Re: tire pressures

[Scott Fisher <sfisher71@domain.elided>]

--- Tessie McMillan <tessmc@domain.elided> wrote:
> Recently I've been driving a friend's 356, 
> We had this one argument about inducing oversteer, 
> which he won after a scientific test, but I still
> can't figure out why.
> 
> Not being able to get the rear end to kick out, he
> wanted to raise the front tire pressure. 
> I said no way, he needed to lower the *fronts* to 
> get more patch so steering input would be greater. 

Raise and lower relative to what, though?  And how do
you know?

There are two things going on here.  First: you only
get more patch if the patch STAYS FLAT as the car
builds up cornering force.  On a skinny tire such as
those used on most 356s, lowering the pressure allows
the tire to roll over farther onto its sidewall,
dropping grip to nothing.  Second: With old-fashioned
cars like the 356 and the 105-series Alfa (there,
we're now magically on topic), the front suspension is
designed to add a bit of positive camber under body
roll, which makes this low-pressure rollover even
WORSE.  If you lower the pressure too much, you end up
scrubbing all the letters off the sidewalls.

Get a bottle of white shoe-polish and try the same
experiment, this time after making a little stripe
right at the "shoulder" of the tire before you make
your run.  Then look at it after the run and see how
much white got rubbed off.  Your goal: to reduce tire
rollover to the point at which only the treaded
portion of the tire (and not the sidewall) scuffs off
the white paint.  (A big block of sidewalk chalk works
almost as well, BTW, and is a lot easier to clean
off.)

> Then he went back to his original starting point, 
> and raised the front just 2 psi. After that we were
> both able to kick the rear end out with barely a 
> sneeze. 
> 
> Why?

See above.  Now, here's an advanced study question for
you:

He raised the fronts 2 PSI and removed a bunch of
understeer.  Let's imagine for the sake of argument
that he raised them 2 more and it stayed about the
same, so he raised them 2 more and the understeer came
back, though milder.  Why?

Answer: because THEN you have only reduced the size of
the contact patch, without also causing the tire to
roll over so much and lift the inside edge of the
patch.  So it's less than optimum grip, but it's not
ALSO causing deformation problems.

One of "the usual suspects" (Fred Puhn or Carroll
Smith) has a graph of tire pressure vs. grip.  It's
very interesting, because there is an area of optimal
pressure at which the tire develops its maximum grip,
and if you go higher or lower than that, grip falls
off.  Makes perfect sense.

But IT'S NOT A SYMMETRICAL CURVE.

That is, grip falls off FASTER on the side of lower
pressures, and SLOWER on the side of higher pressures.
 Here, I'll try it in ASCII art (where the vertical
axis represents grip and the horizontal axis
represents pressure):


|
|       *  *
g    *         *
|  *              *
| *                   *
|*
+---------- PSI ---------

(Note: scale approximate. :-)

What I hope this graph indicates is that while adding
and removing pressure, relative to the optimum, will
both reduce grip, it takes a smaller pressure DROP to
get the same delta-G than it does a pressure RISE.

Two other practical tips: a) it's easier to drop
pressure than to raise it because you can let it out
with the adjuster on the gauge, so consider hitting
the track (if you're still in the tuning phase) with a
couple PSI more than you think you should have.  This
fits nicely with point b), which is that higher
pressures (within a few PSI of optimum) are generally
safer than lower pressures, because there is less
likelihood of the tire separating from the rim.  (Also
behavior with too MUCH pressure is more predictable,
because the contact patch does not deform under side
loading, it's just smaller).

Where this info really comes in handy is in
autocrossing, where you don't know the course ahead of
time and you can't (often) change spring and shock
rates at the track, certainly not in the daily-driver
classes.  So for autoxing, often the only quick change
you CAN make is to adjust tire pressures.

Final tip, especially useful for us'ns here in the
Great Wet North: running slightly higher than normal
pressures is a rain-driving trick, because it raises
the "crown" in the middle of the tire, reducing the
contact patch -- and therefore applying the same
pounds to fewer square inches of rubber.  This can
help force water out from under the tires more
effectively than a wide contact patch, resulting in
more overall grip due to less hydroplaning.

--Scott Fisher
  Tualatin, Oregon
.
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http://sports.yahoo.com/

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