Hi Lon,
What James said. I've played around with a lot of different tires on
several cars (1966 Mustang, '66 Barracuda, and '66 Dart). I in each
case optimal tire pressure varied widely based on tire brand, tire and
wheel size, weight dist, allignment settings, and whether. But the method
of identifying the correct tire pressure and/or correct alignment settings
is the same. your going for pressures that yeild roughly the same temp
across the tire. The method is to take the temps of the outside edge,
center, and inside edge of each tire. What do the temps mean?
- Same temp inside & outside edges, hotter in the middel; tire
is over inflated
- Hotter on the edges; tire is under inflated
- Hot on the inside edge: too much negative camber and/or toe-out
- Hot on the outside edge: not enough negative (hee!) camber and/or
toe-in
You can also see various premutations of incorrect pressure and alignment,
and you can sometimes crutch am alignment issue with a minor varience in
tire pressure. I also like to cross check temps by using the old poor-boy
autocrosser's trick of marking the outside edges of the tires with a dab of
white shoe polish to see how far they are rolling over during a session.
Hope this helps! As an asside my Dart which weighs about 3480 with me and
gas, and is about 53%F/47%R likes 32 Psi F/29 Psi R in its 225 60 15 Hoosier
Street TDs on 15"x8" wheels. My Mustang, which weighs 3150, about 51/49, likes
22.5/22 in its 25"x13" Goodyear Slicks on 16"x12" wheels. The Barracuda, at
3460, 50.5/49.5, with 225 50 15 Toyo RA-1s on 15"x7"s works best for autocross
at 28/28.
At 03:34 PM 9/3/08 -0400, James F Juhas wrote:
>Lon:
>
>What worked for me was specific tire testing, taking temperatures
>(looking for even across the tread) at a test day and during races.
>before that, I depended on advice from fellow racers on similar and
>dissimilar cars, estimates from the tire dealers and manufacturer, and
>in the end, the correct results were achieved through temperatures and
>confirmed with lap times. I bought a contact-probe pyrometer for a
>little over $100 from Racer Parts Wholesale and found it well worth it.
>
>You could be skating because they are too soft at 20 and you lose
>contact at the center; that happened to me running Dunlops at 26. On
>the Hoosiers, on my 2,000 lb car, I run 27/31 Front/rear cold and
>achieved better times at WG than what the dealer's recommendations did.
>
All the Best! Dave
Sr. Software Engineer/Requirements Manager/Systems Assurance Analyst
Aging Racer & Thumb Pound...er...Rule Engineer
http://www.lamehorseracing.com
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