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Are my DIY Electric Car Conversions Correct?

  1. Oct 17, 2008 #1
    Ok, I am not sure if this is the right category to post this in but I am a mechanical engineering "sophomore" student and I want to try my own project, so here goes:

    I want to build a Go-Kart/Electric Car, I don't need everything to be perfect yet but I just want to make sure the calculations I am running are okay.

    I started out from scratch by assuming that all the masses of/in my cart would be multiplied by gravity.

    So running a quick inventory on what my bare average weight will be:

    4 x 5HP Motors = 4 x 83 lbs = 332 lb or 151 kg
    400 lb occupant capacity = 182 kg
    4 x 10kg wheels/tires = 40kg
    Material making up the actual car = 40kg
    (No braking system or steering system accounted for yet but that's fine.)

    Total = 413kg

    Total Weight = (413 kg) * (9.8 m/s2) = 4047 N

    Now 1HP = 746W so (4)*(5HP)*(746W/HP) = 14920W (14.92kW).

    14.92kW = 14920(J/s) and 14920(J/s) = 14920([N*m]/s)
    and
    14920([N*m]/s) / 4047(N) = 3.7(m/s) * 3600(s/h) = 13.32(km/h) / 1.6(km/mi) = 8.3 mph


    Are these conversions being used correctly? Am I therefore correct in assuming that at full motor capacity if one motor is attached to each of the four wheels that the vehicle will be able to travel a maximum speed of 8.3 mph?
     
  2. jcsd
  3. Oct 17, 2008 #2
    You can't convert horsepower into speed. The max speed your car will reach is decided by the point when the drag of the vehicle equals the force put out by the motors. Without knowing your rolling drag, can't help.
     
  4. Oct 17, 2008 #3
    So if the coefficient of kinetic friction were to be 0.7 of rubber on street pavement, I can get the drag by multiplying that times the normal force 4047 N, right?

    (0.7)*(4047 N) = 2895 N of rolling drag?
     
  5. Oct 17, 2008 #4

    mgb_phys

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    Only if you assume the brakes are on and the wheels are locked.
     
  6. Oct 17, 2008 #5
    So I first have to overcome the rolling friction to start moving?

    If so then let's say I apply 4000 N of force and I want to travel a distance of 48270 m (30 mi).
    I would then do 4000 N - 2895 N = 1105 N * 48270 m = 53338 kJ.
    I would need to apply 53338 kJ of work to move it 48270 m (30 mi).
    Then let's say I want to do it within a certain time, say one hour or 3600 s.
    I would then say 53338000 J / 3600 s = 14816 W. That is my power in watts.
    Then I would convert Watts to Horsepower by dividing it by 746 to get ~ 20 HP.

    Is that not correct?
     
  7. Oct 17, 2008 #6

    mgb_phys

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    The rubber-road friction and weight only affect how much torque you can apply to the wheels before they spin.
    The speed of your car will depend on the energy lost to friction in the drie train and (mostly) aerodynamic drag.
    You can't calculate these without a detailed computer simulation, the nearest you can do is drag a similair size/shape vehicle and measure the force needed.
     
  8. Oct 17, 2008 #7
    I'm getting the feeling that I should hold off this project until I go more into depth with physics. :biggrin:

    Thanks anyways guys :-)
     
  9. Oct 17, 2008 #8

    russ_watters

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    Is this project for school or just for fun? That's a really big go-kart/car you're trying to build if just for fun (almost full-sized). Expensive, time consuming, and complicated for a hobby.

    Anyway, the only real way to avoid the calculations is to use off-the-shelf components or a kit.
     
  10. Oct 19, 2008 #9
    This is just a for fun project. I see all these people making their own electric cars and everything on the news and it just got me interested in making my own, even if it starts out as just a go kart.

    The idea I had was to just build it out of wood with the motors attached to each of the four wheels. I hadn't thought about steering or braking yet :-)

    All I have had so far is Physics I, which is what I am currently taking, so I guess I got a little bit ahead of myself with the calculations.

    Maybe I will look into the kits they have out there, thanks. :-) (But as you mentioned it is probably expensive so maybe I will hold off altogether and just stick to researching what has been done already)
     
  11. Oct 20, 2008 #10

    Mech_Engineer

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    Aside from the obviously incorrect calculations, you're forgetting a very import (and very heavy) part of an electric go-kart: batteries!

    Also, you're planning on way too much power for this thing IMO. 5-10 hp should be sufficient for a personal-use, and you should probably use a single motor with a simple drivetrain to the rear wheels.
     
  12. Oct 20, 2008 #11
    :rolleyes: Oh yes, batteries - small detail :-)

    The more I look into this project, the more complicated it's looking...

    I was thinking of using a gear system (which I assume is a simple drivetrain) to move the wheels from a single motor, and I even considered making wooden gears at one point (Because I don't have access to metal working tools yet) - but won't that overheat and combust or something?

    I did see a story a while back about a crew that was building an entire sports car out of wood.
     
  13. Oct 20, 2008 #12
  14. Oct 20, 2008 #13

    mgb_phys

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    ps. tips if you are going to launch a small uncovered electric vehicle.
    Don't do the product launch in November in England.
    Don't launch it in the evening (in the dark) in the middle of London in rush hour traffic surrounded by buses / trucks.
    Don't let the British media try one after a well lubricated lunch.
     
  15. Oct 20, 2008 #14
    Who said the electric car is the car of the future. Always will be.???

    Then there is the Gjoke.........I mean the Gwiz.
     
  16. Oct 20, 2008 #15

    mheslep

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    Adding gears would throw away one advantage of EVs. Electric motors enjoy flat torque curves, providing near maximum torque at stall. Combustion engines do not. Dump (most of) the gears, save the weight.
    http://www.teslamotors.com/performance/acceleration_and_torque.php
     
  17. Oct 20, 2008 #16
    You will have to have gear reduction in one form or another. But if your refering to gears as in different gear ratios, you might want to have a two speed gear box. While true, EMs do like flat torque curves they also like high speeds. Typically at higher RPMs, EMs become the most efficient.
     
  18. Oct 20, 2008 #17

    mheslep

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    Yes, some speed reduction is required. But an EV does not need a big 4-5 speed transmission, does not a differential, does not need axles.
     
  19. Oct 20, 2008 #18
    Yeah, I clearly need to go back to the drawing board. :)
     
  20. Oct 21, 2008 #19
    If the car only has a single motor, how does it not need a differential. :confused:
     
  21. Oct 21, 2008 #20

    mheslep

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    Multiple motors, one per wheel. That has its own difficulties, but it's feasible. Here's an exotic example:
    http://www.drive.com.au/Editorial/ArticleDetail.aspx?ArticleID=8754&vf=23
     
    Last edited: Oct 21, 2008
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