I understand the basics:

Pros:
- Less side wall flex since one can use shorter tires
- The look
- Equals stiffer suspension since a rim won't deform as much as a tire

Cons:
- May be more weight
- Easier to bend

However, is there someting abuot the center of gravity? i.e. If one can get really big rims and big tires to match, the height of the center of tire can be close to the heigth of center of gravity of the car (F1?). Not sure it has any effect. Can anybody shed some light?

Also, at least on TV, both NASCAR and F1 tires don't seem to have very short side walls. How come they are not shorter?

 

YP, the stiffer tires may or may not be a pro for bigger rims. It depends upon the rest of yoyur suspsenion. For our cars with PSS9's or similar, I think the 18 will handle better because the tire compliance actually helps grip. If you design the suspension for 19's, it could and should be better than 18's.

Another drawback for 19 is that they have higher rorational mass (they tend to be heavier, farther from the axle)l. This is more difficult to rotate and the car is a little slower in accelereating and stopping (all other things being equal). It is the same idea as a lightweght clutch, but the opposite.

 

more cons, fewer pros:
less side wall means less forgiveness, snappier breakaway.

'stiffer suspension' means more shockload on shocks not
designed for it, and more range of motion of suspension
arms than designed for. May induce undesirable changes in
wheel alignment at max compression or droop.

'The look' - Let me preface this with emphasizing that this is
my individual opinion, but when I see 996tts with 19" wheels,
it's like seeing gorgeous female track stars showing up in high
heels. Undeniably sexy-looking but also palpably compromised
as far as actual functionality. Of course I am exagerating the
functional difference, but 19"ers are a step back from what the
car can do, IMHO.
Joe

 

I agree with Joe 100%.

 

Not that I am disagreeing.....because I am not........but what would your opinons be of the 997 19" wheel choice by Porsche? I was originally led to believe the overall rolling diameter would be larger, in effect preserving side wall height, but the latest photos and specs indicate the use of 235/35/19's in front and 295/30/19's in the rear.
What do you think Porsche did to the suspension to accommodate this choice?

 

I would assume Porsche designed the "sport" suspension option(standard on the S) to work with 19" wheels. I also assume the 997 final drive ratio (or certain individual tranny gear ratios) were changed from the 996 setup, since the 997 19" rear wheel/tire combo will, in deed, be over an inch taller than Porsche's current 18" offerings.

 

Jack,
I thought the heights of the 997 19" tires and 996 18" tires were roughly equal (Compare the spec sizes). I don't think the 1" height difference is correct.

 

Dittos.

 

I don't know about the option, but if may be either:

1 - the suspension geometry and/or shocks were altered so
the effects of the 19" wheel option do not have any bad effect.
2 - The negative effect was deemed small enough to allow
Porsche to ignore, and be able to offer options that a segment
of their market is clearly willing to pay for.

This would not be the first wheel performance compromise
that Porsche offered for aesthetic reasons. Their 'Sport Techno'
wheel is an 'art' wheel for the 996tt. It is narrower than stock
in the rear, and actually requires wheel spacers to run on the
996tt, but money is money...

I have known cars whose suspension geometries were
benign within the 0-95% physical range of motion, but at 100%
such as full compression plus a bump, the arms suddenly went
from inducing toe-in in the rear (safe and stable) to adding
toe-out, which is demanding sudden oversteer just when it
is most dangerous.
Joe

 

Ken: I can always be wrong. However, if current 996's are running 285 and 295x30 18" tires, then a 295x30 19" tire should be approx. 1" taller. Don't you think so? It's true that the front tire/wheel combo will be approx. the same (235x35 19" vs 225x40 18").