[pengyou]

I would like to make a vehicle that looks like the attached - an atv type with either 8 wheels or tracks, leaning towards wheels. Flat on the top so that it can haul a variety of goods but much bigger than this, weighing about 6,000 pounds with a front loading bucket, back hoe etc. It doesn't need to be fast - top speed of 15 mph is more than adequate - but it needs to be able to crawl with a heavy load on top or pulling a rototiller, etc The wheels will be 24". Length of the vehicle will be about 10 feet.

Most vehicles this size only use 40 hp to move around, or even less. What kind/size of motors would be appropriate? I am willing to use gear reduction to get the low, low speed with adequate torque.

 

[Emyr]

Most vehicles this size only use 40 hp to move around, or even less. What kind/size of motors would be appropriate? I am willing to use gear reduction to get the low, low speed with adequate torque.

You'll need to take it apart and see what find of gearbox is used. A belt-type torque converter might not work well with a motor that doesn't need to rev to produce torque.

 

[steveob]

at 15mph air drag is largely negligible, what you need to make a guess at is what the rolling resistance is (varies by surface and load) at 15mph, and worst case incline.

From that you can make an educated guess at power requirements at 15mph (including going up hills and/or on soft surfaces).

What I don't know how to answer is how well it will turn on dry pavement.

Also how are you getting the power to the wheels?

 

[pengyou]

Thanks. I was looking at electric motor connected via half shaft to the wheel, with some kind of gear reduction in between. One electric motor per wheel. I used the picture of the vehicle to show the basic type. It will not have the same drive system.

How much torque is possible from a 20 hp motor?



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[steveob]

How much torque is possible from a 20 hp motor?

Well with enough gearing, as much as you want (till the reduction drive eats up all the torque). Plus factor in size/weight of larger gear reduction. Also different motors make different torque at different rpms.

gearing doesn't change power (aside from losses), it trades rpm for torque.

Lets say you have 12" radius tires, and 6000 lbs (750lbs/wheel), flat road. So very roughly each wheel can take 750 foot lbs before spinning. Now you don't need 750 foot/lbs per wheel at 15mph, lets say you are fine with 4mph for climbing hills and dry pavement skid steering.

4mph ~6fps, at about 750 foot lbs is about 8 hp per wheel (not including stuff like rolling resistance).

fyi looking at a powered vehicle
https://www.argoadventure.com/ARGO-OUTFITTER-8X8_p_12828.html

it has 30 hp, but it only weighs 1825 lbs. I don't see it scaling up to 6000 lbs myself, not without a huge budget and a fair bit of research.

 

[brian_]

One electric motor per wheel.

I like the idea of minimizing mechanical complexity, and one motor per wheel can help do that in a conventional 4-wheeled vehicle; however, in a skid-steered vehicle all wheels on the same side are driven at the same speed, so it is easy to drive the entire side from one motor with connecting chains. Using only two motors instead of 8 means only two reduction gearboxes and two controllers instead of a 8 of each - that could save effort and cost. It also simplifies coordinating the speed of the wheels on the same side, although some controllers would handle that for you.

For an example, skid-steer loaders (such as the common Bobcat brand) use hydrostatic drive, and could use one hydraulic motor per wheel, but they typically use only one motor per side and drive both wheels by chains.

On the other hand...
We often discuss hub motors in this forum, with the general theme being that for anything other than a bike they are not really effective. If this vehicle doesn't have a suspension or doesn't need to make suspension work at any significant speed, then unsprung weight doesn't matter and it may be a rare case where geared hub-motors make sense. If the motors are inside the wheels, mounted outside the hull of the vehicle, packaging a massive battery inside would be easier.

 

[brian_]

The wheels will be 24". Length of the vehicle will be about 10 feet.

So the tires (not the wheels) would have an overall diameter of 24". This is the size on a typical Argo: 24x10.0-8 (meaning 24" overall tire diameter, 10.0" wide tire, and 8" wheel diameter).