Awesome posts/info guys!

 

Sloth,

You mention you prefer 17" tires at the track. This is what I ran with my 996 cab. Are there 17" wheels that fit the TT? If so, do you have any suggestions?

Thanks,

Bart

 

At the end of the day it all revolves around the manufacture of the tires. Until high end companys do the R&D, ie. Michelin PS2's. There wont be high quality 19" tire fittments that compliment some cars. But I tell you this from years of experience, the new tire technology is good enough to justify the 19/20inch fittments for Porsche, and other high end sports cars. Now it is up to us as enthusiasts to let the technology prove it's merit.

 

18" --> optimum performance setup.

19" --> optimum G-unit setup.

I go for the latter over the former since 99% of my driving is done on public roads.

 

There are three main factors:

Changing tire sidewall size alters the amount of compliance/
'spring rate' that the tire supplies to the suspension. A narrower
aspect 19" tire will necessarily provide less spring, and this
is counter to the design of the suspension, which assumed the
amount of compliance of an 18" tire. The suspension arms will
go through a greater arc for a given bump than they would with
the stock tire/wheel.
The second issue is weight and rotional distribution. *If* the
19" wheel is heavier and/or if the weight is farther out from
the axle line, the suspension will not have as quick a rebound
and won't spin up as fast on acceleration.

If a car is designed for 19", that'll be OK. It's not 19" per se, it's
an issue of going different than stock.
Joe

 

Also look at what real racing teams do. Tires are more
forgiving and controllable with the give that a decent
tire wall supplies. No one is running huge wheels and
tiny thin tires. That's mainly the province of the cars-as-bling
folks.

 

I think you are a bit off on the spring rate to tire aspect ratio claim. If your claim was accurate the air pressure of the tire could have the same effect. Tires are designed around a specific load range, and the sidewalls are reinforced to accomodate the tires anticipated load, speed and temperature requirements. What you are reffering to is more related to your second issue, unsprung weight. In which case yes, a lighter wheel will result in different handling, as will a heavier wheel.

 

Thanks Marc! You are correct! Tire pressure *does* affect the
'spring rate' of a tire, but not so much in the typical pressure
ranges nowadays, especially with the stiff sidewalls we have.
Here's a link to Michelin's Pilot Sport Cup racing tire care tips
(pdf file). See item 8, which corroborates what I am saying.
Maybe I didn't express it well.
Joe

http://www.michelinman.com/assets/pd...otsportcup.pdf

 

Thank You for the clarification Joe, your comments are much clearer now that I know the context they were intended. I have worked very closely with the fine folks at Michelin throughout the development of the Sport Cups, they are a phenominal product. Interestingly enough, if you all notice the new sizes at the bottom of the app. chart. Those are the 19" sizes for the CSL M3, they are very close in O.D. to Porsche. I seem to recall that that car does fairly well in hard track use.

 

no problem. The point is that given a particular outer
diameter, shortening the sidewall is effectively increasing
the spring rate of the tire, imparting more shock load and
travel requirements to the suspension. If a car is designed
for this, no problem. If it was designed for a bigger-sidewall
tire, there will be different behavior than intended.
Joe

 

intriguing new development from Michelin. "The Michelin Pilot Sport PS2 Max Performance Summer tire was initially developed for use as Original Equipment on some of the world's most prestigious sports cars and sports sedans by combining world-class wet and dry handling with confident and predictable cornering. Some of the first fitments include the BMW Alpina Z8, Chrysler Crossfire, McLaren SLR and Porsche GT2 & GT3 models."
This is a statement related to the 315/25/19 tire. Which is 25.25 inches in overall diameter, while it is roughly.25 inches larger than the 295/30/18. It is .25 smaller than the 315/30/18 that is a Porsche OEM fittment. That seems like a pretty dead on compromise for overall diameter.

 

The reason it is a quicker car at the track has little to do with the wheels, it has to do with the higher horsepower and lower weight.

-- Michael