# Help calculating rotational inertia

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?

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

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
Staff Emeritus
Gold Member
abawp said:
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.

abawp said:
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