# How efficient would an electric rock crawler be?



## eou_edu (Apr 27, 2011)

I've been reading this forum for a while now but first time poster and haven't built anything yet. 

I hang out with this group of four wheelers and I'm looking to buy one soon. I rather go the alternative route and build and all electric jeep. These guys use a crazy amount of gas on the trail. All the starting, stopping, fast throttle respond, basically anything that would make a vehicle inefficient. This idea is currently at the thinking stage only so I'm trying to figure out what kind of range I would have on the trail? I wouldn't want to be stuck in the middle of nowhere but usually we do out and back type runs. On one way of thinking it wouldn't have the range to do just because it suck so much energy getting it going and stopping over and over again rather just keep it moving down the freeway. But on another mode of thinking I think it would be a lot more efficient as a gas engine. No idle time to worry about, or dumping fuel into when your romp on the throttle, or fighting much wind resistance. 

So the big question. Would you say an all off road, rock crawler 4x4 the range gap would be closer to a gas engine on the trail than it would be on the road or about the same?

I realize there's a lot of other complicated questions here, but like I said I'm just trying to get an idea weather it's worth even thinking about.


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## Jan (Oct 5, 2009)

I think you're right. And only a real life experiment will tell how much. I know, did didn't help.


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## m38mike (Dec 27, 2008)

I built an electric jeep and kept the 4WD. Electrics are great for rock crawling because of the low end torque. I've used mine off road, not rock crawling, and for slow speed travel they are great. But just like thier ICE counterparts, they use lots of power to get up and around. That means it would take a large battery pack. And you would definitely want to go with LiFePO batteries vs anything else. With all the steep side slopes and tilted angles that you would traverse you would need to keep your weight down and your power up. 

Would it work? Sure! How far do you normally drive in your rock crawler? 10 miles? 20 miles? 50 miles? That would be a major consideration in how big a battery pack you'd need to stay up with the other machines.


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## Woodsmith (Jun 5, 2008)

Also you will need to factor in how far and fast you drive to get to the rocks before you start playing. The you need enough to get home after.

The alternative is to tow it there, play, and then tow it home. That would make the design easier as you don't need high road speeds or range.


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## Ziggythewiz (May 16, 2010)

I would think you could use much less power due to better control. I haven't done rock climbing, but to get on rhino ramps with my gasser I have to give it pretty good power, then slam brakes to not overshoot. With the ev it's just creep up at whatever speed you need.

It would be awesome to make a motorized battery pack that could significantly shift your center of gravity.


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## Jimdear2 (Oct 12, 2008)

Maybe for a very rough idea how big a battery you would need to do the rock crawler part (consider towing or trucking to and from.) You would find out how much fuel a vehicle similar in size and weight to what you want to build uses in a typical event. you need volume and time.

Get either gallons or pounds or liters or grams. Subtract a percentage to cover times a vehicle is idling. Take that remaining figure and convert it to kilowatt hours. That is the beginning of needed battery size calculations. From that you need to cover inefficiencies and losses in power conversion, you have to leave something in the battery at the end or you will destroy it quickly You also want to add quite a bit of capacity for "Just In Case".

Once you have a handle on kilowatts then you decide on the voltage you want to run from there you can work down to amp hours needed. you will also need some form of sealed battery, flooded wet cells just wouldn't work.

In the end I think you will find that battery technology is not quite there yet. The EV torque is wonderful and of course getting rid of complex trannies and gear boxes would be good. you are still running up to that same old problem. A cubic foot of gasoline just contains more energy than a cubic foot of battery no matter what chemistry and it's lighter too.

JIM


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## Jan (Oct 5, 2009)

Jimdear2 said:


> Subtract a percentage to cover times a vehicle is idling. Take that remaining figure and convert it to kilowatt hours. That is the beginning of needed battery size calculations. From that you need to cover inefficiencies and losses in power conversion...


That is his big question. We know that an ICE is the least efficient at low speed, especially at crawling low speed. But we don't know how much more efficient an EV can be under those circumstances. The question is: how much.

A hybrid like the Prius scores best in contrast to its ICE competetives in city driving: low speed and lots of pulls up and stops. Rock crawling might be an exponent of that fact. But how much? Noone can tell I reckon.


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## Woodsmith (Jun 5, 2008)

I think I would be tempted to go with the 'suck it and see' method. 

Strip an old 48v forklift and fit the motor and controller to a jeep and then add four sealed lead acid batteries to it.
Install a cycle analyst or voltmeter and ammeter.

None of it needs to be tidy, just safe for a trial run.

Tow it to a play area and measure the current draw while playing a little to get an idea of what it will do. You'd only need to see what current it draws hauling itself over a few boulders to get an idea of what it would do.

I reckon it is too complex to calculate as it will depend on too many vehicle and terrain variables.

The other way is for m38mike to drive his jeep up a 45deg ramp and measure the current draw. Compare to driving over a small boulder and then extrapolate a guess from there.


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## boekel (Nov 10, 2010)

I'd love an electric 4x4, but trails in the Netherlands usually involve lots of water


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## dtbaker (Jan 5, 2008)

eou_edu said:


> I hang out with this group of four wheelers and I'm looking to buy one soon. I rather go the alternative route and build and all electric jeep. These guys use a crazy amount of gas on the trail.



I think you'd have a GREAT time with the low-end torque and 'unstallable' characteristics. It would totally save your clutch in those times when you'd be tempted to slip it or go too fast in a gasser to prevent stalling.

I think this application may make sense to consider AC rather than DC and design in a variable regenerative braking load to assist. It would probably make a significant extension to range in this case and help modulate use of the brakes.

There is a fella here that just built a car with AC, and set up his accellerator such that foot-off the pedal is full regen and about halfway is 'zero coast'. He hardly touches the mechanical brakes...


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## gte718p (Jul 30, 2009)

I am a rock crawler.









I would love to build an electric crawler. Think r/c crawler. It's the best of all worlds, low end torque, weight balance, flexibility with component locations, and simplicity.

The problem still comes down to energy density. I use 12-18 gallons wheeling the Toyota in a weekend. 90% of the time I'm idling the other 10% I'm at full throttle. There are some efficiencies to be had in electric, but think it would be hard to make up the energy in batteries.

My perfect rig is a 4 linked buggy. Dana 44 steering axles front and rear. 8" motors mounted directly to the axles with a single chain reduction to the pinion. Dual solution mr controllers. A321 battery packs strapped to the wheels. All the weight would be low. Without having to worry about drive line the articulation could be insane.

Problem is build cost is 50k on the low side, and 100k on the high side. It still has range limits. Probably not going to rock places like moab with very long trails, but places like Johnson valley, moon rocks, or Hollister would be incredible. It would also be unbeatable for park racing.


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## zeroexcelcior (Aug 2, 2011)

It should first be known that this is purely speculative on my part, but what if it were a series hybrid? I know it would still be a dinosaur burner but I would image the efficiency to be greater than that of a pure ICEV. The engine would only need to operate in the peak efficiency rpm band and the electric motor would act as a CVT, allowing for the performance of an EV with the range of an ICEV.

Either way, this sounds like a cool project.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> There is a fella here that just built a car with AC, and set up his accellerator such that foot-off the pedal is full regen and about halfway is 'zero coast'. He hardly touches the mechanical brakes...


That's pretty much how all A-pedal regen works, you simply modulate how much current goes into or out of the motor with your foot. I agree that regen would be quite valuable in an off road vehicle. Liquid cooling would be nice too, but you could get around that with some ducting and blowers.


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## Jimdear2 (Oct 12, 2008)

Sorry I came on a bit negative with my comments. I didn't mean to be, after all I'm the fool who strapped a 13 inch 300 pound GE motor to the back of a modified garden tractor for tractor pulling. I think electric is the way to go for a lot of things. 

Talk about a need for torque, pulling is one place where massive torque is great. Crawling would for sure be another.

I did roughly go through the math I described to figured if batteries could contain enough energy to do at least 2 hooks consecutively (in case of a pull off). 

I've also considered if electric would work in crawling but I would want to know I had enough energy to get through the entire course. Stranding a heavy vehicle on top of a rock without the power to get off OOPPSS.

I guess in the end do the math to get close but you just have to do it to find out.

Jim


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## Lordwacky (Jan 28, 2009)

I agree electric rock would be sweet and it should work great if properly designed. and can be a good deal more rugged too. I disagree with the ac comments. Though. If this is a deidcate rock crawler then you will not general be traveling a high enough speeds to take advantage of regen. And also if memory serves correctly dc motors generally have more low end torque then AC. I would be worried about getting crap in the brushes of a series wound DC motor though.... If you could find a BLDC big enough... that would one sweet crawler.


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## JRP3 (Mar 7, 2008)

Speeds are low but descents are steep, you have to expend a lot of energy to go up you might as well get some back on the way down. I agree that BLDC would be the way to go in this case.


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## Jan (Oct 5, 2009)

Why would BLDC be better than AC in this case? What's the difference?


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## JRP3 (Mar 7, 2008)

I think they have higher low end torque because the magnetism is inherent where with induction motors it must be induced. There is a reason the induction motor in a Tesla Roadster turns 14,000 RPM.


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## gte718p (Jul 30, 2009)

I'm pretty sure perminate magnet is the way to go. Low end torque is key. My current crawler never goes faster then 35 mph. There is not a lot of kinetic energy to be recovered by regen.

If your a pure California rock crawler I wouldn't worry about dust. In California my rig was almost never dirty. If your east coast and going to do more trails then rocks dirt might be a big deal. Either way I would be very tempted to seal up the motor and rig up some type of liquid cooling.

Im not keen on the series hybrid. I'm sure it could work, but one of the beautys of an electric crawler is not having to carry around an engine. You also introduce an additional state change for the energy with its associated ineffeciencies. I don't have hard numbers, but my SWAG says its not worth it. Your also adding sprung weight, which unlike every other racing is your enemy.
i do have tenenative plans to build a crawler down the road. My plan is to cheat a little. I never wheel alone. I'm goning to take a Lincon Ranger, a large welder/generator and mount it on the back of my current gas powered rig. I'm going to bring it along as "welding support" You break a lot of crap rock crawling. It just happens to also be able to charge the electric crawler if I get myself stranded.


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## eou_edu (Apr 27, 2011)

Wow very interesting feedback! Thanks for all the input. So many things to comment on. I like the idea of cheating a little bit and having your buddy carry the gas generator. Even if it's a little one and you have to limp home tethered into a small gas generator on a very small amount of power, it definitely wouldn't be the first time I'm limped home from a four wheeling trip. It would also create a whole new brand of four wheelers trash talk, "my ford pulled out your chevy." Instead it would be, "My 92 octane charged your lithium ion."

My limited knowledge I would also say a DC motor would be much more efficient for this set up, not be mention cheaper and easier. 

Energy density is a problem but like it was mentioned when 90% of the time wheeling is idling, you can close a pretty big range gap with ICE. Add that to much more throttle control, lighter motor than an ICE, and drop the transmission and some other unused parts. You could also evenly distribute weight better you could save some weight building a tube frame that didn't have to put the blunt of it's weight in one spot. Even with weight of batteries with enough to get you good range I think you would almost build one the same weight or even lighter. 

Again my limited knowledge speaking here would tell me the most efficient and even least expensive route would be mating the motor to a divorced transfer case, no transmission. Keep the motor small and light and you could easily get power to front and back and still have a good crawl ratio. Even though you don't need near the crawl ratio a typical rock crawler of 50:1-100:1 or even more, I don't think you can get away with direct drive.........The fabled hub motors would be ideal and you could do a lot of great things with suspension, low center of gravity, and controlling all four wheels independently of each other. But expense, complexity, unsprung weight off road, and not a good enough crawl ratio I believe would disqualify this idea immediately.

In a perfect utopia world I would build myself an all electric personal infrastructure. My daily driver would be an all electric car, with my off road 4x4 toy, my 66 GTO converted to electric (I just have to figure out how to hook up speakers to made it sound like a V-8 is under the hood so I don't get shot by classic car lovers when i do this), an electric lawn mower, and ATV. Spend the money on good battery packs and swap them out to whatever I want to drive at the time..........But currently I'm still at the thinking about it stage with no electric vehicle whatsover.......Except my RC car.


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## eou_edu (Apr 27, 2011)

gte718p said:


> I am a rock crawler.
> 
> 
> 
> ...



Your idea about motors to each differential was interesting. Would the gear ratio be low enough for off road? What would keep them going the exact same speed all the time? What if one was hanging up a little? I guess it would be a lot better off than one hub motor hanging up. Instead of a rapid pull to one side they would just fight themselves a little bit...........Not trying to criticize your idea, that's more of a question than a statement. Cost 50K-100K, I guess it depends how deep you get. But other than cost of batteries you could make it a relatively cheap projects I would think.


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## boekel (Nov 10, 2010)

What about a motor for every wheel, thus creating possibility for big vertical wheel travel without the difficulty of driving axles having to link from differential to wheel, huge ground clearance, etc.


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## Jan (Oct 5, 2009)

JRP3 said:


> I think they have higher low end torque because the magnetism is inherent where with induction motors it must be induced. There is a reason the induction motor in a Tesla Roadster turns 14,000 RPM.


I don't see that much of a difference in the torque/power curve between e.g. the currentevtech's 80kW BLDC and the Siemens Ford AC motor. The BLDC is slightly eralier at its peak power. But not much. The only big difference is the much higher max RPM of the AC, and its abbility to maintain that power. Which makes the AC drive better suited for a single speed transmission. Or do you have better info?


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## Jan (Oct 5, 2009)

Lordwacky said:


> If this is a deidcate rock crawler then you will not general be traveling a high enough speeds to take advantage of regen.


Regen is not only about speed. Going down hill with a lot of mass (which is often the case with 4x4's) is a lot of potential energy. The selected gears will make the motor spinn quite high even going slowly downhill. And in my (little) experience with off roading I would not want to loose engin braking.


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## Jan (Oct 5, 2009)

Oh, and liquid cooling is also not negotiable for me. Properly protected motor and inverter against the harsh environment, and beeing able to cool the motor/inverter to the max at a stand still while needing max power. No, DC is not an option for serieus off roading in my opinion.


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## JAC (Jul 3, 2014)

First post! Here goes...
Sorry if it's long-winded. I typed it over the course of the day and I'm copy/pasting it into the forum now.

I want to build a mild, capable 4x4 along the lines of a 1987 Dodge Raider (same as a Mitsubishi Montero) which is similar in power, weight, size, and gearing to a Jeep Wrangler with a 4-cylinder ICE.

I know this is an old thread, but it is perfectly aligned with my interests. I wanted to add my thoughts here. 

I am just learning about EV tech. But I do have some experience off-roading / four-wheeling / rock-crawling. I'd like to outline some concepts so that the EV community understands the needs of the off-roading community. 

The main concept of rock crawling is using your combustion engine for propulsion and braking at or near idle to maintain slow controlled travel over rocks and boulders both up them and down them. Engine braking is a requirement. (Unless you want to drive with two feet all day.)

Off-roading also involves driving through water and mud, snow, sand, and debris. A sealed, submersible drivetrain is a requirement.

'Wheeling and 'crawling involves long periods of little to no forward motion at high power. Active cooling is a requirement. 

There are times when a burst of power is needed. For sport and for safety. Sudden access to a wide RPM band is a requirement. 

Other thoughts:

Applying conventional concepts of efficiency to vehicles in the sport of off-roading is kind of funny. They are more like machines than cars; like a tractor or skid-steer. it should be measured in hours of run time instead of miles and include descriptions such as "heavy" or "light" use. I have a street-legal lightly modified 4x4 SUV that I take on trails and some obstacles. I do not refer to my odometer while off-roading, I only refer to the fuel-gauge and a clock. 

That said, a mild trail rig that is street-legal could tow a trailer with range-extending batteries and/or a generator to get from home to the trail-head and back. But the actual off-roading could be done without the trailer.


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## akseminole (Jan 5, 2014)

JAC

My thoughts on this subject:

An NP435 transmission coupled with either a gear doubler or a transfer case with a low gear of 5 to 1 plus the differential gears.

With high enough gearing you can output fairly significant torque levels, if you are willing to go slow- 2-6 mph. It's my understanding that AC motors are at their most efficient lowest current draw at high rpm. I figure that if you gear the system to run at the motors constant power output at its highest rpm for the lowest current, this would be the most efficient use of power.


1st 2nd 3rd 4th Rev.
NP435A 4.56 2.28 1.31 1.00 5.64
NP435L 6.68 3.34 1.66 1.00 8.26
NP435D 4.90 2.29 1.19 1.00 6.06
NP435E 6.68 3.34 1.74 1.00 8.26


I am drawn to the NP435L & E for their reverse gear. Why? Because you can reverse your AC motor electrically and go forward in reverse...

And being that you don't Need all the torque conversion for highway speeds, there are gears leftover for that too. I figure that an AC76 or AC35x2 would do pretty well at highway speeds, but why waste Amps to make torque when you can just spin gears to make it with a high revving motor? With the gears and lowered current draw you can just chug along down the trail and when it's time to use your peak motor output on the highway, you just change gears.

I assume that it could be really bad for component wear for you to use full amperage torque power at full gear down, a quick calculation for 183-1 x 250 ftlb of torque comes out to 45,750 ft lb

My assumptions for continuos power output of around 30-60 ft lbs x 183-1 = 5,490 - 10,980 ft lb of torque.

A stock jeep with standard gearing has around 24-1 in first gear/low with (arbitrary) 4.11 diff gears and if we assume that it's engine can output 150 ft lb of torque that makes it output 3,600 ft lb.

With all the gear combos possible with the np435 + a gear doubler + transfer case + diff gears, it seems that a low current draw is possible to motor around on off camber style trails. Maybe it's not really suitable for mud bogging, but do you Really want to get a High voltage vehicle Stuck in the mud?

Speaking of stuck, the np435 has a pto access on its side which you could route to either a hydraulic pump or straight pto powered winch. Win win?


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## gte718p (Jul 30, 2009)

It is funny that this thread popped up again. A guy just built an electric crawler over on Pirate4x4. It is in the toyota section. It is very doable, and should be very capable. However you are going to have to drop a serious amount of money on it.


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## JRP3 (Mar 7, 2008)

Link: http://www.pirate4x4.com/forum/toyo...ctric-4runner-off-road-rig-full-time-4wd.html

Video:


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## rochesterricer (Jan 5, 2011)

That is awesome! Love his other vids too. Been strongly considering converting a zero turn mower for my next house, since we plan to get an acre or two.


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