Your opinion(s) are highly asked of....... as well as other "experts" of the 6speed forum.....

I was thinking of going to 19" HRE wheels - 19 x 8.5" fronts, and 19 x 12" rears for the GT2. Am I making a mistake. Should I leave the wheels stock? I feel if I pay big bucks for aftermarket rims, they should be easy on my eyes with a sharp, low profile look/appeal. Performance wise, what (if any) sacrafice am I making?

-Shank

 

Thought my ears were burning. From a performance standpoint you will not go backwards in any manner. Yet move a bit forward. Typical issue with the 19s is ride, yet I doubt with the setup you have that will be an issue. I can't say it is a mistake. Wheels are a very personal aspect and signature to your car. I say if it makes you turn to do the look back in the parking lot then you should do it. No bad issues even though the PSM is supposedly suppose to have an issue with them. I have yet to see that and it doesn't apply to you anyway. So I say do it!!

 

Stephen, I hope you are not telling me words I want to here. Hit me hard, I do like the style of the stock wheels, Though I am in love with Fred's 543's and Oak's 843's (Originally considered for my AWD.) Seems like the GT2 comes with rolled fenders, but I think thats to accomodate the factory 12" rears.

 

Yes, they are rolled to accommodate the 12, but I will tell you I am running 12s and 315s on a 28 offset and I don't need to roll mine The fronts I am working on. The theory originally was the outward pressure of the 19s caused the wheel bearings to fail prematurely. I dismiss this hog wash something Porsche came up with. Not sure why, but I have run 19s on even my old 79 Twin Turbo. I never had one issue. The only over all thing I noticed was a little less quality in my ride due to the overall sidewall size. Less to absorb going down the road.

I wouldn't tell you want you want to hear. Lord knows I haven't to this date and won't lie to you now I say do it!...

 

Go for the HREs man....I will be getting the 84*R soon (mystery number left out intentionally - LOL)

 

Shank,
I wasn't one of the ones in your list but i figured I would chime in nonetheless. By the way, I am deeply hurt.
Many of us are running 19's and it has been generally stated that you go 19's for looks and 18's for performance. I can't say that I agree with that totally. Following are my thoughts on various aspects of 18/19 comparisons:

Rotational Inertia
This is best defined in laymen's terms as the resistance of an object to spinning. The more massive (generally referred to as heavier) a wheel, the greater the resistance to acceleration. I am speaking of accelerating the wheel, not necessarily the car although they are directly related. Think of it this way, you have a 5 ton satellite floating in space with a 1 pound garbage can floating next to it. An astronaut has a much easier time rotating the lower inertia garbage can compared to the satellite. The same holds true for wheels. There is a property of materials called "polar moment of inertia". This is a calculated numerical value that describes the objects resistance to rotation. The more mass located towards the outermost diameter, the greater the moment of inertia. Since tire weighs far less than rim, the lowest moment of inertia would be a 3" rim with an enormous sidewall. Great for rotational inertia characteristics, lousy for sidewall flex. One other point that needs clarification. The highly regarded "wheel weight" by itself is not a good measure of a wheels performance. Consider this comparison. Two wheels of equal weight. Wheel A has 80% of its weight located near the rim edge whereas Wheel B has 80% of its weight located near the hub. Wheel B will accelerate faster that Wheel A every time! Moreover, a heavier wheel will accelerate faster than a lighter wheel if its weight is distributed better (closer to the hub). This is why I personally prefer wheels that have spokes that are larger at the hub and taper to a smaller cross section at the rim. The golden rule, keep the mass near the hub. The winner depends on Polar Moment of Inertia.

Sidewall Flex
Take a look at that old Weltmeister Camber Truss advertisement and you will get this one right away. They showed a drawing with the car in an extreme turn. The sidewalls were angled as a result of the lateral shear friction forces acting on the tires in a turn. The taller the sidewall, the greater the flex. The greater the flex, the more squishy (engineering term) the feel in a turn. This squishiness equates to a lack of control since the car will shift that much more to each side in turns. The shorter the sidewall the better. 19's win.

Ride Compliance
Ride compliance is the ability of the car to be Lexus-like. Float over the bumps while traveling straight doewn the road. Each bump is absorbed by the tire sidewall flex (both sidewalls flexing outward uniformly as in the radial bulges bulging further), the springs, the seat cushions and your own bodyfat....yeah, your ***. The shorter the sidewall, the less compliant. 18's win if you are looking for a Lexus type of ride, 19's win if you want a sports car ride.

Suspension Compliance
This pertains to the suspension's ability to act quickly. This is totally dependent on the weight of the wheels and irrespective of the distribution of the weight of a wheel within itself. Best case, as light as possible to keep the unsprung weight as low as possible so the suspension doesn't know it has wheels attached.

Looks
19's win in every case. That is probably undisputed.

So, in summary, you want a light wheel with its mass concentrated near its center ideally with tapered spokes.

My advice, go for what gives you chills man!

 

19's bend easier. Makes the car more of a hassle to drive when you have crappy roads.

By the way, how do you know if you have a bent wheel (aside from sticking it on the spin/balance machine, visible damages)? Are there bents that we can't see?

 

Come on guys (especially KPV - you know better), discuss the drawbacks to 19's as well.

#1. There are highly limited tire choices for 19's, especially R-compound tires.

#2. You have to be more sensitive to road hazards as you have less room to absord the shock of pot holes, etc.

#3. The suspension was tuned for 18" tires (weight and compliance) so may need to be re-optimized. I'm not sure of the magnitude of this factor.

#4. Unsprung weight. The mass of the 19" set-up is almost always higher with a 19" set-up compared to the stock 18" hollow spoke. I'm not sure of the magnitude of this factor.

#5. Bigger Brakes. Going to 19's allows bigger brakes, a nice plus.

What does this all add up to? I don't know. The biggest factor I see right now is tire availability. Until that I addressed, I will stay with 18's.

KPV: a better demonstration of moment of inertial is a cannonball and a dumbell. If the both weight 20 lbs, the canon ball is much easier to rotate, but the dumbell with 10 lbs at each end is much more difficult to rotate.

Tim

 

Ryan,

Weather or not the 315's fit without rubbing depends on the brand tire you are using. Yes, some are wider than others. I have had sets on that fit and others that rubbed. There is a difference in the tread width among tire manufacturers.

If you are really looking for the best performance go with the 18's. The 18's presently have far a away the best choice in street legal performance tires. Not to mention dedicated track tires. You You have a chance to try them. It won't do me any good to try to explain the difference. All I can say it is like night and day.

 

this is not an expert comment by any means, just that of a heavy user:

form follows function. i run nasty bumpy roads and high speed and want the best performance, lowest weight and best tyre options: 18's.

some would claim better apperance is a "function" but i don't agree.

 

ColorChange,
It was late!! Cut me some slack. OK, with respect to your comments.........
#1-agree
#2-agree
#3-disagree, the suspension will have been optimized for the sprung weight not the unsprung weight. The springs are sized to support the car (whether it be a fat and heavy TT or a stripped and lightweight GT2), the shocks are designed to dampen the oscillations the springs produce given the spring constant of the the spring used and the mass of the car.
#4 - Agree that 18's are usually lighter thereby reducing unspring weight (See suspension compliance in my previous post), however, if you choose a 19" wheel and tire setup that matches the overall tire diameter of the stock setup the CG (Center of Gravity) is precisely at the same point. The distribution of mass may be different (See Rotational Inertia of my original post).
#5 - Wholeheartedly agree and this is what I am currently looking into with my additional 185 horses.

I agree about tire availability. It is a problem. Fortunately, and arguably, I like Yokohama's. They are on the TT, the MINI and the VehiCROSS. I fell in love with them back in the day of the A008's.

Cannonball vs Dumbbell. Good analogy Tim!! I feel like the dumbbell with my analogy. LOL THanks!

One last thing. Hamman7 enlightened me on this. I made a comparison in the past to the upcoming 997 and its use of 19's. I used that as an argument for 19's. The difference we will probably see, however, is that Porsche will probably increase the rolling diameter of the tire thereby avoiding the very short sidewalls of the current 19" tires that must match the 996's overall diameter. Just another point, although not really pertinent with this conversation.

I am not making a case either way. I am simply stating facts about engineering mechanics. Seeing the height of the sidwall on my car, if I were to track it or pursue high speed events as Watt does, I would opt for 18's. The fear of screwing up a rim in a hard corner at a track would bother me. Likewise for the avoidance of potholes and the like.

If you are after a great look and only partake in normal driving and an occasional thrash, then 19's may be for you.

 

Shank, unless you are going to be tracking it 19 inch all the way!

 

Well I plan on tracking it, but I will keep the stock wheels for that. I am strictly talking for the street.

 

I Agree. 19" offers a better look. I am running 19" on my 996 and would not trade them for 18's. Why else would you pay over $500 for rear tires--Bling Bling

 

KPV - you can catch me when I screw up too. The only reason I dinged you is I have seen a lot of your posts in the past and you are usually quite good. You may have me on point 3 but I don't think so. I can't get a hold of my racing engineer so I will try on my own.

The suspension not only supports sprung weight (yes the majority), but also absorbs/dampens wheel deflections (unsprung weight). The dynamics of the wheel deflections will change now that the mass is reduced. This means different suspension dynamics and dampening/rebound (shock valving) would probably be optimal. The high frequency (unspring) of the suspension has changed, the low frequency (sprung) has not.

I'll bet Porsche tunes the suspension differently for the new 19's on the 997.

I will check with my tech later if I'm wrong or he has more to offer.