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Homework Help: A simple question (but I don't knwo the answer)

  1. May 1, 2007 #1
    I have a bicycle wheel (A) with a circumference of 78 inches.

    I want to attach a wheel (B) to it that will spin FIVE times (for each of the ONE revolutions of A).

    What is the circumference of wheel B?

    (I honestly do not know how to calculate wheel/circle rotation)

    thank you.
  2. jcsd
  3. May 1, 2007 #2
    I would guess at 78 / .20

    Is this correct?
  4. May 1, 2007 #3
    Ooops...I meant 78 x .20

    And to further my problem:

    the circumference of the wheel (with tire on it) is 78" but I have a gear that is 75" in circumference which is attached at the hub. If I need a wheel to spin five times more for every revolution of the tire do I use the 78" or the 75" in the calculation to find the right size gear?

  5. May 2, 2007 #4


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    78 x .20 will do it unless you are connecting the second wheel to the gear and not the wheel.
  6. May 2, 2007 #5
    okay...so my assumption...

    was correct, in that it's the circumference of the circle (gear) is the source for the calculation.
    A gear half the size of another gear would spin twice the rotations, on fifth the size, five time, etc.
  7. May 2, 2007 #6


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    With gears it is better to use gear tooth counts rather than diameter (or circumference).
    Gears mesh so the effective circumference is not on the surface unlike a bicycle tire.
  8. May 2, 2007 #7

    yes...i am using the count of teeth...(chain driven) is there a formula? or just divide the smaller into the bigger?
  9. May 3, 2007 #8


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    Which way depends on the answer you want.
    But, divide one by the other is pretty much it.
  10. May 3, 2007 #9

    cool beans...

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