19" vs. 18" Wheels/Tires

[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 .