Here is a somewhat crude illustration.

The 360 Modena uses a 45 series sidewall in front and 40 in the rear. Not a bad aspect ratio.

The Challenge Stradale uses 19" wheels with 35 series aspect ratio all the way around. Still not bad.

If you go 19" on a Turbo or GT2, you are forced to going to 25 SERIES ASPECT RATIO, therefore the tire is much, much thinner in order to maintain the same rolling diameter. At 18", you are already at a 30 series aspect ratio in the rear for a GT2 and Turbo, already thin, even at 18".

Obviously, it's impossible to really look at this pound for pound, since aspect ratio relates to the width of the tire, but it gives you some general idea of how thin the sidewall is.

The point is that Ferrari 360 chassis is designed around a larger rolling diameter than the 996. Therefore, there is more rubber to work with even at 19".

 

Just my 0.02$. In the saloon car racing series in Europe anyway they are using 16" or 17" first for acceleration (rotational mass) secondly for stability and thirdly for the tyre to last longer.

I'm talking about ETCC, BTCC or in the case where i have some experience STCC. The former owner of my M3 won STCC last year and I visited a couple of trainings and races last year and spent quite some time in the pits.

FWIW

 

The 996 really needs more wheel well room so taller tires can be used. I'd bet 35 or 40 series rear tires will outperform the stock 30's. I think the taller sidewall will keep more rubber on the road. I've seriously thought of going to a 17x12 rear wheel with a 315/35-17 tire size. You could probably cram in a 335 with minor clearance work(?) Tire rack has 13 different interesting tires in 315/35-17. However the stock hollow spokes are so light durable and relatively inexpensive I hate to change them.

Edit: I'm thinking more in terms of the best for street use.

 

If you want to run an increased rim diameter and maintain the rolling diameter, it is possible to decrease the comprimises you will suffer by adjusting your suspension accordingly, but you will never make up for the loss completley. Rim size and tire wall is designed and engineered to fit not just the wheel well remember. Many other factors come into consideration when selecting aspect ratios, rim size, offest, and tire compound. Keep that in mind when making selecting your new tire diameter. With all due respect, I am sure that Porsche engineers are better suited at selecting rim and tire specifications for your car. Although, if you are changing rim size primarily for aesthetics, you may want to consider changing your offset and adding a bit of lip to your rims for some increased bling

 

the smaller the wheel, the faster they can spin. Its all in physics

 

The diameter of the wheel and tire together are still equal. I am still not convinced that it is any more than just a poor selection of tires that are causing the handling problems. For a smooth track surface a 19" will have a crisper turn in due to less sidewall flex. Hence the Enzo, Stradale, Gallardo, CGT all have the larger diameter rims. The tire manufacturers are all coming out soon with better compound tires and the price is dropping accordingly.

 

First of all, as previously pointed out, it is absolutey irrelevant that "Enzo, Stadale, Gallardo, CGT all have larger diameter rims" as applicable to the Turbo given the fact that the diameter of 996 wheel/tire setup is around 25.1", which is not the case with the aforementioned cars. If you look at every single one of those cars you mentioned, the reason that they can run larger diameter tires is because their overall tire diameter is, in some cases, much greater than the 996 chasis'.

The stock Turbo's unloaded sidewall height is about 3.5", and if you look at those other cars, they run around that or greater. None of them run 3" or less sidewalls as we'd be forced to with 19" wheels.

And remember, despite the already short 3" sidewall of the 19" setup, the loaded static sidewall height will be even less... a reduction of .5" would not be unexpected. How much give can the resultant 2.5" of sidewall have, especially given that the low-profile tires are designed ultra stiff to begin with for hazzard protection.

Secondly, it is absolutely a misconception that some think the less sidewall you have the better. Think about it this way... a car's suspension gradually loads up and transfers the forces experienced in the transfering of forces in acceleration, decelleration, changing of directions, and any combination of as well as undulations in the road surface due to irregularities, turns, elevation changes, variance in coeficient of friction, etc. to the chasis/driver.

Well, you can think of your tire's sidewalls as its own "suspension". In other words, if you had zero give, the loading of forces on the sidewall would be ultra aggresive... almost instaneous, allowing it's grip limits to be reached much more aggresively. And once exceeded, it is much harder to regain. Sidewall flex allows for the forces to gradually (relatively speaking) loaded up over a given time period.

What does all this mean? Ultimately, this "give" allows for handling predictability and recoverability, giving more time to react to what the contact patch, suspension, etc is communicating to the driver.

You need sidewall flex... you need a suspension... we don't drive on glass-smooth surfaces with infinite grip.

Now, the question as to how aggresive (stiff) a particular suspension is setup or how much/little sidewall is required, depends on one's driving style and talents. Every car has a "sweet spot", and every driver has his/her "sweet spot" in terms of their capabilities. While Michael Schumachaer or JPM can probably coax a better time with a stiffer setup, you and I would probably fair better with a more forgiving setup.

I have driven many cars with all kinds of wheel/tire combo, and I can tell you without a doubt that having less sidewall is not always better. Given tires with identical coeficient of friction, I'd venture an experienced guess that 99.99% drivers will lap faster in a a properly setup 18" vs. 19" in our ~25.1" diameter setup.

Hope that helps.

Stephen
'01 Turbo Ti Upsolute
1stephen1.com

 

When I first read this I understood the question “why do 18” track better than 19” to mean “track in a straight line” If that was the question the first thing to check is the weight of the wheels, typically 19” are heavier and with the mass of the rim is further out causing the centrifugal forces to increase making an exaggerated reaction to any of its function. So, if the tire is out of balance… you’ll notice it more… it the car “tracks” a little going down the road … you will notice it more.

On the subject of tire sidewall this is far more complex than we have time to talk about but ASSUMING a 30 series tire has less flex than a 35 series tire is equivalent to making a blanket statement that all 3.8 turbo motors make more power than a 3.6 turbo. It all depends on the motor.

A tire is a 4 dimensional spring with many many variables. Some of the considerations in tire designs are:

Rolling Radius
Forces Acting on the Tire
Friction Coefficients
Vertical, Lateral, Longitudinal and Torsion Tire Deformation under load
Longitudinal and Lateral grip VS Vertical Load
Vertical Tire Deflection at a given Camber.
Lateral Grip, Self Alignment Moment, Vertical Load and Slip Angle

All of these and more are variables DESIGNED into the tire and CAN be extremely manipulated. The bottom line is, using laser ride height indicators and shock potentiometers on a race car, we found we had to go much stiffer on the springs to maintain the same body roll when we switched from 16” slicks to 18” slicks. This was solely due to that fact that the 18” tires with a lower aspect ratio had a much softer tire spring rate making them roll much more than the 16’s.

I can also tell you we did a lot of testing on a Speed Vision 996 cup car which had to run 315/18’s tires on the rear and we were able to talk them into testing with 335/17’s. In that application the wider 17” tires were noticeable slower at the 3 different tracks we tested.

I’m not trying to say I know all the answers here but, I have enough data that I could write about 100 pages on this subject and still I’m shocked every time I make a logical assumption and more times than not find myself wrong after testing.

What I have learned for sure is, the aspect ratio has very little to do with the spring rate of the tire ... don’t fool yourself. If you switch to a lower tire and it's stiffer it's only because it was designed that way ... Simply meaning, it's the wrong tire for your car.

 

Cary and stephan ... Bravo! Well said.

 

Great information, Cary. Just curious, with regards to the tires that were tested... were they R-compound road racing tires? Specific to the test you had mentioned, I would love to seen it carried out one more step with a 19" setup (not sure if it even exists for that particular tire if any) to further define the "sweetspot" of tire choice.

Track rubber (road racing, not drag) have much stiffer sidewalls than street tires so as to minimize sidewall deflection due to the nature of its intended use, and as their sidewall height increases, a stiffer carcas design may be used for compensation, which is not always the case with street tire designs.

Also, I neglected to mention previously that the width of a wheel on which a particular tire is mounted will also affect the amount of deformation a tire can have.

In general, going plus sizes has its advantages (ex. response) up to a certain point. Then, you start to introduce characteristics into the equation that are undersireable and counter-productive for handling.

Stephen
'01 Turbo Ti Upsolute
1stephen1.com

 

The point that I was trying to make with the new highest permoance cars coming out is that they are ALL at the 19" diameter. What are the changes to the suspension that were made between the "standard" 360 Modena and the Stradale to allow the Stradale to handle better on the 19" tires? The frame geometry is the same, so it wasn't designed with 19" tires in mind when it first came out. Everyone seems to agree that it is a faster car at the track.

 

On the then Speed Vision (now Speed GT) 996 the spec tire was a B. F. Goodrich R compound. No other sizes or brands were legal for racing so, we didn’t test with anything else.

On the 16” VS the 18” they were full race slicks. The 16” was a Goodyear and the 18” were Yokohoma. I forgot that I had recently tested some 18” rear Goodyears as well that were taller and stiffer than the Yokohoma’s, I personally really like the feel of those. They weren’t necessarily faster but for sure stiffer and didn’t like as much bumpsteer where the Yokohoma’s seem to be more comfortable with a little compression toe gain.

Code Red,

Again, I’m far from an expert on this however, I would bet that it’s in the tire design more than the rim size on the 360. Ferrari will give the tire manufacture specs to that information I listed above and the manufacture builds a tire to suit. For instance, if you look at a 275/35-18 from Michelin I think they have 4 different Pilot Sports in that size. If you look closer you’ll see one is a Viper OEM, one is a BMW OEM there is a standard YR rated and a standard ZR rated and may even be one more. Those will all react differently on the same car yet they are not only the exact same size but, the same brand and even the same MODEL!

I’ll see if I can find some technical info on changes to the 360 however, now days I think it’s almost easier to spec a tire to the manufacture than to do too much redesign of the suspension. With that said, don’t necessarily run out and buy those for your car that requires a different slip angle blah blah blah… That’s a double a-arm suspension mid-engined car. It’s all in the side wall and the compound.

 

For the Ferrari, it is very possible that both 18" and 19" setups are both within the "sweet spot" of handling for that car... iirc, the Stradale's tire sizes are front 225/35ZR-19 and rear 285/35ZR-19. For our Turbo, we'd have to go with 315/25/19. That's a difference of ~21mm in sidewall. Some have gone with 295/30-19, but that changes your gearing.

Again, the overall diameter of the Modena allows for an effective 19" setup, whereas for our Turbo, a 19" is not as effective because we have a smaller overall diameter. Just because a Modena can be effective with both 18" and 19" doesn't mean that our Turbo would be the same.

I know everyone wants a 19" setup for looks... who doesn't. That's why the advice is always 19" for looks, and 18" for performance.

Stephen

 

Cary,

What you say is true and as usual I will defer to your vast knowledge of suspension topics.

However, from my experience, given the same brand of tire (even though this is another complex issue you bring up), there is a noticeable difference in stiffness and ride quality between stock and plus sizing-- or in this case 18" vs. 19"

Spring rate or not, the tires with less sidewall seem to break more suddenly and ride worse.

And I suppose we can only make generalization-- but think about it, everything else being similar, aren't most 3.8 turbo motors putting out more power (area under the curve) than a 3.6? Larger displacement usually means more torque across the powerband. Ergo, less sidewall as the only variable, i.e. with the same carcass structure and compound will usually affect the spring rate, wouldn't it?

 

Tyson,

You're exactly right, assuming the sidewall is exactly the same construction and the only difference is the aspect ratio. I was just trying to point out not to assume this in all cases because, most of those differences can be compensated. However, in the case of a 315-25-19, what was it designed to used on? My guess would be, not a 3400 lb rear engine car. It’s not that they couldn’t make a 19 to work but that tire is not spec’ed for a 996. So, people are putting the wrong tire on the car.