I understand very well that from a true accuracy perspective, only the chasis-point measurements will do. And as you've stated, it is unrealistic to do that for our purposes.

However, I don't think measuring from the center is any more difficult than measuring from the ground. As Porsche owners, I've gotta give us a little more credit than that! Owners of Miataes, S2000, Civics, Supras, M3s, etc... on the various forums do it all the time to collect data.

While it's normally no big deal, with the variances of the different size wheels and tires, upgrading from 18" to 19" can yield close to 1/2" difference in radius!

Case in point, when comparing the stock 225/40-18 vs. 245/35-19 (popular front plus one upgrade), we get a difference in radius of approx .4"!

Now, if we are talking about lowering the car an inch, I think we can all agree that .4" is very significant when trying to compare an 1" suspension drop. A stock-tired car will read .4 inch lower than a car shod with 245/35-19 tires, despite the fact that both cars are dropped the same amount!

And a lot of people run 295/30-19s in the rear when going Plus One. Given equal suspension drops, that would measure .25" taller than a stock Turbo with the method of measuring from the ground. Again, .25" variance in a 1" drop is very significant.

Data points collected via measuring from the floor will have to be qualified with + or - .25~.4 inch!

By measuring from the center of the wheel to the lip, you take out all wheel/tire factors.

Peace!
Stephen
'01 Turbo Ti

 

Stephen,
If you wish to collect and publish the data you are suggesting, please post a request for members to send you the data. I for one would be interested.
Respectfully, I would like to correct you on a few aspects of your previous post.
In the time that I have been a member here, I have listened and viewed what tires people are generally running on their 19 inch wheel setups. In general, I would be pretty safe saying that most people running 19's are using 235/35/19's in the front and 315/25/19's in the rear. Most people are using either the Yokohama AVS Sport (Only TUV approved in the 315 size) or the Continental Sport Contacts (Recently DOT approved in the 315 size).
The comparison of the overall tire diameters between factory supplied Pirelli P-Zeros and Yokohama AVS Sport's is as follows:

Front:
Pirelli P Zero 225/40/18 = 25.2 inches
Yokohama AVS Sport 235/35/19 = 25.43 inches

Rear:
Pirelli P Zero 295/30/18 = 25.0 inches
Yokohama AVS Sport 315/25/19 = 25.2 inches

As you can see the overall diameter difference is only .23 inches in the front and .20 inches in the rear.

That means the radius differential is less than 1/8".
Of course different tire setups will yield different results, but I believe this is a fairly common setup.
Again, your method is more accurate and I am not arguing that point so please take the initiative and collect the data.

OT:
As an unrelated side comment, if you look at the ratio between front/rear for the 18 vs 19 setups, the VC should not receive any additional stress.
Stock 18" setup = 1.008
19" Setup = 1.009
These numbers represent the rotational variation percentage experienced by the viscous coupling when changing from 18's to 19's of the sizes indicated. Insignificant by any standard.

 

KPV,

From the short time that I've been on this forum, I was under the impression that more people were running 295/30 vs. 315/25 for the rear 19". And I did not have intentions of starting a new list, as I was merely making suggestions on how the existing list could be improved. Nevertheless, good discussion.

OT: Just curious, when going "plus one", what's the general consensus for chasis balance when going 19" with 235/315mm F/R?

The front width is increased by 10mm, while the rear by 20mm. Through a few spirited laps at Gingerman, and watching other 996s, I noticed that the car's basic behavior was initial understeer in, and power-on oversteer out. And turn-in was biased towards understeer as well.

Does the 235 front and 315 front exacerbate the understeering tencencies? Any info gathered on this?

While logic points to 18" as the sweet spot in terms of wheel size for our cars, our cars simply look much more "bling" with 19". I don't road race much anymore, so the possibility is there for me to go plus one.

Stephen
'01 Turbo Ti

 

my "bling" will arrive this week. can't wait...

 

Stephen,
Regarding widths, I agree, the car has understeer built in. I would agree that the 235/315 setup does exacerbate this. If you go to the 19 setup with these tire choices, and engage in spirited driving or track work, it is important to add a rear sway bar to counteract Porsche's built in understeer as well as the added understeer resulting from greater grip in the rear. However, and this is a big however, the various rear tire section widths are all over the map. Make sure to compare the actual width specs because there are tires with lesser numerically spec'd tires that are actually wider!!
In any case, adding a rear sway bar is usually a very good idea to force a more neutral handling car. Of course this is only an issue if you drive in situations wherein the traction is at or near the limit. Poseurs need not apply. One caveat though. If you have upgraded your power in any way, throttle induced oversteer becomes another very efficient way of countering the inherent understeer.