Well, there's a problem with that. Tire pressure at racing temp is really
what you want. If you start off with a cold temperature set with those two
tires that were sitting in the sun, then they will be at least two pounds
softer than the others when you come up to full temp.
You're right, the calculations won't help. Nitrogen really does help--I
wasn't kidding about that.
The process I apply to my Radical (Vintage racing doesn't generally enable
this kind of pit work) is to go out on the test, tune and qualify days
before a race with tires set to some ballpark pressure, drive the bejesus
out of the car for a few laps, dash into the hot pits where my pit crew
yanks off the body, checks tire temps across the tires, checks pressure,
and then sets all to the pressure we arrived at from our prior testing.
Then I do a few more hot laps and come back in for more readings. We're
tweaking suspension to get inside and outside tire temps correct for the
track, taking lap times to make sure we're going in the right direction,
and comparing inside edge to center temps to make sure the pressure is
right. We also play with corner weighting, spring rates, and which side of
my mouth I keep my gum at.
Then we put the car away 'till the morning and check the cold temps. That
gives us a baseline cold pressure reading (assuming we don't have some
kind of a leak--any difference hot to cold that's way out of line is
suspect). We use those pressures for each tire as the cold pressure for
that track. Until (and if) we get the time to test it all again.
For example, at PIR I use RF 17#, LF 16# RR 18# LR 19#. Fully warmed those
pressures translate to 24# all around with Nitrogen in the tires. PIR is a
predominantly right turn track--no surprise--the lf tire is working hard.
With air I used to see pressure go at least 3-4 pounds higher between hot
and cold. And it seemed a good bit more unpredictable.
Fussy stuff, but worth a few seconds if you have a good crew to help. The
radical can do PIR with the chicane and with my fat a** in it in 1:19:
something with tweaking, but 1:23 is about as good as I used to get
without all the effort in setup. I didn't have any help at the last race,
I still had the setup from last year's enduro, and 1:23 was the best I
could do. That's my excuse and I'm stickin' to it.
Of course vintage cars aren't nearly as fussy, cuz the tires suck to begin
with, and I do 1:31 at PIR in Peyote, slipping and sliding like a lunatic
with a minimum of fussing. There's probably a few seconds left in there,
but who cares.
-----Original Message-----
From: Rocky Entriken [mailto:rocky@tri.net]
Sent: Monday, July 22, 2002 4:01 PM
To: Wes Dayton; Friends of Triumph
Subject: Re: Tyre Pressure/Temperature
> Maybe I should ask this another way? Has anyone else out there spent
> Saturday getting their pre-race tyre pressures just right. Then you
> arrive at the track on Sunday morning (after a nice crisp night) and
> found they
are
> four pounds lower and you say to yourself "do I have a leak, or are
> things as they left them or what"?
>
> Get my drift? And if I'm missing something very obvious, just be
> gentle
when
> you let me know.
>
IMHO, yes you are missing something very obvious. The time to make the
definitive check on pressures is just before you go out, not the day
before. Yesterday's readings are irrelevant. Even if you raced yesterday.
Of all the various "settings" for a car, this one is too easy. Trying to
figure out a bunch of mathematical formulae is just overcomplicating a
simple task. Read the gauge, add or bleed air as necessary, done.
I've had tires gain a pound or two on one side within an hour just because
that's the side that was in the sun that morning.
And screw all that other chatter about exotic gases, etc. An air bubble
and a good tire gauge (the kind with a bleeder valve) are cheap and
useful. "Getting it just right" is just pssssssssssssssst, psst, psst.
(And I'll set mine to half-pound increments at times).
--Rocky Entriken
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