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Old 09-21-2015, 05:56 PM   #21 (permalink)
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Consider what kind of performance a Bobcat Skid-steer style loader has, this will give you a Weight ( though some of that can be done away with removal or the counterweight), a Horsepower, and performance. These machines will spin all four wheels on pavement, so power to the wheels is plenty. Top speed is the limiting factor according to your parameters. CAT 216B Skidsteer: HP...51, Top Speed....7.9, Weight...5690.

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Old 09-21-2015, 08:59 PM   #22 (permalink)
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Quote:
Originally Posted by CFECO View Post
...These machines will spin all four wheels on pavement, so power to the wheels is plenty.
Sorry I have taken it upon myself to try to steer the discussion towards actual engineering details, but I think you mean torque. power would be a function of torque * rpm. A slow turning wheel may not have much power (which is why I'm not balking at light rock climbing with a 10hp engine).

an ironic example, that you see on may rock crawlers is a winch, 4 hp and can pull it straight up a cliff. So in a way, they are already electric hybrids
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Old 09-22-2015, 12:19 AM   #23 (permalink)
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Yes and no! To get get a 5+ thousand pound skid steer loader to spin the tires on pavement, one must have the throttle wide open, using all the horsepower!
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Old 09-22-2015, 09:51 AM   #24 (permalink)
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P-Hack, you obviously have some strong feelings about this, tell us how you really feel? Did you even read the first sentence of the first post? I dont know if you are intentionally trying to be Debbie Downer, but you are being very antagonistic.

The biggest reason I like electric for this application, is because I think it would be fun to design and build, period. The second biggest reason is because in my experience, most ICE's don't produce sufficient torque at idle, which is where the majority of the time is spent. Instead, they have to be oversized.

No where have I said this would be practical, or cost efficient. I have also not said that gearboxes would not be used, I had just asked a question about the electric motors having both sides of the spectrum.
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Old 09-22-2015, 07:32 PM   #25 (permalink)
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lol, you have a lot to learn, so you might want to get started. physics is fun, daydreaming is a waste of time, on a tech forum anyway.

the problem isn't impossible, things have weight, they make torque, they fly apart at a certain rpm (or torque, or volts/pressure/current/???), things have different energy/power densities (volume/weight), tires make traction based on weight, instantaneous. For any idea you have in your head, you should be able to figure out how fast it can go and what kind of slope it can climb and for how long.

There's no reason, or excuse, for wishful thinking, or disregarding experienced answers that you happen to not like.
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Old 09-25-2015, 12:33 PM   #26 (permalink)
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Punching a few numbers into the calculator on this site.

A few assumed figures that should be somewhat close

frontal area 25ft^2
Cd 0.6
Crr 0.1 (Wiki has Crr for tires on sand at 0.3, but that would requre all of my engine power to do 35, and I can hit 50+ on sand so I went with something more reasonable.)
4000lbs
Drivetrain efficiency 75% (full 4x4 transmission and all)
500w of paracitic load (probably quite a bit low)

All of that gives a requirement of 35hp.

One site listed hydraulic pumps as having 80% efficiency X2 gives a 64% conversion efficiency. So you would need an engine that could put out 54hp continuously to hold 30mph, or up to 27hphr of storage for the 30 minutes.


Efficient, most likely not.

What he would gain would be a butt load of low end torque, exactly what every rock crawler wants more of.

As far as weight, my rig is pretty light at about 3200lbs, with an engine that weighs in at 650lbs (210ftlbs at 1800rpm, and 115hp at 3200rpm). Depending on how he builds his rig and how big even with hydaulics he shouldnt be over 4000lbs.

Edit to add: I am rarely even at 1800rpm, often below 1000. When I do run above 1500 on the trail I am also feathering the clutch to control the speed.
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Old 09-25-2015, 01:19 PM   #27 (permalink)
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Now if he was able to keep the weight down to 2000lbs with a 2 seat tube buggy the hydraulic motors/gear reduction at the wheels etc the engine would only have to produce 30hp to maintain 30mph.
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Old 09-25-2015, 02:47 PM   #28 (permalink)
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that sounds like a reasonable approximation. Now if we add hydraulic storage:
per: http://www.hydac.com/fileadmin/pdb/p...0140000011.pdf
A 50 litre accumulator
with 400 bar maximum
pressure can supply
approx. 360 kJ = 0.1 kWh
(adiabatic).
50liters is about 100 pounds.

plus it needs a low pressure tank, so that is roughly 100lbs of fluid for 100watt hours of energy, plus tanks and stuff. It's enough energy to run the headlights for an hour maybe. to make 25hp (plus 10 from the ICE) for a half hour would be like 10,000 lbs of fluid. And you would have to sit there going nowhere for over an hour to charge it first.

Last edited by P-hack; 09-25-2015 at 02:53 PM..
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Old 09-25-2015, 02:56 PM   #29 (permalink)
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I'd shoot for the light side first. Say two drivetrains out of a couple of Zero Turn riding lawn mowers, use a frame from one of the 2 seat adult go carts. Wide'n the suspension to get more wheel travel, use one engine and have an extra drive motor geared for the desired top speed. Keep the weight to 1,000 lbs dry would vastly increase climbing ability. My old go cart went 30mph with 3 HP, though it was not hyd. drive.
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Old 09-25-2015, 03:42 PM   #30 (permalink)
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According to an online sizing calculator

35hp at 5000psi = 10gpm

The engine would definitely have to be powerful enough to maintain speed on its own.

It wouldn't be efficient, but I would likely be able to be a good slow steady crawler.

The reason I prefer hydraulic to electric is that when you bind up your drive train and stall the motor. The electric motor draws way much more current when stalled greatly increasing your chances of burning out your motor controller, batteries, motor, wiring etc. Burn it out with no spare and your rig gets dragged out, which so far I have been able to avoid.

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