Treefrog
Book Wheeler
With all the new EV trucks and hybrid 4x4s, heck an Ultra4 class, on the horizon, maybe it’s time to get our head out of the oil well and look at where we are ultimately headed. It has been mentioned and briefly discussed a few times here, but its worth the time to look a bit closer.
But before we go looking at some concepts behind building one, first we should take a trip to the past and look at what has been done before. I’ve only been able to find any information on 3 rigs. TonyK’s buggy on PBB, tipover’s electric Samurai, also on PBB, and a few videos by Evcrawler Buggy on Youtube.
Sorry in advanced about the mixed units systems. EV stuff seems to love mixing them.
TonyK’s shop built one on PBB back in 04/05. This is the one I'll go most in depth on, because it has enough info available to, and because it’s a buggy.
They went with a powertrain layout of motor -> crawl box -> t-case -> differentials. Interestingly, no transmission.
By the time it was parted out, it had a Klune V (pretty sure it is the 4:1 version), 5.0 atlas, and 6.5 gears turning 40s.
They said it had 85 hp at 4000 rpm and could output 300 ft*lb at 0 RPM. However, I was unable to find a motor that meets those specs. I did find the one shown below that meets everything except the torque, which is on 108 ft*lb. They may have had a setup that let them push the high amperage to it at low RPM.
Energy wise it had 3 16 Ah car batteries in parallel for steering and 12 42 Ah car batteries in series for drive systems. Claimed it had a 1.5 hr charge time. The pack weighed in at 400 lbs. Current cost for the exact same setup is somewhere around $4350.
A bit of math says he had 6.1 Kwh for drive and 0.6 Kwh for steering. A claimed run time of 2.5 hours at comp level and 4-4.5 hours in normal offroad driving. These means that we can estimate .7 wh/lb of vehicle when crawling for an hour for driving and steering.
Some other relevant info is that it weighed 2378 lbs without water with a 50/50 split.
A note or two before we dig into the details: this is a buggy designed for slower rock crawling and a few looser climbs, but not bombing through at high speed. And that we are converting a buggy. In all likely hood it would be a ground up build, but a conversion gives us easy numbers to start from.
The conversion has two main parts to it, the motor and the battery. Let’s look at the motor first
Motor
The motor selection is driven by the minimum max rpm and the torque at 0 RPM. RPM is the easy one.
Estimating torque we need is harder. I figure the approach is to find the torque needed to do a standing burn out in the driveway. Most of the values are easy but, we are forced to assume a tire coefficient of friction. I figured 1.5 would be a safe estimate for a sticky compound tire on dry pavement.
Now over to our theoretical buggy. 40” tires with 5.38 axle gearing. For the transfer case let’s go with a 4.0 Atlas 4sp. A loaded down trail weight of at 4000 lbs with a weight of 3000 lbs after the engine, radiator, transmission, fuel system, etc. have been removed. Assuming that it used an iron block V8 and an automatic. Let’s go with an even weight distribution of 50/50. For now, let’s assume that after the conversion we have the same weight.
Assuming we want a max speed of 10 mph in the lowest gear combination, about 40 mph with the crawl box in high, we need a motor capable of at least 5000 rpm.
Also, if we want to be able to break the tires free on pavement with the transfer case in low and the crawl box in high, we need at least 90 ft*lb at the motor.
Of interest is the torque and therefore the HP requirements seem low. If we want all of that torque at max speed, we will need a motor with 87 HP. Not a lot. The big thing to remember is that that torque is available from 0, no revving the engine, no torque converter, no delay.
It may be better to go with a clutch-less setup with a lighter duty manual transmission like an AX15 instead of the crawl box. Lower low and more options to play with.
But all this assumes that we are using a single motor in a conventional drivetrain layout. But what about separate motors at each end of the vehicle like Tesla’s pickup or the new Hummer? Or one per wheel like with hub motors? The weight and room savings of getting rid of the transfer case and driveshafts would be significant.
One motor per axle
Sounds great. FWD, RWD, 4WD, just two switches on the dash. That is where the easy stops. We would be looking at a lot of custom components. No junkyard axles here. Two speeds would be feasible using a planetary set out of a pair of transfer cases and a solenoid. Plenty of off the shelf lockers we could use as a base. Just replace the ring and pinion with some external gears that have a total ratio equivalent to the ring and pinion and t-case low. Seems reasonable so far. Heck, front high and rear low could be useful. The big issue here is cost. All the custom parts, and even more so, the motors and their controllers. 2x the cost at least.
Hub Motors
The issue here is the torque and RPM constraints. With the other two configurations, the locker and/or t-case split the torque as needed. And the selectable low ranges give a selection of speed vs precision, so we didn’t need a motor that had tons of low torque but that can also rev out. With hub motors, each corner needs to be able to generate 50% of the torque needed and reach the wheel RPMs needed.
Going off the hypothetical conversion, we need 5000 ft*lb between the rear wheels to break the tires free. So, 2500 ft*lb per corner. Big tires really screw ya here. But at the same time, to reach that 10 mph max crawl speed, we have to spin up to 84 RPM. To reach the 40 mph speed would take 340 RPM. There are no current electric hub motors that can output that torque or spin that slow. The ones designed for buses and such only have 1100 ft*lb. This means we need a small motor and a reduction.
This leads to a cost issue. 4 slightly smaller motors, each with its own controller, reduction gearing, hub, and a bunch of essentially one-off parts for steering and the axle beam, protecting the exposed motors, etc. Gets really expensive really fast. On top of that you need a motor coordinator or to put up with a system that acts like it has limited slips at both axles and the center locker and torque limiting clutches at all 4 corners. And you have to deal with the steering forces of a rock crawler. How often does a stock knuckle break on a dana 60 with hydro steering and 40s compared to under normal abuse?
Battery
On to the battery. Much less numbers here. Just need to make sure that you can support the amperage and voltages needed, both at fully charged and near fully discharged. The big thing here is that unlike 15 years ago, running lithium batteries is a reasonable option, though it is more expensive, complex, and takes more work to do. The saving grace of an electric crawler will likely be the extremely low idle power draw, if it kept spinning like an ICE when waiting for its turn on an obstacle, it would die quick.
The big question is how many Kwh or Ah are needed.
he new hybrid Wrangler (5100 lbs) has 17.3 Kwh, about 15 usable, and a range of 20 miles in town. TFLoffroad found it got 4ish miles going up Red Cone pass before the motor kicked on. Once we account for the 5.6 Kwh that went it took to take that 5000 lbs up 3000 ft, that leaves us with 2.5 kwh/mile trail riding.
TonyK had 6.7 Kwh and could get 4 miles of southwest crawling. I have found one other electric buggy, that gets 4 hours of eastern wheeling out of 16-18 kwh battery. An EV Samurai over on PBB gets a couple hours hours out of 13.3 Kwh. He believes it can do the Rubicon in one charge, but takes a generator and charges it just to be safe. Also says that wheeling in low range he averages 25 A or 2.4 kw draw. With his 96 V pack, that puts him at 5-6 hours. That makes me believe that 25 A is the average when moving and not over the entire ride.
But before we go looking at some concepts behind building one, first we should take a trip to the past and look at what has been done before. I’ve only been able to find any information on 3 rigs. TonyK’s buggy on PBB, tipover’s electric Samurai, also on PBB, and a few videos by Evcrawler Buggy on Youtube.
Sorry in advanced about the mixed units systems. EV stuff seems to love mixing them.
What has been done before
TonyK’s shop built one on PBB back in 04/05. This is the one I'll go most in depth on, because it has enough info available to, and because it’s a buggy.
They went with a powertrain layout of motor -> crawl box -> t-case -> differentials. Interestingly, no transmission.
By the time it was parted out, it had a Klune V (pretty sure it is the 4:1 version), 5.0 atlas, and 6.5 gears turning 40s.
They said it had 85 hp at 4000 rpm and could output 300 ft*lb at 0 RPM. However, I was unable to find a motor that meets those specs. I did find the one shown below that meets everything except the torque, which is on 108 ft*lb. They may have had a setup that let them push the high amperage to it at low RPM.
Energy wise it had 3 16 Ah car batteries in parallel for steering and 12 42 Ah car batteries in series for drive systems. Claimed it had a 1.5 hr charge time. The pack weighed in at 400 lbs. Current cost for the exact same setup is somewhere around $4350.
A bit of math says he had 6.1 Kwh for drive and 0.6 Kwh for steering. A claimed run time of 2.5 hours at comp level and 4-4.5 hours in normal offroad driving. These means that we can estimate .7 wh/lb of vehicle when crawling for an hour for driving and steering.
Some other relevant info is that it weighed 2378 lbs without water with a 50/50 split.
A hypothetical trail buggy conversion
A note or two before we dig into the details: this is a buggy designed for slower rock crawling and a few looser climbs, but not bombing through at high speed. And that we are converting a buggy. In all likely hood it would be a ground up build, but a conversion gives us easy numbers to start from.
The conversion has two main parts to it, the motor and the battery. Let’s look at the motor first
Motor
The motor selection is driven by the minimum max rpm and the torque at 0 RPM. RPM is the easy one.
Motor RPM = desired speed(mph) * gearing * 336.1 / tire diameter(inches)
Estimating torque we need is harder. I figure the approach is to find the torque needed to do a standing burn out in the driveway. Most of the values are easy but, we are forced to assume a tire coefficient of friction. I figured 1.5 would be a safe estimate for a sticky compound tire on dry pavement.
Motor torque needed(ft*lb) = Weight(lbs) * weight bias(1 being all weight on rear) * tire size(inches) / (gearing *20)
Motor torque needed(N*m) = Weight(lbs) * weight bias(1 being all weight on rear) * tire size(inches) / (gearing *14.75)
Motor torque needed(N*m) = Weight(lbs) * weight bias(1 being all weight on rear) * tire size(inches) / (gearing *14.75)
Now over to our theoretical buggy. 40” tires with 5.38 axle gearing. For the transfer case let’s go with a 4.0 Atlas 4sp. A loaded down trail weight of at 4000 lbs with a weight of 3000 lbs after the engine, radiator, transmission, fuel system, etc. have been removed. Assuming that it used an iron block V8 and an automatic. Let’s go with an even weight distribution of 50/50. For now, let’s assume that after the conversion we have the same weight.
Assuming we want a max speed of 10 mph in the lowest gear combination, about 40 mph with the crawl box in high, we need a motor capable of at least 5000 rpm.
Also, if we want to be able to break the tires free on pavement with the transfer case in low and the crawl box in high, we need at least 90 ft*lb at the motor.
Of interest is the torque and therefore the HP requirements seem low. If we want all of that torque at max speed, we will need a motor with 87 HP. Not a lot. The big thing to remember is that that torque is available from 0, no revving the engine, no torque converter, no delay.
It may be better to go with a clutch-less setup with a lighter duty manual transmission like an AX15 instead of the crawl box. Lower low and more options to play with.
But all this assumes that we are using a single motor in a conventional drivetrain layout. But what about separate motors at each end of the vehicle like Tesla’s pickup or the new Hummer? Or one per wheel like with hub motors? The weight and room savings of getting rid of the transfer case and driveshafts would be significant.
One motor per axle
Sounds great. FWD, RWD, 4WD, just two switches on the dash. That is where the easy stops. We would be looking at a lot of custom components. No junkyard axles here. Two speeds would be feasible using a planetary set out of a pair of transfer cases and a solenoid. Plenty of off the shelf lockers we could use as a base. Just replace the ring and pinion with some external gears that have a total ratio equivalent to the ring and pinion and t-case low. Seems reasonable so far. Heck, front high and rear low could be useful. The big issue here is cost. All the custom parts, and even more so, the motors and their controllers. 2x the cost at least.
Hub Motors
The issue here is the torque and RPM constraints. With the other two configurations, the locker and/or t-case split the torque as needed. And the selectable low ranges give a selection of speed vs precision, so we didn’t need a motor that had tons of low torque but that can also rev out. With hub motors, each corner needs to be able to generate 50% of the torque needed and reach the wheel RPMs needed.
Going off the hypothetical conversion, we need 5000 ft*lb between the rear wheels to break the tires free. So, 2500 ft*lb per corner. Big tires really screw ya here. But at the same time, to reach that 10 mph max crawl speed, we have to spin up to 84 RPM. To reach the 40 mph speed would take 340 RPM. There are no current electric hub motors that can output that torque or spin that slow. The ones designed for buses and such only have 1100 ft*lb. This means we need a small motor and a reduction.
This leads to a cost issue. 4 slightly smaller motors, each with its own controller, reduction gearing, hub, and a bunch of essentially one-off parts for steering and the axle beam, protecting the exposed motors, etc. Gets really expensive really fast. On top of that you need a motor coordinator or to put up with a system that acts like it has limited slips at both axles and the center locker and torque limiting clutches at all 4 corners. And you have to deal with the steering forces of a rock crawler. How often does a stock knuckle break on a dana 60 with hydro steering and 40s compared to under normal abuse?
Battery
On to the battery. Much less numbers here. Just need to make sure that you can support the amperage and voltages needed, both at fully charged and near fully discharged. The big thing here is that unlike 15 years ago, running lithium batteries is a reasonable option, though it is more expensive, complex, and takes more work to do. The saving grace of an electric crawler will likely be the extremely low idle power draw, if it kept spinning like an ICE when waiting for its turn on an obstacle, it would die quick.
The big question is how many Kwh or Ah are needed.
he new hybrid Wrangler (5100 lbs) has 17.3 Kwh, about 15 usable, and a range of 20 miles in town. TFLoffroad found it got 4ish miles going up Red Cone pass before the motor kicked on. Once we account for the 5.6 Kwh that went it took to take that 5000 lbs up 3000 ft, that leaves us with 2.5 kwh/mile trail riding.
TonyK had 6.7 Kwh and could get 4 miles of southwest crawling. I have found one other electric buggy, that gets 4 hours of eastern wheeling out of 16-18 kwh battery. An EV Samurai over on PBB gets a couple hours hours out of 13.3 Kwh. He believes it can do the Rubicon in one charge, but takes a generator and charges it just to be safe. Also says that wheeling in low range he averages 25 A or 2.4 kw draw. With his 96 V pack, that puts him at 5-6 hours. That makes me believe that 25 A is the average when moving and not over the entire ride.
We had to install shocks with helper springs and max out the preload adjustment to get it off the bumpstops 
It had a top speed of about 50-55MPH, but it didn't seem to matter what gear you were in, so the motor was probably limited by both RPM and power. I can tell you that the 5 speed was entirely too many gears for the electric motor; you only really needed two of them, and if the ratios were optimized, you may have been able to make effective use of 3 gears. It was weird only using the clutch to shift gears at speed and being able to take off just fine in any gear including 5th 
