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Help with Torque values

  1. Dec 2, 2015 #1
    I am building a remote controlled toy vehicle, tricycle style powered by a single wheel. Weight is 150lbs drive tire is 24in dia when calculating torque needed and HP
    So far I have got acceleration as 7.04 ft/sec that times mass of 150lbs should give me torque, at 2112 ft lbs that times rpm of 300 is 633600 divided by 5252 should give me HP of 120.6? That seems high to me. I was thinking it would be closer to 10 HP... Did I do something wrong? Thanks for any help..
     
  2. jcsd
  3. Dec 2, 2015 #2
    I forgot top speed should be 25 mph and should be able to reach that speed in 5 sec.
     
  4. Dec 3, 2015 #3

    billy_joule

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    Science Advisor

    Nope. ( ft/sec) *(lbs) = lb ft/sec Not just ft lb.
    The equation you've used is newtons second: F=ma

    If you combine that with the most convenient definition of power:
    P = Fv
    you get:
    P=mav
    In words, to have constant acceleration, the engine power must increase linearly with vehicle speed.
    Not at all what happens in reality, I'm sure you've noticed that the faster a car is going the slower it's acceleration is; Power is relatively constant so acceleration must decreases as velocity increases.
    In other words, your assumption that acceleration is constant is incorrect.
     
  5. Dec 3, 2015 #4
    Ok, but I was thinking that my end calculation of 120 HP was really high, can someone give me the right wheel torque and HP? Just a rough idea so I can order a motor, and figure gear ratio's.
     
  6. Dec 3, 2015 #5

    CWatters

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    Sorry I prefer to work in SI units..

    25mph = 11m/s
    150lbs = 68kg

    The acceleration from post #2 works out at about 2.2m/s2

    You could estimate the power required to keep accelerating at 2.2m/s2 at a speed of around 11m/s using

    P = FV
    P = maV
    p = 68*2.2*11 = 1645W or about 2.2HP

    Note this is just the power needed to keep accelerating at 2.2m/s2 at around 11m/s. At lower speeds less power is required.

    It does not include any extra power required to overcome rolling resistance or drag as you haven't specified this. You could measure it by towing the cart at the required speed and measure tension in the tow rope.

    Google suggests typical go-carts have around 2-3 times the above so perhaps shoot for around 4-6 HP. But it's your decision! Racing carts are in the range 20-30HP I believe.
     
  7. Dec 3, 2015 #6
    Thanks! It helps a lot!!
     
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