Ferrari Mailing List: Re: Tire Pressure for Aftermarket Wheels

[Rick Lindsay (rolindsay]

Thanks Dennis!  I run a bit over 30 all around.  Stock asks for 30 front, 34 
rear - I think - but I never have anything in the boot.  The really big 
difference is that the car now feels like a sports car where before, the ride 
was too soft.  Remember, these cars here in the US are set up for 
post-soccer-moms who want the look but not the feel of a sports car.  (They are 
also the kind who drive with the top down, windows up and quaff-protector in 
place!)  Now, with 315hp and a tight ride, the car feels and performs great!  I 
do, however, avoid road hazards fanatically.  Life is too short for a boring 
daily driver!

ric

----- Original Message ----
From: Dennis Liu <bigheaddennis [at] gmail.com>
To: Rick Lindsay <rolindsay [at] yahoo.com>
Cc: The FerrariList <ferrari [at] ferrarilist.com>
Sent: Sunday, November 4, 2007 12:24:21 PM
Subject: RE: [Ferrari] Tire Pressure for Aftermarket Wheels

Pretty trick, Rick!

Re your specific question - IIRC, it's something of a brain teaser/physics
exam question.  In the hypothetical world, if a tire/wheel is inflated to,
say, 30 psi, the SIZE of the contact patch is the SAME as a bigger/wider
tire/wheel inflated to the same pressure.  Or so I'm led to believe - I've
debated this a fair bit, and I've come to the conclusion that it's correct,
at least in the theoretical world.  You can change the SHAPE of the contact
patch with different tires, but the overall AREA remains the same, given a
constant pressure.  

And then, of coursem we move into the real world!

I'm a little bit in doubt that manufacturers use a sidewall height ratio to
calculate pressures.  Rather, the overall performance of the package is
tuned to what the manufacturer targets - given the BRAND/TYPE of the OEM
tire, the size of the wheel, the total unspring weight, the springs, the
suspension settings, the shock settings, the weight of the car, etc.   too
many variables for you to calculate at this point.  IIRC, there are
rule-of-thumb formulae for calculating effective spring pressure for a
change in tire pressure, but I'm not nearly smart enough to take that into
account for street driving.

Keep a couple of things in mind, in addition to what I wrote to Chuckles
below.

A.  a 19" rim results in an awfully low sidewall, which will in turn result
in lots of bent rims and blown tires - ask how many 360 CS owners have had
to write big checks for new wheels.  Keeping a higher pressure will help
reduce the number of ouch incidents you suffer.

B.  While it's fun to chat about this, the easier/better way to go about
this is to set your pressures, then go for a spirited drive.  See how it
feels.  Then reduce/add pressure until you're happy.

vty,

--Dennis

P.S.  I'll bet that you end up happiest, in your SL500, somewhere around
34/34 all-around, COLD.  :-)

-----Original Message-----
From: Rick Lindsay [mailto:rolindsay [at] yahoo.com] 
Sent: Sunday, November 04, 2007 1:15 PM
To: BigHeadDennis [at] gmail.com
Cc: The FerrariList
Subject: Re: [Ferrari] Tire Pressure for Aftermarket Wheels

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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