Ferrari Mailing List: Re: Tire Pressure for Aftermarket Wheels

[Rick Lindsay (rolindsay]

Dennis, CG, 
   Thanks for asking this question.  I just put 19" wheels on my SL500 and 
asked the SL List the same question CG just asked.  Instead of useful 
information like B.H.Dennis offered, I got a lecture on tuning the pressures in 
on the track.  That is, of course, after all the ethnic slurs died down.
   My question was:  Since I have increased the area of the contact patch, 
should the pressure be lowered so as to put the SAME pressure on the road, per 
square inch.  OR, is there perhaps an ideal sidewall deflection?  Say; the 
ratio of sidewall height AT THE ROAD to the actual sidewall height?  I'm sure 
the manufacturers have an 'ideal' deflection percentage.
   So as we go forward, please keep my little SL in your list of cars of 
interest, and let us all know what you learn.
BTW, here's how the car looks today:
 
http://www.aubard.us/SL500/HPIM0731.JPG
 
rick

----- Original Message ----
From: Dennis Liu <bigheaddennis [at] gmail.com>
To: rolindsay <rolindsay [at] yahoo.com>
Cc: The FerrariList <ferrari [at] ferrarilist.com>
Sent: Sunday, November 4, 2007 9:18:53 AM
Subject: Re: [Ferrari] Tire Pressure for Aftermarket Wheels

Charles wrote:

>What is the list's opinion on picking a tire pressure for aftermarket
wheels where the wheel differs signficantly from stock?

I ask because of my TR. It had the metric wheels when I bought it, so I
bought a set of 18" HRE wheels from Karl early on. They are obviously
very different from the stock wheels in both sidewall height and
construction, so I always picked the tire pressure by looking at the max
rating on the tire and backing it down some (like if the tire said 51
psi max, I would run them around 44-46. This is very unscientific. I
haven't seen uneven tire wear, so by that measure I assume it was OK.

Is there a way to calculate the best tire pressure? Should I pick the
factory recommendation from the 512TR since its wheels are nearly
identical to my current ones in terms of size?

I did Google the subject but only seem to get car-specific wheel
recommendations.

======================

Good initiative, Chuckles.  A few things to keep in mind.

First off, the Michelin TRX system is ancient, ancient technology - at least
five generations behind, if not more.  Hey, think about it, I've slept with
girls born after the TRX metric system was introduced!  (Well, I haven't,
but Dave Handa has.)  Assuming you've bought the current, new generation of
tires, they're still worlds better than that of even 4-5 years ago.   Toss in
the fact that sidewalls are getting ever lower and stiffer, and the
manufacturer's recommendations are absolutely, positively worthless now.
Those recommendations were written for tire technology of 25 years ago.
Tire composition, design and sizing have changed dramatically over the last
30 years.  Arguably they are the component of a car that has made the MOST
advancements in 25 years.

Secondly, what the factory wants is not necessarily what YOU want.  The
factory makes certain recommendations on tire pressure because they want to
avoid liability for Chuckles oversteering the car off the road.  They feel
much safer if Chuckles understeers the car into a tree.  Accordingly and
notoriously, most if not all manufacturers set up their cars, in factory
spec, to have nice, safe, predictable understeer.  That's why on many sports
cars, particularly mid and rear engined ones where you're most likely to
suffer the deleterious effects of oversteer, particularly TTO, you'll see a
bias in pressures towards the rear (e.g., 28/34).  Manufacturers believe
that for a novice driver, he or she can handle understeer much better than
oversteer, or even if it's set up to be neutral, if the car starts a four
wheel drift.

Thus...  You need to consider the type of tire, the conditions under which
it will operate, AND what you want the car to do.

For example, if you're going with modern 18" rubber, particularly if it's a
"max performance" or "ultra high performance" compound, and you're driving
it mostly in warm temps (60F-80F ambient), and you'd like the car to be more
neutral in handling, I'd go with 34/32, front to rear (measured cold).   If
it's hotter than that, drop a pound or two (tires will heat up more).
Maximum stick on street tires come around 40 lbs hot, and if you drive
aggressively on the street, it's easy enough to add 4+ pounds (10+ lbs on
the track).  More pressure increase in the rear for a mid-engined car,
particularly on a fat pig Testarossa.  For street use, I'd like to shoot for
a target temp of 36-38 lbs HOT.  The ride will be a little stiffer, but
you'll enjoy yourself more.

BTW, the old trick of chalking the sidewalls and measuring roll over that
way?  Forget it.  Not nearly as useful in the day of rubber-band tiny
sidewalls.

Just my $0.02.

Vty,

--Dennis

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