18" Wheels SC

[Ayles]

Recently I have seen a couple sets of Jongbloed and CCW 18" wheels that were built for an SC. I think it looks pretty good and least for those models weight wouldn't seem to be an issue.

I read a ton of threads on the topic but there isn't anything recent and they don't seem to address wheels that were custom built for this application.

Looking over tire availability, the number of options goes WAY up with 18" wheels.

[Driven97]

I think the biggest issue is overall diameter. A 255/40-17 is 25" tall, which nearly matches the OE 25.1" 215/60-15.

A 255/40-18 is 26" tall. That's a significant impact to our already long factory gearing.

Also, when comparing wheels, note it's not just weight that matters, but also where the weight is. A 20lb 17" wheel is going to have less rotational inertia than a 20lb 18" wheel, as all the weight (the rim section) is further inboard on the smaller wheel.

Not trying to totally poo-poo your idea, but trying to make you aware of possible issues.

[Quicksilver]

I'm really wondering when the style fad of big wheels and no sidewalls is going to turn. There is no justifiable engineering reason for the elimination of sidewalls. Comfort, compliance, and safety are improved with a larger sidewall. It is like every idiot who ever had HotWheels® as a kid grew up and bought a car based on them.

[Ayles]

Not worried about Poo pooing at all! Seems like people are running them, but at what cost and risk to the vehicle... I don't want to invest in wheels that end up having a wide range of possible problems.

I'm just doing research at this point. Mostly due to being a little discouraged by what it costs to get into a pair of 8 or 9 inch Fuchs these days. I'd like to avoid replicas at this point.

[Bill Verburg]

Originally Posted by Ayles

Recently I have seen a couple sets of Jongbloed and CCW 18" wheels that were built for an SC. I think it looks pretty good and least for those models weight wouldn't seem to be an issue.

I read a ton of threads on the topic but there isn't anything recent and they don't seem to address wheels that were custom built for this application.

Looking over tire availability, the number of options goes WAY up with 18" wheels.

It depends on what you want, the larger the wheel diameter the shorter the tire sidewall can be, this is generally good for performance but bad for ride quality.

911s are generally geared and have torque curves that work well w/ a drive tire OD of ~25"
rear tires w/ ~25"+/- OD are
225/50 x16 on 7 or 8 @~24.9"
245/45 x16 on 8 or 9 @~24.7"

245/40 x17 on 8 to 9.5 @~24.7"
255/40 x17 on 8 to 9.5 @~25.0"
275/35 x17 on 8 -10 @~24.7"

245/35 x18 on 8 to 9.5 @~24.7"
255/35 x18 on 8.5 to 10 @~25"
265/35 x18 on 9 to 10 @~25.3"
285/30 x18 on 10 @~24.9"

There are others a bit taller but shorter is generally preferable to taller and there are several really short 15" options too.

all will fit SC/Carrera rear flares w/ varying amounts of difficulty, the lower the profile and the wider the wheel the better for performance and vice versa

w/ some it's tough to get matching fronts and one size the elusive 275/35 x17 hasn't been generally available in decades

[Antonov]

I'm not advocating running 18's as I currently run 17's and 16's but just wanted to touch on a few points.

Not sure there are many tire manufacturers that make a 255/40r18 worth running, but there are plenty that make a 255/35r18 which are also 25" in diameter.

With regard to your statement on inertia, you are correct. However, I looked into this sometime ago, and when comparing tire weight size for size (255/40r17 vs 255/35r18) of the same manufacturer and model, I found that usually (not always) the 18" tire ended up being lighter than the 17" which would tip the advantage towards the 18" setup.

Originally Posted by Driven97

I think the biggest issue is overall diameter. A 255/40-17 is 25" tall, which nearly matches the OE 25.1" 215/60-15.

A 255/40-18 is 26" tall. That's a significant impact to our already long factory gearing.

Also, when comparing wheels, note it's not just weight that matters, but also where the weight is. A 20lb 17" wheel is going to have less rotational inertia than a 20lb 18" wheel, as all the weight (the rim section) is further inboard on the smaller wheel.

Not trying to totally poo-poo your idea, but trying to make you aware of possible issues.

[Bill Verburg]

Inertia is tough to nail down it depends not only on the total mass but also on the mass distribution away from the axle center-line, another reason shorter is better, whatever the absolute #s

[Ayles]

Thanks again for all the info!