Help calculating rotational inertia

[abawp]

I need to compare a wheel and tire package on my car and I need help calculating for rotational inertia. I am comparing the weights and size of two different wheels and tires, and so far, this is what I have gathered:

14" wheel & tire (tire is a 195-60-R146):
Overal Diameter - 23.2126"
Wheel weight - 17 lbs
tire weight - 21 lbs

15" wheel & tire (tire is a 195-45-R15):
Overal Diameter - 21.9094"
Wheel weight - 17.5 lbs
tire weight - 16 lbs

Now, where do I begin?

 

[z-component]

Start by looking at some rotational inertia equations for disks/rings. What are they?

 

[abawp]

Don't think this is correct, but this is what I have come up with.

formula for rotational inertia for a disk:

I = 1/2 M * R^2, where M = mass, and R = radius?

14" wheel:

I = 1/2 * 17lbs * 7"^2
I = 416.5 in^2 lbs (not sure what the units are)

195-60-R14 tire:

I = 1/2 * 21 * 11.6063^2
I = 1,414.4172

14" wheel w/ tire:

I = 1/2 * 38 * 11.6063^2
I = 2,559.4178

Now the 15" wheel:

I = 1/2 * 17.5 * 7.5^2
I = 492.1875

195-45-R15 tire:
I = 1/2 * 16 * 10.9647^2
I = 960.0436

15" wheel w/ tire:
I = 1/2 * 33.5 * 10.9547^2
I = 2010.0921

Did I do this correct?

 

[Hootenanny]

Don't think this is correct, but this is what I have come up with.

formula for rotational inertia for a disk:

I = 1/2 M * R^2, where M = mass, and R = radius?

That's right.

I = 416.5 in^2 lbs (not sure what the units are)

Your units are also correct. The SI units for moment of inertia are kg.m², but as you are using imperial units (pounds, inches), your units are fine.

~H