You're exactly right, Bill, and I do much the same albeit with a lot less
thrashing about it. Club racing with a crew of "self" mandates that. In my
case, through many years of doing this, I have a baseline of where to start.
I want a racing pressure of about 24F 27R. I know I will go up about three
pounds from cold on an autocross run, about six pounds from cold in several
racing laps, and I set my pressures accordingly. My target pressure may vary
depending on the track or course, especially if it is predominantly right or
left. And I may adjust between autox runs or between practice and race
depending on what I feel the handling doing -- usually a factor of the
track, the surface, the weather, etc., even just how I am driving the car
that day.
But my point to Mr. Dayton was really, I do that before I go out, not the
previous day! Yesterday is irrelevant.
I've basically found if my sun-side tires have gained a couple of pounds,
they are still "cold" as far as the tire is concerned, compared to what will
be "hot" after the tire has been raced. So I generally even out the
pressures to where I want them to start just before I go out. Seems to work.
--Rocky
----- Original Message -----
From: "Bill Babcock" <BillB@bnj.com>
To: "'Rocky Entriken'" <rocky@tri.net>; "Wes Dayton" <oilyrag@hsrca.org.au>;
"Friends of Triumph" <fot@autox.team.net>
Sent: Monday, July 22, 2002 6:41 PM
Subject: RE: Tyre Pressure/Temperature
> 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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