ///M susp., Ints., R1's

> Art Borin wrote:
> Sorry, I disagree.The primary factor in tire speed ratings is the tire's
> capacity to dissipate heat.  The faster the tire rotates, the more heat
> (friction) is generated over a period of time.  The speed ratings are for
> sustained speed, not a quick run like FlyinVR6 stated.  Thus, a H rated
> tire can withstand a claimed 144 mph blast, as long as that speed is not
> held for a long duration.
> 
> Khan Klatt wrote:
> Hi, and welcome. You may be right (I'm not an expert on these things) but
> from my conversations with a friend who is a distributor for
> Kelly/Springfield tires, I was led to believe it was the speed factor,
> that you are indeed OK to go faster than the rating, as long as you
> weren't making any significant turns (meaning a really straight road).
> 
> Perhaps we are both right, and what might build the greatest heat in a
> tire are angular friction (that caused by the gripping and turning of the
> tire against the pavement) rather than longitudinal friction (that caused
> by the tire moving in a straight line). If this is the case, you're still
> more technically right, as the turns are simply a factor of heat. If this
> isn't the case, then I've been misinformed, or it's a function of both
> factors.