My NB 993 has now the original Cup II wheels (205/50 and 255/40). I have a long time admired the RUF 993 wheels, and the new HRE Classic Series are great as well. A move to 18” wheels would then be required. How would such wheels change the driving feeling etc.?

 

In general terms, one will find 18" wheel/tire to be slightly more responsive due to the smaller sidewall. The trade off is that the tire will also be less forgiving and will transmit bumps in the road. Another (+) is that you may be able to fit a wider tire on the rim. The stock 18" wheel set uses 225/40/18 and 265/35/18 tires front and back.

 

The wheels will not change anything. What will change "feeling" is your choice of tires.

If you keep the same tire circumference and width, the traction will be the same. What will change is the sidewall height, which will make the ride stiffer. The 18" wheels are usually wider than 7/9 Cups, usually in 8/10 range for NB. That means that you can go with 225/265 combo or even a bit wider (I would not recommend going wider).

If you go with 18" wheels you will need steering rack brace.

 

17" tires are generally less expensive, so there is that going for 17s.

Rufs are heavy compared to Cup IIs. Mostly the ride will be affected. as stated above, more responsive, harsher ride.

 

Having overhauled the suspension recently on my 993 (w/ H&R Strusts and Elephant Racing Sport hardness bushings among other bits) and still riding on Cup 2 17" wheels, I am becoming a bit more hesitant to switch to 18" wheels for street use. I feel plenty of bumps and jolts on Northern California's less than perfect streets.

I might reserve the use of 18s for ideal road surfaces only (i.e. track use only). Or I might finally give in to the urge and mount some 18s after all...

OP, are you asking with the intended use on street, track, or both?

 

Me too, although I'm still riding on my original suspension at 30K miles. I really like the ride with the 17's and I'm hesitant to mount 18's.

 

I recently just switched to 18s and can hardly tell a difference to my old 17s. Based on posts here, I was expecting a much rougher ride, but it's suble. I actually prefer the ride in the 18s, in my limited exposure so far.

 

Someone who has done this switch from 17" to 18" recently - please comment on the seat of the pants change in feel, if any, in the change/increase in unsprung weight for a street-driven car.

Can you feel the added weight slowing down initial acceleration from a dead stop?

Can you feel the extra braking time/distance needed to slow the car once moving?

 

Why do you assume added unsprung weight??

 

I changed from 17"s to 18"s two years ago on my 96 targa. Can hardly tell any difference. Tire longevity seems to be about the same and in spite of hearing the glass targa top cannot take the additional stiffness, I've experienced no problems. I also do not notice much change in braking. I have to say that I like how the 18"s look on my car.

 

Assumption is that there is added weight in a bigger tire. Please educate me if this is a wrong assumption.

 

I just did this switch last week and am very pleased. Ride quality is barely different, much to my pleasant surprise. As for unsprung weight, there are several factors when comparing different tires and wheels since the weight varies greatly. Doing this for me was at least as much about performance as appearance, so I chose wheels (O Z Allegerita) that were lighter than the Cup II's. When it was all said and done I saved about 3 to 4 lbs per corner and it is definitely a difference that I can feel. My car accelerates a bit snappier and turn in is much sharper with the shorter sidewall and I have to believe less rotating mass to change direction.

 

Why is it a bigger tire?

it's fairly easy to find a combination of 18" wheel and tire that wil be lighter than 17" Cup and 17" tire. Don't forget, in 17" tire you have an extra inch of sidewall, which is not light. Hell, let's look at some examples, shall we? These are specs for Dunlop ZII: http://www.tirerack.com/tires/tires....el=Direzza+ZII The difference in weight between the 255/40-17 and 265/35-18, the two tires I've personally used on my 993s, is one lb. About the same holds true for a Michelin PS2: http://www.tirerack.com/tires/tires....ilot+Sport+PS2 Surely, a light 18" wheels can easily be found that would be more than 1 lb lighter than 9" Cup II at 22 lbs. Heck, OZ Alleggerita in 18x10 are 3 lbs lighter.

Having said all that... for a street car... it doesn't matter, in my opinion. I have about 250+ autocross events under my belt, maybe about 2,000 runs in 100+ different cars. Including my own 2,100 lb CSP car that I've been driving for 11 seasons on 25 lb 13x8.5 wheel and tire combo (sic) and 33 lb 15x10 wheel and tire combo. I'll be damned if I can really feel the diference in acceleration due to weight. Maybe others have a much finer calibrated butt dyno, I don't know.

 

When I switched from Cup II's to 18" MY02's I never noticed a difference in acceleration or ride. Then I switched to 18" BBS RS-GT wheels and again noticed no difference. Could be because the 18" wheels I switched to are lighter then the cup wheels they replaced.

 

BTW, I feel _clear as day_ differece when I switch to snow tires on 17" 996 10-spoke wheels. Not in acceleration or braking, of course, but the ride is cloud-soft in comparison. Well, d'oh, the snow tires' sidewalls are made of tasty marshmellows.