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Basic electric vehicle calculations?

I am coaching a team of 6th graders in Odyssey of the Mind, and they have decided to do the vehicle problem, which involves building a battery-driven electric vehicle strong enough to carry one of the team members. Unfortunately my software background, even with what remains of my college physics, is not quite sufficient to assist them with their vehicle design (btw, for the OM enthusiasts - I am not designing their vehicle, but I should be able to teach them theory)

Anyway, the problem is as follows: How do I calculate what kind of power/torque I would require in order to move a small vehicle (about 2m long, 1m wide, 70 kilos loaded weight) at about walking speed (5mph)? And is there any text out there that would guide the team to an optimal configuration of motors, batteries and gears to achieve this power/torque/speed requirement?

I would greatly appreciate any assistance!

Boris Pelakh

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pelakh
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Refer to the competitions for light four wheel vehicles. The best of them get well over 1000 mpg (thats 1 and three zeroes) out of an IC engine on a flat track. You can work out the average power consumption from that. The usual design tenets apply: add lightness, specify narrow tires, and apply eggshell between driver's foot and throttle. Don't go (very) fast.

Brian Whatcott Altus OK

Reply to
Brian Whatcott

Boris, Considering your audience, I think the formulas needed are: First off, the energy needed to accelerate the mass of the vehicle and person, in a predetermined time, (easy to do). Second, now add the rolling friction, more difficult, maybe use a red wagon with person and weights, and a fish scale. Verify the readings at a few speeds. Third, calculate the aerodynamic losses, assuming a flying brick and your intended speed. (Note: do this calculation at freeway speeds to impress the fact that MPG is dominatd by aerodynamics.)

Avoid calculations regarding motors and gearing, other than those needed to run your vehicle at 5 mph at about 60% of rated motor rpm. (I pulled that 60% number out of my ass, since I have no idea what your motor will be.)

Sources: donation tread mill motors, motorized wheel chair, ebay. You are going to need ~300 watt motor, using electric scooters as a reference. Dave

Reply to
Mechanical Magic

snipped-for-privacy@yahoo.com wrote in news:1193848099.978606.113740 @o38g2000hse.googlegroups.com:

There is the simple formula

F=m*a+m*g*(Crr+tan(gradient))+1/2*rho*Cd*A*v^2

BUT

that is not really applicable. At this sort of speed you'll find that your problem is getting the thing moving at all, rather than maintaining

5 mph.

So, why not build a trolley with the right wheels and mass and measure how much force it takes to get it moving?

And then measure the force that motors and different gear ratios and wheel diameters and so on can generate?

I suppose I could point you in the direction of more relevant equations, but the experimental approach seems much more likely to be helpful to kids.

WAG - 200N to get it going, 50N to keep it moving at 5 mph on smooth concrete and hard tires.

Fun project,

Greg

Reply to
Greg Locock

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