Need some gearing advice for a slow offroad EV

Electric cars, trucks, ATVs, NEVs - things bigger than a motorcycle.
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madsci   1 mW

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Need some gearing advice for a slow offroad EV

Post by madsci » Jun 25 2017 2:45pm

First off, let me say that I'm a total noob when it comes to designing and building an EV and may be totally wrong about any number of things. I welcome feedback on all aspects of this project.

I'm building an EV to carry 2-4 adults at 5 MPH on level but dusty ground. I've built some pieces of it for testing but mostly right now I'm working on the CAD model. It uses two 36v, 800w MY1020 motors and is set up for 4 wheel drive and differential steering.

Each motor has a 12t #40 sprocket that drives a 48t sprocket on a jackshaft, which has two 12t sprockets each driving a 48t sprocket on a front wheel and a back wheel, so 16:1 overall and about 6.5 MPH at 2700 RPM with 13" mini ATV wheels if I did my math right.

What's giving me the most trouble right now is that 48t #40 sprockets are big - almost 8" OD, leaving me with less than 2.5" ground clearance at best. It'd probably be digging into the dust as soon as there was any weight on it.

I'd like to replace the motors with gear motors, or add a gearbox, and get maybe a 6:1 reduction ratio before the chain drive. Smaller drive sprockets would let me keep everything within the frame and protected by a floor pan.

Trouble is, I can't find any 36v gear motors over 600 watts and I'm not aware of any add-on gearboxes that will work with these motors. I've already got the battery pack and charger, as well as a good brushed DC motor controller, so going to a higher voltage or brushless motors would probably be out of my budget. The other possibility I'm considering is to add a second jackshaft on each side to split the reduction ratio, but that's a lot of chains to mess with for a guy who's never worked with roller chain before.

Any suggestions? I've learned a lot from browsing minibike, kart, and ATV forums, but almost no one builds 5 MPH vehicles and most are going for speed.

Thanks!

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amberwolf   100 GW

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Re: Need some gearing advice for a slow offroad EV

Post by amberwolf » Jun 25 2017 4:48pm

If you're only going 5MPH, then even with 4 adults at say 200lbs each, and say 200lbs for the vehicle, that's only 1000lbs.

Doesn't take much power to move that if it's level ground, as long as the rolling resistance isn't that high.

My SB Cruiser trike, with me on it and one of the dogs in it and pulling a trailer with the other one, weighs several hundred pounds--yet it doesn't even take but 1000w or a bit more to keep it moving on level paved streets, at 20MPH. Much less at only 5. Heck, I could pedal it at 5mph for a short distance, and my legs suck at it these days, as much as the joints hurt I don't use them for it much.

You can calculate how much power you'd need for a particular speed and weight on various terrain, with various power/speed calculators around the web. Then use that as a baseline for how much power you actually need.

That'll tell you what motors, controllers, and batteries to use, for that power level.


Keep in mind that a motor can usually handle more power than it's continuously rated for, for a short time (varies by motor and conditions), so it can deal with startup conditions and getting you up to speed, then settle down to a lower power level keeping you at the 5MPH.

The batteries still have to be able to reliably put out the current needed at that higher level, without voltage sagging too much, and the controller has to be able to do that, too, so those two things might need to be sized bigger/better than the continuous loads suggest, but the motor can handle it if it doesn't have to do it frequently or for long periods, or you can provide sufficient cooling for it.


Personally, for your application, I'd be looking at motors *designed* for such low speeds: powerchair motors; stuff intended for powered wheelchairs.

Most of it is 24v stuff, but you can often run it at 36v if you have to, as long as you don't keep it at full throttle too long (shouldn't need to for your applications).

Most of them are rated for 300-600w continous power, depending on type, and they have to handle much higher loads momentarily while starting up with heavy people on the chair, or going up ramps, etc.

I used to use one with gearbox built into it to drive my CrazyBike2's pedal chain drive.

Depending on height-off-ground, and gearbox design of the motor, you could probably even just use the wheels that come on the gearbox output axles, either with the "solid" tires they come with or pneumatic replacements for a better ride.

madsci   1 mW

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Re: Need some gearing advice for a slow offroad EV

Post by madsci » Jun 29 2017 11:46pm

That's reassuring about the power requirements. I've got a tiny Razor electric ATV with a 350 watt motor and it can carry me over rough ground in the vacant lot across from my shop, so 1600 watts seems like it ought to be plausible for 4 people.

My budget has pretty much evaporated - I'm a small business owner and a slow sales month means very little money for toys. Could get better tomorrow, but I'm short on time. I'm building this thing for Burning Man and it needs to be ready in under 60 days.

The motor controller, batteries, and charger that I have now are probably going to have to stay. New motors would be at the top of my list if it means simplifying the reduction problem. If I can make the existing motors work, that would be better for my budget.

The vehicle is going to be over 200 pounds, mostly because of the batteries. I have three 110 AH deep cycle batteries that weigh about 160 pounds total. (I don't know the Peukert constant and they'll probably suck for their weight, but they're what I have.) I'm building the frame mostly out of 1" square steel tubing, since I already have a few hundred feet of it.

Can you point me to a good calculator for power requirements? Bogging down in the dust is my biggest terrain-related concern. Where the surface is hard packed, it's great. It's where they set the current land speed record, in fact. Currently the area is under water and it's hard to predict what the surface conditions will be like.

I just now found this two stage chain reduction drive (http://buggies.builtforfun.co.uk/Howtoo ... page2.html) and I think I'm going to see if I can fit that in my CAD model. I didn't think of flipping the motor around and having the shaft pass back under it, and I think that might work for me. Still a lot chain, but I already have 50 feet of #40 and the sprockets are already mounted on the motors.

Another consideration is that the machine only needs to survive a week and might be driven 30 miles a day at the outside. If it bogs down occasionally, the passengers can jump off and move it - I'm adding grab bars for that. If the batteries take a lot of abuse, they can be retired to less arduous duty when the week is done.

If it survives even three days, I'll consider the project a success. Upgrades can come later, as the budget permits. Or maybe I'll swear the whole thing off and never try this again, depending on how it goes.

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