Great post. On a similar note, it was interesting/eye-opening when I went and saw the launch of the half-million dollar 2010 Lamborghini LP670-4SV and it was sitting on 18" wheels as the stock wheels. The regular Murci sitting next to it had 19" as stock. Lambo rep said they determined that the 18's were deemed best for performance for the SV.

 

heavy chevy..so true...my car runs great.

 

+++++1 stick with 19's

 

True but the skinny side wall that you will need to fit a 997 will not help with performance...handling. That is what I am getting at.

 

I feel the same way and that was well said.

Ok, well I am glad to see it is not just me. Grrr.

Feel better now to see how many others think along same lines.

Cheers to my fellow enthusiasts!

 

Unfortunately I'm not able to dedicate as much time to 6Speed these days but in the past we always tried to be a technical resource for everyone so I apologize for HRE not having the presence we used to have here.

I just got back from spending a few days in the Bay area with the Ferrari Owner's Club and PCA guys as well as the Hooked on Racing group. We spent a lot of time discussing technical aspects of wheels/tires (Pirelli was introducing their new Trofeo tire as well) and this was often a topic with the different groups.

In general, going to a 20" wheel/tire package is going to do 2 things that are detrimental to the performance of the vehicle over staying with a comparable 19". First it is going to increase the unsprung mass which is going to hurt handling by introducing higher loads to the suspension and requiring the suspension use more force to get the wheel back down to the asphalt. You want to keep your wheel attached to the ground as much as possible when you hit a disturbance and the lighter it is, the easier it is for your suspension to do that.

The 2nd thing it will do is increase your rotational inertia. This is a function of the mass as well but has to do with the distribution of mass about the axis of rotation. The more mass you have farther away from the axis of rotation, the higher your rotational inertia. This means it is going to require more energy to get the wheel turning under acceleration and stopped under deceleration. Reducing your rotational inertia is easily seen by an ice skater pulling their arms into their sides to spin faster and moving them out to spin more slowly. Same basic concept. This is why putting 26" wheels on SUVs usually increases their stopping distances and why we don't offer heavy 24" and 26" wheels as our goal is to not hurt the performance of the vehicle, even when upsizing.

For an HRE wheel, our goal is to always offer the strongest and stiffest wheel we can but to keep the mass and rotational inertia low. Now that means if you're comparing a 19" P40 to a 20" P40 the effect is obvious as the 20" is going to be heavier. However, the tire does come into play as well but in general the tire is not going to be lighter in 20" than in 19". The sidewall will be stiffer which will hurt ride comfort and again add more force to the wheel and suspension if you hit a pothole. Now you want a stiff sidewall for a racing tire, but you're going to get that if you buy a 18" or 19" race oriented tire because it is designed into the tire, not because it is necessary due to the ultra-low sidewall.

Now that all being said, often we see our upsized wheel/tire packages still weighing significantly less than OE. On a full-size Range Rover, our wheel tire package was ~20lbs per corner lighter in 22" than the OE 20" package. That's a RR and not a 911 so don't expect savings to be that extreme but we saved quite a bit per corner using our C93s on a 997 GT3RS a while back but that was staying with 19" and the OE tire. We saved 4lbs/front and 7lb/rear PER corner.

At the end of the day, if you are truly looking for performance for a 997 then stick with 19" or even consider go down to 18" if you're going to be tracking it a lot (we're coming out with new 18" race oriented Monobloks very soon so consider yourselves the first to know). If you really like the look of the 20", feel comfortable knowing a 20" HRE is designed to minimize these negative effects and may not be noticeably different than the factory 19". I have 19" P40s on my 997 so I can't 100% say from direct experience about the 20" but we've not heard any complaints and I think this is because the 20" wheels are still lighter than my OE 19" (Sport 5-spoke).

If you have any more questions, just let me know and we'll try to answer them.

 

Kudos Alan and HRE for a great response!

 

Actually increasing tire size does not affect the size of the contact patch. It can reshape it, but that's all. Contact patch size is based on only two variables: vehicle weight and tire air pressure.

 

Interesting posts, and another illustration that there's a lack of basic physics knowledge among many Porsche owners. To say nothing of spelling skills.
Regardless, I just went through a lengthy process of purchasing a new set of wheels for my 997. To be fair, I just didn't like the looks of the lobster claws (the longer I owned them, the less I liked them). I had my heart set on a set of 18" wheel/tires to reduce unsprung weight and increase compliance, but was frustrated with the limited options of wheels and tires from which to choose. I might not have gone about it properly, but I ended up with a (very nice) set of GT3 wheels. I know they are anything but light, but they look great and I'm sure they pass the strength test.
Overall, a bit disappointing in that I wanted to get a set of Michelin Cup tires for the 18" wheels, but they don't seem to exist in the right size.
All that said, 20" wheels seem like a bad idea unless you just have to have the look. And for those who want to sacrifice performance for looks, I suspect that they won't miss what they don't appreciate. And likely won't keep the car long enough for the wear and tear to matter to them.

 

No width and diameter change contact patch size.

I'm talking abou wheel diameter, not just the tire.

 

you could put 165-15s from the original 901 on a modern 997 (yes, I know they wouldn't fit - I'm trying to make a point) and if you inflate both sets of tires to the same psi, the tire patch size would be exactly the same as 305s. Only the 165's patch would by long/narrow and the 305's would be short/wide, but the area of the patch would be the same.

I used to think the same as you. It only makes sense that the tire patch should increase with size, but physics proves otherwise.

 

Might be true in theory but I'd emplore you to do some research on the actual findings. Tires over different makes do not flex like balloons. They have sidwalls that remain upright and the contact patch has a lot to do with their deflection. Not only that but decreasing PSI does not always equal an increase in contact patch, thereby making your simple theory void.

Take a look at this ACTUAL TEST (not theory) of contact patch.

http://www.performancesimulations.co...on-tires-1.htm



"In essence, these tables are likely a rather conservative view and understate the dramatic change in contact patch pressure, further indicating that contact patch pressure is not very well tied to air pressure at all."

 

+1

The ironic thing is that if you put 20"s on the car to improve the looks - unless you have PCCBs or an S the base Carrera rotors look tiny.

I use my OEMs for r-comps and have some 19s for the street. Very happy with that setup.

 

That is funny as hell. ::

Possible signature material right there!

 

Pretty funny bunch of posts from my fellow enthusiasts. I'm with you 110%. Nevertheless, I'm sure our friend at HRE sells a boatload of 20's for 997's. We have all undoubtedly bought plus 1 wheel sizes for other cars. Doesn't make much sense for the 997, but to each his own and I guess it can look cool. For 90% of owners, they'll never notice the performance difference. Hey, I ain't bolting 23's on a RR either, but that isn't stopping a lot of people. To each his own. Do whatever turns you on. There are plenty of very good options available, from HRE and others. Lasting damage to the car, I really doubt it.