Re: Tire Pressures

[Scott Fisher <sefisher@cisco.com>]

> Janet Scruggs wrote:

Gross -- I know you wrote to me personally, but the more I wrote here,
the more I thought some of the other Audisti here would benefit from
this systematic, empirical approach (as well as having more fun making
their cars go around offramps more quickly!).  Hope you don't mind me
including some excerpts from your question to provide a little context.

> Earlier this year I asked
> how does one arrive at the equivalent of the 'recommended' tire
> pressure when you change from the original tire size. [...]
> Finally, I decided to go to the max allowable tire pressure, drop a
> pound or two per axle and do a subjective evaluation.  Works.... but I
> wanted some understanding.

Ah.  Here's the best way to find out:

1.  Draw a tight, twisty race course in a biiiiiiiig empty parking lot
(or have the local autocross club draw one for you :-).  

2.  Take an air compressor or bottle of compressed air to the parking
lot with you.

3.  Pump your tires up to about 40-42 PSI.  At this point, you can opt
for The Cheap Method (which works really well) or The Expensive Method
(which is high-tech and therefore MUCH better for bench-racing points
afterward).

3a.  The cheap method: paint the edges of your tire treads with white
shoe polish.  Yep, the same stuff your mom used to put on your little
white shoes when you were a toddler.  You want to have it just touching
the part of the tread that runs on the ground, and wrapping a bit around
but not very far up the sidewall.  Typically, this is not as far as the
letters.  Do this at 120-degree intervals around the tire.

3b.  The high-tech method: Acquire a pyrometer -- basically, a
thermocouple hooked onto a DMM used for measuring temperatures.  

4.  Now, drive as fast as you can around the course, getting the hang of
it.  Come in off the run and do one of the following:

4a.  The cheap method: see how much of the paint has worn off the
corners of your tires.  You want a minimal amount of rollover, but you
want *some* so you know you're getting compliance.  Here, it helps to
have someone who's seen it once before to point you in the right
direction; an experienced competitor can tell you, "I like to see the
paint rubbed off down to the Michelin Man's bellybutton" or the like.
:-)

If you have too much rollover (that is, if a LOT of the white paint has
rubbed off), increase your tire pressure.

If you don't have enough rollover (that is, if NONE of the white paint
has rubbed off), decrease your tire pressure. 

In either case, move in 2-lb increments till you overshoot, then back up
1 lb.

4b.  The high-tech method: Take temperature measurements at three points
across the tire's tread -- the outside edge, the center, and the inside
edge.  

If the middle is hotter than the edges, drop your pressure.

If the outside edge is hotter than the middle, raise your pressure.

If the temps are within 5 degrees or so F across the tire, you're
golden.

As in the other method, move in 2-psi increments till you overshoot,
then go back 1 psi.

5.  WRITE DOWN THE BEST SETTINGS FOR EACH CORNER.

Yes, this does mean you have to do it four times per run.  As I once
told the autox list, "Winning is hard.  That's why they call it
winning." :-)

There may be different settings for the front and the rear tires,
because of different weight/dynamic loads on each tire.  Also, of
course, you can get much the same effect by just doing a skidpad -- a
large circle which you drive around to get the tire scrub/heat working. 
If so, make sure you run it in both directions; you may find that the
car is happier with different settings at each corner.  But an actual
autocross course with right turns, left turns, and straights, is
superior because it allows you to test your car under dynamic loads as
well as static lateral grip.  And if you have a friend there with a
stopwatch, you can time yourself.  Note that, especially if you've never
done this before, there is a HUGE learning curve factor to consider;
your third lap will almost certainly be faster than your first, even if
your first lap had perfect pressures and your third was off by 20%.  So
don't just go by the clock, go by the tire.

Finally... note that there is an optimum tire pressure setting (assuming
all you're after is grip, and you don't care about road noise) for any
tire/roll rate/alignment setting; change ANY of these and the pressure
setting changes too.  Even more interesting: if you vary from this
optimum setting, grip falls.  But it falls away *more slowly* if you go
*higher* than the optimum pressure.  So if you're 1 or 2 PSI too high,
you're closer to maximum grip than if you're 1 or 2 PSI too low.

(Why?  Well, because when you're below the optimum pressure, the tire
will roll onto the sidewall, the contact patch can deform, and grip
falls away rather suddenly.  When you're above optimum, however, the
tire "crowns" somewhat, reducing the size of the contact patch and
therefore the amount of available grip -- BUT the tire stays solid and
doesn't roll over, so breakaway is more gradual.)

Whew.  That's it.  Time to go home...

--Scott "the things I think of while cleaning out my inbox" Fisher