PCA SDR

I noticed that the new Porsche Cup cars and RSRs use 18" wheels, not the 19s like the street cars come with.

I have often wondered what effect diameter has on patch size and grip (between 15" and 16"), but I guess that wasn't a concern of of the study :-\

BF Goodrich, before they were bought by Michelin, had a service where they sent technical articles to interested parties. One of the articles was about the effect of lowering tire profile on contact patch shape and performance.

The article said that as profile number goes down, tire patch becomes shorter and wider as one would expect.

However this trend in shape puts all the "working rubber" closer to being on a transverse single line, rather than on a longitudinal single line as going toward a larger profile number does.

The upshot is "control ability" for the driver changes with profile number as does ultimate grip.

With all the working rubber on a transverse line (low profile), the tire becomes less "user friendly" with optimum slip angle being harder to dial in by the driver although ultimate grip is improved- it is just harder to exploit.

With the rubber on a more longitudinal line as in a higher profile tire, the opposite occurs there is less ultimate grip but that grip is easier to exploit by the driver and the car is easier to control.

So, the take home lesson I have is, if you are Sebastian Vettel, go for low profile, if you are a "normal" person go for a less low profile and you will probably be faster in an autocross with your lighter, easier to drive tire kit.

I think a lot of people forget that their tires are really considered to be the most important part of your suspension. Bigger is not always better, just like stiffer is not always better. I always thought it was silly when you see kids with their muscle cars and hot rods putting 19" rims on thinking and bragging about their improved performance. Especially in drag racing where you want the sidewall to wrinkle and increase the footprint.
One thing to look at when thinking about chassis, suspension and tire dynamics is: what do the fastest cars use?
In drag racing it's typically a 15" rim and Formula 1 cars run 13" diameter wheels (328-333mm) and no good off-road vehicles use larger than 17" rims. In my opinion the only good reason to go bigger than 17's is if you need clearance for larger rims, like on a RUF Porsche which requires 19" rims to clear the rotors.
Jut my 2 cents though, I am not a tire engineer. If someone here is, I would love to hear from an expert what the "best" rim size is for a 911.

In F1, the rules call out their 13" diameter, I suspect same situation in some other racing series as well, so this has to be considered. I got to talk to Leo Mehl, then director of Good Year racing and asked about the 13" wheels and he commented that in an F1 car the high tire side wall as far as he was concerned was the real suspension of the car. Watch the slow motions during F1 TV coverage and one can see why he said this.

Bob Gagnon wrote:In F1, the rules call out their 13" diameter, I suspect same situation in some other racing series as well, so this has to be considered. I got to talk to Leo Mehl, then director of Good Year racing and asked about the 13" wheels and he commented that in an F1 car the high tire side wall as far as he was concerned was the real suspension of the car. Watch the slow motions during F1 TV coverage and one can see why he said this.

That's true most modern Formula cars use 13" wheels with exceptions of a few US Formula series which use 15". Indycars are also bigger and heavier than F1 cars. The spring rate of the tire is an important part on a formula car to choose the springs, as they work together for mechanical grip. Of course there are many other variables such as ride height, anti-squat, droop, spring preload, track surface etc.

Personally I think 17" wheels for racing is one of the best size for a Porsche, as the tire choice is bigger than for 18" or 19" wheels and the tire cost is a little less too.

Below is the BF Goodrich article I referred to.

martinreinhardt wrote:Personally I think 17" wheels for racing is one of the best size for a Porsche, as the tire choice is bigger than for 18" or 19" wheels and the tire cost is a little less too.

I agree with you Martin for all the reasons you give above plus I think 17" are easier to drive with.

I checked Porsche Motorsport and all the latest competition 911 cars use 18"wheels, the GT3 Cup, GT3-R and GT3-RSR and I think for these FIA series there is no wheel diameter requirement, so I figure this diameter pleases the Porsche development guys the best.

Edit: I looked at the FIA rules for series production and GT cars and there is an 18" maximum wheel size limit, so much for my speculation!

BTW, I noted when doing this research that the GT3-R is OEM Porsche supplied with 1 piece forged alloys from Ray's engineering in Japan!! Hey, those are the same type wheels I bought at the Nissan dealer who sells them for a 350 Z under their Nismo brand name and they are great quality, forged, shop peened and hard anodized and, for a 350Z, only $1700 (discounted) for a full set of 4 (don't expect this pricing from Porsche!!!). (18 pounds for an 18 X 9.5 wheel). Just tootin' my horn here since I am a fan of Ray's wheels (also used by Williams F1). The Cup and GT3-RSR are still OEM supplied with BBS 3 piece with forged centers.

Here is Ray's wheel on Porsche