# Motor Sizing Math ?



## rillip3 (Jun 19, 2009)

For your application, wind resistance is negligable. It's arguable, but generally speaking, this only matters if you're pushing something ridiculously un-aero, like a box, or going at around 50 mph, which won't matter for your app either.

Gearing is not much of a concern because electric motors have fabulous torque across a fairly broad band of RPMs, including 0 RPM. Some mechanical advantage is a must, but being able to change the gearing is not that important. As a rule of thumb most people use second gear for their cars, and that's about 6:!. Not that helpful for your app maybe but it gives you a baseline.

Tire diameter could affect the speed, because of how much more ground a large tire will cover at the same RPMs as a small tire, but this is going to be relatively minor.

Occupancy is really just weight.

So basically, I think you should focus on weight and possibly tire diameter, though honestly, unless you're building a hot-rod for him, I doubt this is going to give so much of a difference that you'll even notice.

I'm not really good at the mathy bits, but this link from the wiki might help:

http://www.diyelectriccar.com/forums/showthread.php?t=15508


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## frederic (Jul 4, 2009)

I'm not building a hot-rod, but I would like it to move (from zero) without the little scooter motors catching on fire, which is where my desire for math came in. A lot of the scooter motors I seem to be able to choose from do not have specifications except for wattage (and that's probably peak power), voltage, RPM and housing diameter. I want of course size the motors large enough to move the thing but not so large that I spend more than necessary.

That post was the magical one I was unable to find, so thank you for digging it out for me. Awesome ;-)


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## wakinyantanka (Apr 8, 2008)

frederic said:


> Hello, and first I'd like to thank those in power for a great forum.
> 
> I've been doing some extensive searching here and other forums, as well as on Google without much luck. I imagine my search keywords aren't correct enough to yield something useful, so I figured I'd just ask.
> 
> ...


Here is the link to the calculator I use, just plug in your numbers. Hope it helps. 
http://www.bgsoflex.com/auto.html


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## tomofreno (Mar 3, 2009)

Bob Brant's book "Build Your Own Electric Vehicle" has the equations used to calculate the forces on a vehicle, required motor torque and power, etc. An Excel spreadsheet based on these equations can be downloaded from electricnevada.org. The only part of the spreadsheet you need would be the first part where forces, torques, and required motor power are calculated, just below the input variables. The rest considers specific motor/controller combinations. I recommend reading the chapter of Bob's book where he discusses the equations and going through the calculations yourself for the mental excercise and better understanding.


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## frederic (Jul 4, 2009)

Thank you all for your input and help. Most useful, got some numbers going already.

BTW, I found this last night, here on this site totally by accident!

http://www.diyelectriccar.com/forums/showthread.php?t=15508


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## Amberwolf (May 29, 2009)

How fast do you plan for it to go? If it doesn't weigh a *lot*, you could do what I did with my bike, and use powerchair/wheelchair motors. Some of them are pretty powerful, for small things like this, and have gearboxes "built-in" that put them at the 4MPH range with 10" wheels (typical size). Some of the gearboxes are designed for 8MPH full throttle. 

I have one that is labelled for 120RPM at 24V, and one that is 135RPM at 24V. One of them is also a 4-pole motor that is probably around 650W, but the smaller ones are more like 350W (guessing--there are no specs on them except for a no-load current rating of 3.6A at 24VDC. Invacare hasn't answered my attempts to ask about them, and probably never will). 

My bike is about 120 pounds with batteries and motor (half that without them), plus my 150 pounds and up to another 10-20 pounds of stuff, and I can get up to 15MPH with just the motor driving the bike's normal drivetrain (so I can shift gears). 

If you can find some old powerchair at an estate sale, yard sale, thriftstore, or junkyard, you could easily get the whole package pretty cheap, and they often have two motors (one for each main wheel) and two gearboxes. Some use a transaxle and one more powerful motor. 

Ebay also has them, but I don't know what prices run.
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## frederic (Jul 4, 2009)

Amberwolf said:


> How fast do you plan for it to go?


Not very fast at this point, only because my son who will be driving this most of the time is four. He'll be five by the time I complete the project, considering the pace I tend to do things.

It's going to be quite large, as my son requested I build him a "truck like Daddy's" which is an F350 crew cab. So it will be able to seat four children his size or two full size adults, based on my deciding it cannot be larger than 4'x8' as that dimension is what I can roll into the bed of my F350 crewcab without issue, allowing me to take it places for him to tear up someone else's grass ;-)

I am going to wildly guess this will exceed 500lbs unloaded, about what an average riding mower would weigh.

Once he's older (and able to clutch and shift) I'll re-power it with other parts I have already (which are not electric)

Based my start in the math I believe that four little 1200 watt peak (about 800 watt avg) scooter motors, one driving each wheel individually, is more than enough power but I am concerned about gearing. The tires are Carlysle 18x9.50-8's therefore have an 18.5" diameter. My concern at this point is "take off". These motors are 4" diameter with 4" of depth, and even though they are rated at 1.5HP I simply cannot believe that's not marketing nonesense or worse, and outright lie.

The larger the gear on each wheel's axle stub, the taller the spindles have to be in order to have room for the gear and chain with enough clearance for upper and lower a-arms.

The attraction to these particular motors is the physical size - I can install them on homemade spindles and drive through sprockets and chains, the axle stub that will be suspended in the spindle.

I'm trying to figure out if 4 of them is enough, and if a 12 (motor) to 55 (wheel) sprocket ratio is enough.

I've looked at gear motors from wheelchairs as well as from industrial sources and while the prices were reasonable the issue was how to "simply" fabricate them in with a IRS/IFS suspension. Miniature CV's are difficult to find and my attempt to make one as an experiment out of aluminum didn't really work out that well.

I've attached a crude respresentation of the spindle-mounted motor idea - all four wheels would be built this way, with the rear spindle's tie-rods fixed to the chassis in the right spot so caster doesn't change with articulation.

Green - live axle "stub" with woodruff key for mower wheels, and scooter 55-tooth sprocket.

Red - double-row ball bearings, 2" OD and 3/4" ID

White - Spindle and motor mount

Blue - 48V 1200 watt scooter motor

While the parts will be the same on all four wheels, the motors will always be mounted on the side of the spindle that faces the back of the vehicle so the right side will mirror the left side.

I know that electric motors have instant torque when full power is applied however with the original car in the youtube video in my first post, I found that a 350W scooter motor geared in this same fashion (12 to 55) didn't really accelerate well at all. This is fine because of my son's age, but I'd rather have more power than necessary and have my controller limit acceleration and speed, so as he gets older and is more attentive, I can "dial" up more power for him.

Another option that may play out is I'm looking at a used electric golf kart this coming thursday, knowing that the batteries hold just enough of a charge to test drive it about 100'. If I can negotiate the seller to my price point, I'll forgo the complexity of AWD and spindle-mounted motors and instead use the rear axle of the golf cart "as is" and suspend it with a 4-link with a panhand rod, something I'm more familiar with anyway. At least I know a 4 HP currie motor with the rear-axle gearing that's typical, can turn 18" wheels at a decent pace.

I've been going through this mental exercise of AWD because I'm not so sure I can negotiate the seller down far enough where I'll end up with the cart.

Again, I'm trying to avoid fabrication half-shafts and CV's even though with some practice and much wasted material, I can do it. I have a milling machine and a lathe, so if I "had" to I could cut gears, sprockets and so on.


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