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Wheels and Crazy Spinning Things

  1. Dec 17, 2003 #1
    Wheels and Crazy Spinning Things!!!

    For the illustration of the following model, go to http://www.corniceventures.com/images/wheels.JPG

    If the 2 wheels ("B1" and "B2") are the same diameter (40 inches) and are rolling forward at 4 miles per hour...Would belts "D1" and "D2" rotate hubs "C1" and "C2" respectively at the same rate (rpm's)?

    Here are all the parameters:

    "B1" and "B2" Diameter >> 40 inches
    "A" Diameter >> 12 inches
    "C1" and "C2" Diameter >> 1 inch

    Here is what my calculations told me:

    "B2" Circumference >> ~125.66 inches
    "A" Circumference >> ~37.70 inches
    "C1" and "C2" Circumference >> ~3.14 inches
    "B2" RPMs at 4 MPH >> ~33.61 rpm
    "A" RPMs at 4 MPH >> ~112.05 rpm
    "C1" and "C2" RPMs at 4 MPH >> ~1344.54 rpm

    To me it seems that belt "D2" would rotate hub "C2" more quickly than belt "D1" would rotate hub "C1" given the same forward speed, but maybe I should trust my calculations! Any help would be appreciated, thanks in advance!
    Last edited: Dec 17, 2003
  2. jcsd
  3. Dec 17, 2003 #2


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    Gold Member

    Both big wheels spin with ω = 4mph/40in. (convert that into rpm)

    Assuming the belts don't slip:
    ωA*rA = ωC1*rC1


    ωB2*rB2 = ωC2*rC2

    Note that ωA = ωB2 = ω

    Rearrange to find one hub speed in terms of the other:

    [itex] \omega_{C1} = \frac{r_A}{r_{C1}}\,\frac{r_{C2}}{r_{B2}}\omega_{C2} [/itex]

    and since the hubs (C1 & C2) are the same size:

    [itex] \omega_{C1} = \frac{r_A}{r_{B2}}\omega_{C2} [/itex]

    So the hubs spin at different rates; since the radius (diameter) of B2 is greater than A, Hub C2 will spin faster than hub C1. So, your intuition was good, but there was some error in your calculations (I can't say where you went wrong without seeing your work). You should follow this line of thought out yourself to understand the idea and to double-check the algebra.
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