For everyday street use...

What are the tradeoffs/benefits of 17" vs. 18" wheels? Do these benefits change for a C4 vs. C2?

Thanks,
Eric

 

Ive found that on standard suspension, the 18" give a firmer/harder ride whereas 17" are softer.

However 18" wheels can look so much better than the 17".

 

Eric
Been there, done that. After careful analysis a few months ago, I found no logical reason to go from my 17" Cups (w/Pilots) to 18s.

... except, I think the car looks better w/18s & I really like the turbo twist look. So my car now rides on 18s (w/Assimetricos) & I'll use up the Pilots at DEs & then either sell the wheels or mount R tires

Moral: forget the "pros and cons" - the best P car expenditures are emotional. <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

 

18” wheels are wider, so you get more rubber in contact with the road, which improves handling. They are also heavier, so you get reduced acceleration, and have shorter (stiffer) sidewalls, so you have a firmer ride.

If I recall, someone did a test with a Boxster, with 16”, 17” and 18” wheels around a track someplace. The 17’s had the quickest overall lap time.

 

Related question:

I've always been curious why all those circa 80s 911s that are seriously track/race prepped stick to the 16" and even 15" rims? Obviously, they could choose any size, yet practically every well-prepped pre-90s car I've seen wear the Fuchs, and most in 15"...there must be performance reasons, right?

Edward

 

Read <a href="http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=3&t=001620&p=" target="_blank">wheels discussion</a> and be even more conflicted. The 18" give good grip and have minimal weight penalty if they are hollow spoke, but the trade off is somewhat harsher ride, depending on the tires you choose. The only way I could understand a pre-90's doing better with smaller wheels is that the weight penalty becomes significant with lower HP and torque.

 

I recall reading somewhere that the all-wheel drive system is calibrated for the wheels on the car, so using different tire sizes can confuse the system (wheels would spin at slightly different rates). If that's true, would changing to 18s on a narrow body C4 that came with 17s be an issue? Obviously, the C4S has 18s, as does the Turbo, but is the computer programmed accordingly? Is the computer even involved in that viscous coupling?

Vic
95 C4

 

I have 18" hollow spoke wheels on my 97-993 C2. My only complaint is there is not as good a selection of 18" tires as there are 17" tires but I expect that will change as more cars have 18" wheels.

Regards,
George
In sunny Arizona

 

Vic,

Do you think each car's 4WD system was individually calibrated based on which wheels were going on the car? I don't know anything, yes, literally nothing about this, so I can't even be skeptical... I've seen C4 with both 17" and 18" wheels, and most seem to be original equipment. Hmmm...

Thanks,
Eric

 

I believe the issue with the C4 is that you need to keep the wheel's roll out distance (or circumference) the same as the factory wheel and tire combinations. I believe that on the 964s the roll out has to be within 6% of the factory's spec. I am not sure if the 993 has a similar requirement or not.

 

Edward,
I was told that many of the 80's 911 guys opted for 15" wheels because it was an easy way to lower their car for track events.

 

To "Lower their cars"? I don't think so, 993RS. They had torsion bars to adjust for height, no? Still, I'm curious why they uses the size that they do.

Edward

 

I think that the 80's Carrera guys used 15 inch wheels because they got lower gearing. A 225:50:15 has a smaller diameter than the same size with a 16 inch diameter. Same width, more rev's per mile.

 

Though I've never seen any dire warnings about changes to the C4 wheel/tire, it's true that efficient operation of the AWD system relies on a certain difference in rolling radius front to rear to pre-load the viscous coupling that transmits torque from the gearbox to the front.
That's about the extent of my knowledge ( EEs don't study ME after freshman year ), but I have seen an excellent write-up on this by Paul Frere in one of his books. Net: whatever changes you do carry out to the C4/C4S, make sure that the ratio of the front/rear rolling radii are close to stock.

 

You guys are amazing. I understand less than half of what you are talking about, yet I keep coming back!

'95 Triple Black Cab