Ferrari Mailing List: Re: Tire Pressure for Aftermarket Wheels

[Dennis Liu (bigheaddennis]

Yup, that's what manufacturers say.  After all, who wants to be
responsible/sued if some idiot spins the car off the road?  Not to mention
that it's the easiest way to give "safe" advice to all of your buyers.
 
Then again, if you choose to live and die solely by manufacturer
recommendations, no one would ever buy a bigger rim, or even brands of tires
not specified in the manual.  E.g., the 355 manual specifies only certain
tires from certain manufacturers - you may have to hunt down NOS in the back
of the warehouse.  And not to mention all of the TRX buyers out there.  Or
Porsche owners who have to deal with the "n-spec" marketing BS.
 
Think about it - Ferrari made certain recommendations for, say, an older car
still running bias-ply tires.  do you really think that same advice applies
to modern rubber?
 
vty,
 
--Dennis

  _____  

From: Brian E. Buxton [mailto:BrianBuxton [at] BuxtonMotorsports.com] 
Sent: Monday, November 05, 2007 2:02 AM
To: BigHeadDennis [at] gmail.com
Cc: 'The FerrariList'
Subject: Re: [Ferrari] Tire Pressure for Aftermarket Wheels


Everyone I have ever bought rims or tires (Tire Rack, Kinesis, HRE, BBS,
ForgeLine, etc.) from says to run the factory recommended pressures even
when you go up in rim and tire size.

Brian




Dennis Liu wrote: 

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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