Not having a temp probe I used the Autocrossers guide... set the front tire
pressures such that at the extreme you're not rolling onto the sidewalls,
then set the rear pressures to get the handling balance that you want. You
judge roll over on the tires using white shoe polish. Usually called
"chalking" the tires.
On a Mini Cooper S running 13" wheels with Yoko A008Gs I was running 34 to
36psi front and 28 psi rear. A local car prep specialist insisted I was on
drugs to use such pressures... then he took temp readings for me and found
the tire temps were right where they needed to be. And the car handled
perfectly at TWS.
The basic idea is to keep the front tire tread evenly flat onto the road
surface for traction. Then the rear tires are adjusted to get the
oversteer/understeer characteristics you want.
Most tires will tend to slip on lower pressures (rolling onto sidewalls) and
grip better with higher pressures.. until you hit the point where the tire
starts to balloon out and you wind up wearing out the center tread with no
wear on the edges. So the basic autocrosser rule was to keep adding
pressure to get them to stick, until you stopped using the shoulder tread.
Then you were at max front pressure.
With rear wheel drive you have other forces working on the tires so you'll
tend to run a few more PSI in the back. Same concepts apply. Unless you
like lighting up the tires without moving the car.
One benefit to chalking... you can check it any time, not just right after a
hot run session.
Cheers,
Jim
Dallas
'61 Morris Mini vintage racer
'76 Spitfire DM autocrosser
-----Original Message-----
From: vintage-race-bounces+britbits=netzero.net@autox.team.net
[mailto:vintage-race-bounces+britbits=netzero.net@autox.team.net] On Behalf
Of Dave Mapes
Sent: Wednesday, September 03, 2008 6:56 PM
To: vintage-race@autox.team.net
Cc: vintage-race@autox.team.net
Subject: Re: [Vintage-race] Vintage-race Digest, Vol 17, Issue 1
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.
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