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Re: A4Q tires & Tahrs (H, Z, etc) -- sorta long
In regards to "skittishness" on grooved roads and metals bridges:
My brother is a custom tire cutter for one of the tire firms in Akron, Ohio. He
"cuts" designs for developmental tires so the engineers can real-world test
prototypes before going to the expense of making molds for them. He is not an
engineer, but has accumulated a working knowledge of some of the dynamics of
tires. Sometimes the engineers tell the cutters what they are trying to
achieve, sometimes it's just "Ummm, we'd rather not say."
According to him, several tire manufacturers (he didn't say names) were trying
to reduce the "skittish" nature of their tires on grooved roads. This was
apparently done in an attempt to get the lower priced tires accepted as OEM
tires for certain Japanese cars (Toyota, for sure, maybe Mazda, too). From the
different tire manufacturers all trying to solve a similar problem at the same
time we assumed that car manufacturers periodically spec'd their new
requirements: resistance to "skittishness" (stability/straight line tracking)
just happened to be the topic of the day. Since they generally don't "groove"
roads in Germany, "skittishness" was probably low on the spec sheet for the A4Q
I don't know how the story ended; ie which tires were most stable. I'm sure it
was a trade-off. Stability vs mileage vs treadwear vs turn-in response vs tread
width vs compound vs etc.
In regards to H-rated vs V-rated vs Z-rated Tahrs.
In Germany if your car can exceed a speed limit, then you MUST have tires that
are rated for that speed. There are exceptions for winter tires/etc. This is
why the '91 200's had the sticker warning (complete with "manuel" mis-spelled)
on the dashboard next to the ignition. Audi was covering their corporate
posteriors: they built a car capable of exceeding 130 mph/210 kph and put
H-rated tires on it. This is one reason for electronic speed governors on hi-po
I have the H-rated tires and I "try" to keep it under 130 mph. Al Powell was
correct when he said the rating was a measure of the ability to sustain the high
temperatures created by high speed driving. The centrifugal (centripetal?)
forces acting on the tire are also a factor. The ability to control the
rotational stresses changes with temperature which changes with speed (damn,
more physics/chemistry). A Z/V rated tire _officially_ is only a rating of the
ability to handle those stresses. Correct tire pressure is a HUGE factor here
-- more is GENERALLY better (nomex on!).
As has already been said: there may be H rated tires that stick better and give
better control/feel in your unique driving enviroment: wet/dry,
hot/temperate/cool climate, good/bad roads, driving style, etc.
In regards to sidewall stiffness:
I have read that low profile tires often have more flexible sidewalls then their
tall brethren. This is due to the need to have some flex/spring in the tire to
maintain a good contact patch. I'm sure Eric F. can explain it clearer than I.