19" vs. 18" Wheels/Tires

[Johnny Danger]

Originally Posted by DenverSteve

Johnny,

I am looking at a set of tires/wheels for winter wear that are 19" and the rear tires are currently 295/30's. The tire calculator shows them to be comparable to my stock 255/40/17's. The offset would go from 55 to 67. You seem to have much more tire/wheel knowledge than I. Do you think these will fit okay?

S.

Well, to begin with, in my opinion 295 is too wide a tire for any application regarding a 986 boxster; let alone for winter use. As far as offset in concern, +67 (I suspect these are 996 wheels) is way too high for your vehicle. Therefore, if you're determined to use them, then you're going to have to bring the offset down to around +45, and replace the tires with something more suitable . In fact, you'll be hard pressed to find much in the area of 19 inch winter tires. Personally, I would stick with a 17 inch wheel package for winter driving .

[DenverSteve]

My plan is actually use my existing 17's for snow and use new wider for Summer/Performance tires. I want the best wheel-well fit (best filling the well) on best performing wheels such as Carrera Lights or similarly lighter wheels.

[cheetah]

Even if the 19" weigh less than the 17's, the effective mass (moment of inertia) is much higher. Simply put, the weight is farther from the axis of rotation.

I currently have 18's 225-F / 265-R on my 02 S, and am going back to 17's. The 18's were great until I did all my suspension mods (PSS9's, solid everything, etc.) but now I want a little more sidewall. When it breaks loose, it happens suddenly.

Also, I am getting some lightweight, Forged rims so the wheel weight will go down significantly. Weight reduction is my plan of attack for now, since I have a heavily modded 3.4L which puts out a lot of power already.

The big rims look great but have their downsides.

[Johnny Danger]

Originally Posted by cheetah

Even if the 19" weigh less than the 17's, the effective mass (moment of inertia) is much higher. Simply put, the weight is farther from the axis of rotation.

I currently have 18's 225-F / 265-R on my 02 S, and am going back to 17's. The 18's were great until I did all my suspension mods (PSS9's, solid everything, etc.) but now I want a little more sidewall. When it breaks loose, it happens suddenly.

Also, I am getting some lightweight, Forged rims so the wheel weight will go down significantly. Weight reduction is my plan of attack for now, since I have a heavily modded 3.4L which puts out a lot of power already.

The big rims look great but have their downsides.

I disagree with you assertion . Ultimately, it comes to reducing unsprung weight and rotational mass. If the overall circumference is consistent, regardless of diameter the the lighter wheel will create less inertia .

[BYprodriver]

Originally Posted by Johnny Danger

I disagree with you assertion . Ultimately, it comes to reducing unsprung weight and rotational mass. If the overall circumference is consistent, regardless of diameter the the lighter wheel will create less inertia .

What would be the correct assertion if the overall circumference is inconsistent?

[Johnny Danger]

Originally Posted by BYprodriver

What would be the correct assertion if the overall circumference is inconsistent?

Here's an example:

The original factory set-up on my vehicle consisted of 18 inch Sport Design wheels

18 x 7.5 225/40/18
18 x 9 265/35/18

Current wheel set-up

19 x 8 235/35/19
19 x 10 275/30/19

Practically speaking, the overall circumference of the wheel and tire combination is similar (had I kept the fronts at 225 and the rears at 265 the circumference would have been virtually the same) . However, I have reduced approximately 60 lbs.+ of unsprung weight and rotational mass ! As far as acceleration, braking and overall handling is concerned, this has proven to be a night and day difference over the oem set-up . And, ride quality has not suffered one bit.

[cheetah]

Originally Posted by Johnny Danger

I disagree with you assertion . Ultimately, it comes to reducing unsprung weight and rotational mass. If the overall circumference is consistent, regardless of diameter the the lighter wheel will create less inertia .

Disagree or not, physics shows that with all else being equal (same tire circumference, same width, same rim style & density), the larger rim setup will have a larger rotational mass.

Most of the tires mass is on the tread (which diameter & weight doesn't change with rim size). Most of the mass of the rim is the on the circumference of the rim (which does change with rim size). Moving this large weight farther from the axis has an effect on the rotational mass. More of the weight differences comes from the metal rim vs the rubber tire changes.

I have enclosed a simple display of this from a 3D modeling program. The units are converted to simple rotational mass, but they show the comparison of an 16x8 rim with a 25" diameter tire to an 18x8 rim with a 25" diameter tire. This is a good test for our Boxsters.

The 18" rim setup shows a 23% increase in effective mass of the wheels, even though the overall circumference of the wheels stays the same.

[Johnny Danger]

The point that you're missing - is aside from comparing wheel/tire fitment of equal overall specifications, when there's a significant difference in weight between the two, this results
in a major reduction of unsprung weight and rotational mass .

[blue2000s]

Originally Posted by cheetah

Even if the 19" weigh less than the 17's, the effective mass (moment of inertia) is much higher. Simply put, the weight is farther from the axis of rotation.

I currently have 18's 225-F / 265-R on my 02 S, and am going back to 17's. The 18's were great until I did all my suspension mods (PSS9's, solid everything, etc.) but now I want a little more sidewall. When it breaks loose, it happens suddenly.

Also, I am getting some lightweight, Forged rims so the wheel weight will go down significantly. Weight reduction is my plan of attack for now, since I have a heavily modded 3.4L which puts out a lot of power already.

The big rims look great but have their downsides.

+1. I was going to post the same thing until I came across your post.

[Johnny Danger]

Originally Posted by DenverSteve

My plan is actually use my existing 17's for snow and use new wider for Summer/Performance tires. I want the best wheel-well fit (best filling the well) on best performing wheels such as Carrera Lights or similarly lighter wheels.

Here are some guidelines to follow if you're looking to upgrade to 19 inch wheels:

Front Fitment :

Wheels - 19 X 8 to 8.5 with offsets ranging from +35 to 45 depending how aggressive you want your set-up to be .

Tires - 225 to 235 width

Rear Fitment :

Wheels - 19 X 9.5 to 10 10 with offsets ranging from +35 to 45 . Again, depending how aggressive you want your set-up to be .

Tires - 275 to 285 width

This is what my set-up consists of :

19 X 8 +31 235/35/19

19 X 10 +31 275/30/19