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Help calculating rotational inertia

  1. Mar 20, 2006 #1
    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?
     
  2. jcsd
  3. Mar 20, 2006 #2
    Start by looking at some rotational inertia equations for disks/rings. What are they?
     
  4. May 10, 2006 #3
    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?
     
  5. May 10, 2006 #4

    Hootenanny

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    Staff Emeritus
    Science Advisor
    Gold Member

    That's right.

    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
     
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