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Calculating max velocity of car

  1. Jun 23, 2013 #1
    Hey! I have a project where I'm building a car running from direct sun-light only using solar-panels and bike hub motors.

    I would like to determine max velocity before cashing out on motors and panels. I have some numbers from the panels and motors I looked on, how do I turn them into the max velocity of my vehicle?

    Mass: 200 kg
    Tire diameter: 12" or 30.48 cm
    Rolling resistance: 13.748

    Torque: 33 N.m
    RPM: 120 or 240
    Gear ratio: Not sure if it's necessary since there is no gears and the wheel is running directly on the motors axis.
    Diff ratio: I guess it's 1 since there is no gears.

    Some of the numbers like the torque may be inaccurate since I have got it from some calculators in the internet.


  2. jcsd
  3. Jun 23, 2013 #2

    Andrew Mason

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    Science Advisor
    Homework Helper


    At what speed?

    The maximum speed will depend on the aerodynamics of the body of your car as well as the rolling resistance. You will find that the torque produced by the electric motors depends on their speed and load. I would suggest that you work on making the car as aerodynamic as possible first. You will need enough surface area to generate the power you will need and you need to incorporate those panels into your car while maintaining an aerodynamic shape.

  4. Jun 24, 2013 #3
    The rolling resistance is in N so only 13 newtons.

    Aerodynamics will not be so much of a problem I think, because it will have two panels as a roof and light-weight steel frame with four wheels, a steering-wheel and a seat.

    I have some diagrams which maybe can be of help:

    http://www.electro-mobile.se/uploaded/project/documents/281/Datablad 4033.jpg This is a 12" wheel which uses a hubmotor.

    http://www.electro-mobile.se/uploaded/project/documents/282/Pie48V.pdf This is a 16" hub motor with a wheel attached.

    And this is the last one, a 12" hub motor with a wheel attached: http://www.electro-mobile.se/uploaded/project/documents/359/E4037-211data_a.pdf

    The wattage my panels will generate is 820 watts and max amp of 16, and 48 volts.
  5. Jun 25, 2013 #4

    Philip Wood

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    Gold Member

    Just wondering about one of your figures: 820 W is quite high. The best commercial panels produce about 175 W per exposed square metre in sunshine. This would suggest that you have a panel area of over 4 square metres (that is over 40 square feet).

    The car's maximum speed is the power output from the motor (which will be somewhat less than the power from the panels) divided by the resistive force, Fres, on the car. Fres: is partly made up of rolling resistance (roughly constant) and partly of air resistance of various sorts, which increases with the car's speed. Unfortunately, it's quite difficult to estimate how large the air resistance will be at different speeds, but you've got to have some sort of estimate in order to find the maximum speed of the car!

    Sorry not to be more helpful.
    Last edited: Jun 25, 2013
  6. Jun 25, 2013 #5
    Every bit helps. Thanks for replying. Alright, well I used a physics program called Algoodo and I used the numbers 18 Nm as torque and 240 rpm to the wheel-axis and it did a speed of 7 m/s or 35 km/h, 21 miles/h and that program has air-resistance built in and rolling resistance. Is those numbers reasonable? I had 200 kg as load and 16" wheels.

    I know the panels will be big. I have two 410 watt panels which is 2 x 1,64 m each. So the roof will be 6 square meters approximately, big roof!
    Last edited: Jun 25, 2013
  7. Jun 25, 2013 #6


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    Staff: Mentor

    With such a low motor rpm, you may not have to consider anything else! Just multiply it by tire circumference to find speed.
  8. Jun 25, 2013 #7
  9. Jun 25, 2013 #8


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    Staff: Mentor

    That's revolutions per MINUTE, right? So your answer can't be in m/s...
  10. Jun 25, 2013 #9
    Right! So 240 RPM is 4 Revolutions per second if divided by 60.

    However lets say we have 240 RPM and 51.365 and it turns to 12327.6 m/m that would be

    12327/60 = 205.45 m/s Can't be right.

    But if it would be every hour then it would be:

    (12327/60)/60 which is 3.42 m/s

    But this can't be right either.
  11. Jun 25, 2013 #10


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    Somehow when you found your minutes, you lost your millimeters:

    12,237 mm/min / 60 sec/min = 0.20 m/sec.

    Still, not sure how you found circumference. Converting diameter to circumference just requires multiplying by pi. So I get 30.5*3.14=95.7 cm. So 95.7/100*240/60= 3.8 m/sec.
    Last edited: Jun 25, 2013
  12. Jun 26, 2013 #11
    Ah, circumference it's just the length around the circle, didn't know that word in english. Sweet. That seems like an ok velocity, not looking for high speed.

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