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Discussion Starter #1
Long time listener, first time caller.
Thought about tacking this onto a recent thread about a CJ7 conversion but decided it was different enough to start fresh.

Been thinking about an EV conversion for a LONG time and one of the ideas that keeps bubbling up to the surface is an old CJ5. One of the reasons is because it may be one of the easiest ways to achieve the mythical direct drive conversion.

No need to rehash the +/- of dumping the tans and transfer case, it's been covered before. A CJ/Wrangler style could be well suited to this for several reasons:
1) Light weight (even lighter with all the fiberglass tubs available)
2) Lots of very high ratio axle gears available (or axle swaps)
3) Can fit 2 electric motors side-by-side (I think) where the transfer case sits (one facing rear, one facing forward. Front diff is already offset)
4) Easy to get custom drive shafts

So I put together a spreadsheet to check some numbers and here are the highlights:


For "stock" Jeep numbers I just used very generic stuff as there are so many combinations out there,
28" tires
3.31 axle ratio
3.52 1st gear
2.27 2nd gear
4.0 I6 motor
220 ft lb max torque at 2800 rpm

With 31" tires and a 4.88 axle ratio the EV motor would be spinning 3700 rpm at 70 mph (pretty low rpm:().

I looked at roughly duplicating stock acceleration by matching stock wheel force. (I know, I know, a Cj5 with a glass tub, modern 4.0 and low gears would go like hell. It's a starting point)

To yield the same tire force numbers you would need each EV motor to have 290 ft lbs (1st gear equiv) or 190 ft lbs (2nd gear equiv).

So, most likely a pair of motors with a torque output between the 2 would result in pretty good acceleration. Maybe 200-230 ft lbs. High, but not outrageous. For the highway it probably only takes about 50-60 hp to keep it moving at 65 so that shouldn't be to too hard to get with 2 motors except for the problem of low rpm.

I looked at a couple of motor curves and estimated some efficiency numbers. With a big grain of salt, it looks like it would be getting around 1 mile/kWhr on the highway which seems really poor. I have a Soul EV right now and get 2.5-3 times that on the highway.

I'm trying to wrap my head around the trade-offs. 2 motors doubles the torque to compensate for the torque lost to gearing. But you double the power, which really only matters for top speed. Top speed would be irrelevant as long as you can reach 70mph. Efficiency seems worse running lower rpm. Weight, losses, reliability and complexity are better.

A crawler box could be cheaper than a second motor. But some are very noisy. Maybe high torque/low power motors are an advantage in some way?

Any thoughts on motor selection? Things I'm missing? Anybody tried it?
 

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What are your goals, exactly? Like, how will you use it? The dual motor idea sounds like a neat way to keep 4WD.

You can still get monstrous 6:1 gears for Jeep axles, and that gives you a 60-70mph top speed at 4-5,000rpm on 31s—spooky speeds in your average CJ.
 

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Discussion Starter #3
Overall goal is looking for something classic, topless and 2+2 to convert. Top choice would be Karmann Ghia but open enough to consider other options based on their merits.

If it were a jeep, it would be a fair weather driver. Even 2 wheel drive would be fine. The motivation for double motors is more about overcoming the problems with direct drive than a need for 4 wheel drive. However, it seems like an interesting option to explore because somebody else (such as the OP in the previous Jeep thread) might really need the 4wd capability.

Essentially: [2 motors] = [double torque] = [torque from 2:1 gear reduction on single motor] /= [efficiency of single motor at 2x rpm?]
At least I think that's the trade off. But that's why I'm looking for other folks to weigh in.

On most cars this doesn't work because the axle ratio is too low. With a Jeep you can compensate quite a bit with big gear ratios. I've seen ratio's in the 5's for the smaller Jeep axles (D30, 35, 44) but not into the 6's. Also, with the 5's I've seen, the pinion gear is so small I would be seriously worried about the strength. That's why I proposed the high 4's. Strong, proven, easy to get.

I picked 31" tires just because 31/32's look good on a CJ5 without looking like a monster. For me it would be street tread all the way. Speed targets are based on having built in overhead. I've owned a CJ7 and Wrangler and yes, no doors, swampers and 75mph is hairy.

So really trying to figure out what I'm missing. What's the catch? Seems too simple. Maybe there is no catch, maybe it's just a matter of picking your poison.
 

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Discussion Starter #4
Strange, I posted a reply but it never showed up. Take two...

My particular desire is for a classic, topless, 2+2 EV conversion. Top choice right now is a Karmann Ghia, but a CJ5 (maybe 2 or 7) would fit the bill as well.

For my use it would be a fair weather driver not an off roader. However, it seemed like an interesting enough drivetrain variant and could be useful to the OP of the previous Jeep thread and maybe the new one on the CJ2.

I have owned a few jeeps (CJ7, wrangler and Cherokee) and am very familiar with them. I have never seen an axle ratio over 6 for a standard Dana or Chrysler axle. The biggest I've seen were in the 5's and the pinion gear gets so small it seems really weak. High 4's are common and proven.

I picked 31" tires simply for looks. 31/32s on a CJ5 are nicely proportioned without looking like a rock crawler. And yes, 75mph in a CJ with no doors and swampers is quite hair raising. But, if you want to do 60 you need the power to do 75, even if you never go that fast.

Really just looking for feedback on the motor/efficiency/voltage tradeoffs of this concept. Fabricating a steel mount for 2 motors to sit where the transfer case goes would be no problem. Drive shafts would be no problem, might even be able to use the stock ones. Then you have the whole gas tank area and engine bay for everything else.
 

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What I am thinking about for my jeep cherokee is a tesla large rear drive unit with a zero ev quaife 4.5:1 Reduction Gear Set and a welded diff, hooked straight up to my transfer case box or transmission? Damiens Logic Board, HVJB, tesla charger with Damiens Logic board Salvaged production batteries. I got a lot of spare jeeps if I start breaking anything.
 

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What I am thinking about for my jeep cherokee is a tesla large rear drive unit with a zero ev quaife 4.5:1 Reduction Gear Set and a welded diff, hooked straight up to my transfer case box or transmission?
TESLA LARGE DRIVE UNIT GEAR SET 4.5:1
This solves the two big problems of using the Tesla drive unit rotated 90 degrees and placed where the transmission and transfer case would be:
  1. overall gear reduction ratio is reduced by a new gear set to be suitable in combination with final drive reduction in the vehicle's axles
  2. reverse-rotation oil pump (separate item) allows reversed drive rotational direction, to match the vehicle's axles with the Tesla motor in the centre of the vehicle and with the front propeller shaft on the right-hand side.
In case it is not apparent to someone, this replaces the intermediate shaft and gears but the output gear is the same as stock, to work with the stock Tesla final drive section (differential and ring gear). I assume that 4.5:1 is the overall drive ratio of the drive unit after modification, combining the lower-than-stock reduction of this first stage plus the stock reduction of the final stage.

An assumption behind this solution is that simply using an oil pump designed to pump the opposite shaft rotation direction is adequate, without changing anything else about the oil scavenge flow in the gearbox.

This is intended for Land Rovers, in which the propeller shafts to the front and rear axles are in line with each other, offset from the centre line of the vehicle by the transfer case. In a Jeep, this would work for the front, but the rear output would be offset despite the axle's input being centred... that's a problem.

I didn't see anything on the Zero EV site indicating that this is from (or uses components from) Quaife, although they could be the manufacturer.

I am surprised that anyone went to the effort of developing this kit, given what I would guess is a small number of potential customers. I applaud Zero EV both for doing what appears to be a very nice job of the kit, and for providing a clear description in the video.

With this system from Zero EV, the original transmission and transfer case are not needed; the outputs of the Tesla drive unit are connected to the front and rear propeller shafts. That leaves options for the Tesla diff, which now becomes the centre diff of the vehicle:
  • leave it open
    • not great for off-road, as it would need an individual wheel braking system to prevent spinning of any tire which loses traction
  • limited slip
    • better off-road, but still may not be good enough
    • Zero EV offers a Quaife helical gear type limited slip
  • lockable
    • I don't know of a manually lockable diff which has been installed in the Tesla case
  • spool or welded
    • completely disable or eliminate the diff, forcing the front and rear axle speeds to always be the same, but that means that locking hubs on one axle (presumably the front) would need to be unlocked to allow operation on pavement.
    • Welding is possible, but not the most elegant solution; a spool is a part which replaces the entire diff with a single solid part (to which the ring gear is bolted) for a simpler, lighter, and more reliable solution than leaving the diff in and welding it.
If you were to use original transfer case instead of using both drive unit outputs, one to each axle, one output could be connected to a transfer case. In a Jeep, that would fix the problem of alignment of the propeller shafts. Since the transfer case input is on the vehicle centre line, this would offset the bulk of the drive unit, which would be a problem to fit it in - this is not what Zero EV intends, but it would give you the two gear ranges and front axle disconnection of the original transfer case. The drive unit could be placed in the engine compartment, with a shaft to the transfer case (making it a "divorced" transfer case), but that uses up valuable space in the engine compartment which is probably needed for battery.
 

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Oh nice idea, put a spool in instead of a Lincoln locker(weld the diff). Think I got enough room for batteries out back. Thanks Brian! Got to sell a couple ice corrollas first so I can afford a Tesla drive unit to try this. If someone else wants to experiment with it be my guest. Hahaha really want to make my Cherokee Tesla powered. The future is bright!

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Discussion Starter #8
That's a super interesting proposition. Way too much $$$ for me though.

Some thoughts. I'm surprised that the stock Tesla diff is open and doesn't have some kind of traction aid built in. How do they do torque bias and control wheel spin? Is it all with individual brake modulation?

As far as gearing, the Tesla stock ratio is about 9.5:1 so a 4.5:1 replacement would require an axle ratio of about 2:1 for it to spin in it's happy place. Not sure I've ever seen a 4x4 (much less a car) with a ratio that low/high. For a Jeep, if you used a stock-ish 3.2:1 with the 4.5:1 you would have quite a lot of wheel torque! Maybe with tall tires you are back to Tesla equivalent.

For installation in a Jeep with the offset front diff I don't think it would be too much of a problem. Just draw a line from the rear pumpkin yoke to the front pumpkin yoke and install the Tesla unit on that line.

The reason you have u-joints is for the vertical misalignment from axle to transfer case. There is no reason some of that misalignment can't be horizontal. The u-joint doesn't know the difference. Just split the difference front to back.
 

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Yeah I got double cardan joint front driveshafts front and rear anyways right now. Definitely love big tires!! Would be awesome to not have a transmission and transfer case. But for a low price solution I think I am gonna get a Tesla large drive unit probably on ebay for less than 3500 bucks with my tax return, weld the diff, and mate it directly to the transfer case.
High voltage junction box $250
Damian's control board $650(could open source if you know how to do that stuff, I don't...)
Use scrap steal from the metal pile at work for motor mounts.
Run stock drivetrain and try not to spin the tires till I can afford to beef up my drivetrain. Got parts jeeps. Really just want a Tesla motor. Should be twice as efficient as my Advanced D.C. motor. So may be able to get like 50 miles on a charge?????
Already have 20 kwh of L.G. chem cells.
I wonder if my zivan ng3 can handle a high voltage battery pack? If not...
Tesla charger gen 2 $250 bucks on ebay
Damian's logic board $450. (Could open source on here for free if ya want.)

Yeah It's crazy looking but I can easily put 10,000 dollars a year in fuel if I drive my jeep to go skiing and mountain biking everywhere I want. Kinda feel like I can afford it. We'll see.





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I'm surprised that the stock Tesla diff is open and doesn't have some kind of traction aid built in. How do they do torque bias and control wheel spin? Is it all with individual brake modulation?
Yes, it's all done with the brakes. With the now-universal inclusion of ABS and stability control in cars, brake-based traction control is just a software feature, and so mechanically controlled differentials are becoming less common.

Of course this doesn't work for a DIY builder who is not retaining the stock controls and is using a different vehicle, so Zero EV (and perhaps others) have been fitting a Quaife limited-slip differential for an entirely passive mechanical solution.

For installation in a Jeep with the offset front diff I don't think it would be too much of a problem. Just draw a line from the rear pumpkin yoke to the front pumpkin yoke and install the Tesla unit on that line.

The reason you have u-joints is for the vertical misalignment from axle to transfer case. There is no reason some of that misalignment can't be horizontal. The u-joint doesn't know the difference. Just split the difference front to back.
I think that's a valid approach, but if the drive unit were midway between axles it would put the drive unit outputs closer to the centre line and thus put the motor and inverter off-centre, so they would not likely fit in the transmission tunnel. It would likely work with the drive unit in the rear part of the engine location, if you're willing to give up on using that whole space for battery modules.

It would be worth checking how much static angle is permissible for the U-joints.
 

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As far as gearing, the Tesla stock ratio is about 9.5:1 so a 4.5:1 replacement would require an axle ratio of about 2:1 for it to spin in it's happy place. Not sure I've ever seen a 4x4 (much less a car) with a ratio that low/high. For a Jeep, if you used a stock-ish 3.2:1 with the 4.5:1 you would have quite a lot of wheel torque! Maybe with tall tires you are back to Tesla equivalent.
I think there's an assumption that the top speed of the converted Land Rover (or other 4X4) will be much lower than the top speed of the donor Tesla, so a greater overall reduction ratio (resulting in a lower road speed at the motor's rotational speed limit) is appropriate. They're shooting for suitable, not equivalent.
 

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4.5 is a pretty nice gear ratio for a Range Rover - with the stock axle ratio of 3.54 you get a top speed with stock tires at 97mph or up to 107mph with 32’s. Range would suffer greatly at those speeds in such a brick. So probably not bad since it would cover a normal speed range up to 75/80 easily.

Older jeeps like Rover’s had offset front and rear differentials, I believe even the CJ5’s came this way at first, later in the production run converting to a center rear diff. Now a large Tesla motor maybe overkill for a small jeep, but doing the math with the stock ratio in a small Tesla motor and with a 3.31 axle ratio (available option) along with 32’s you would have a top speed of 55mph. Not your 60 or 75 requirement but maybe sufficient. My GPW (WW2 jeep) may have a stock top speed of 60 but feels so much more comfortable at 45. I hadn’t done this math before but seems like I just found the path for an easier conversion.
 

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Discussion Starter #14
Ok so we got a little off topic, but into some really interesting sidebars.
Since the transverse (longitudinal?) tesla option is way past a reasonable $ amount, back to the original question. I would love to hear from some guys that really understand motor tradeoffs.

Direct drive has a problem because you generally don't have enough torque for traditional gear reduction ratios in existing cars. The easy solution is use the tranny and you have any gearing you need to generate the torque.

Rather than double the torque with gearing, my question is how about doubling the torque with a second motor. This halves the RPM at cruising speed. But is also cuts the power required for each motor. What is the downside to this?

Would AC motors or DC motors handle the situation better? Would this allow for a lower system voltage or would a higher voltage still be better? How does motor efficiency trend when you are running way below the rated power level? Could you use a single controller to run 2 motors?

As I said, my simple comparison spreadsheet shows that to roughly duplicate a stock jeep's acceleration you would need each motor to have about 220 lb-ft (300 Nm) of torque. That's a pretty big number based on the limited data sheets I've looked at. Am I missing a source or type of motor that would work better?

Thanks
 

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Discussion Starter #15
Just to put a stake in the ground:

Pair of transwarp 9 motors and high amp controller(s).

Trouble is, I can't find any info on what a max torque would be for high amps. Their chart only show up to 350A but I know some guys put over 1000A (or more) through them for short periods.

My hunch is that this setup would be easy cruising at speed, but really high Amps every time you leave a stop light.
 

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Rather than double the torque with gearing, my question is how about doubling the torque with a second motor. This halves the RPM at cruising speed. But is also cuts the power required for each motor. What is the downside to this?
The assumption here is that the motor(s) would be used at half the speed that they can handle.

To me, the downsides are
  • high motor bulk, weight and cost... simply because you have twice as much motor as you would need with suitable gearing
  • double the complexity of motor controllers, wiring, coupling, and mounting
Could you use a single controller to run 2 motors?
While in theory two induction motors could be run from one controller, and even two synchronous motors could be run from one controller if they are mechanically connected precisely in phase, I don't think anyone does this - they get their own controllers. Two DC motors could run in parallel from one controller, but if they take unequal current you have lost your ability to individually limit motor current to a safe level.

Normal practice, even with DC motors, appears to be to use separate controllers, and that's what I would assume.

Would AC motors or DC motors handle the situation better? Would this allow for a lower system voltage or would a higher voltage still be better?
Given that they have separate controllers, the motors won't care whether they are run singly or in tandem, so they don't have any situation to handle. The AC versus DC choice is independent of the single or tandem configuration choice.

Would this allow for a lower system voltage or would a higher voltage still be better?
Voltage is required to overcome back EMF (proportional to speed) and winding resistance (proportional to current). For one motor, whether lower speed means less voltage requirement depends on where in the motor's operating range you are working; I don't think you can make an assumption without considering the individual motor and application. If comparing one motor to two of the same motors at half the speed and the same torque and current each, the two-motor setup will need substantially less voltage for the same performance (although more than half as much because only the speed-related voltage requirement has been reduced).

In the Spark EV, GM chose an unusually slow motor configuration, with a much lower reduction ratio and larger (higher torque capability) motor than more typical modern EVs such as the Bolt which replaced it. The system voltage is the same for the Spark EV and Bolt... but of course this is comparing a single motor in each case, designed for the corresponding application, not simply using one versus two of the same motor.

How does motor efficiency trend when you are running way below the rated power level?
Motor efficiency is better at relatively high load and moderate speed, but for most motors the rated power level is largely about cooling (rather than electromagnetic factors), so the rating might not tell you much about the most efficient operating point, especially with motors rated by aftermarket suppliers (rather than industrial ratings).


I find it strange that the conventional wisdom expressed (sometimes very aggressively) in this forum is that two separate motors on one axle is a bad idea due to motor and controller cost and complexity, while the conventional approach for high power with cheap motors which is widely supported is to use two motors in tandem... with the same motor and dual controller cost. :confused:

I think in practice it makes sense to use two motors in tandem when what you've got is only half the size of what you need, and it's important to use what you've got instead of what would be the most effective solution.
 

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It still seems like the dual motor approach is sound.

Driveshaft is a solved problem. The hard part is modifying driveshaft length and adapting a joint to the EV motor output shaft. If the angles are too high, there are numerous double-U-joint kits available that are trail-tested by those who run absurdly high lifts.

Power is a solved problem. The power is in the batteries to a large degree...I would be surprised if even a pair of lowly Warp 9s couldn't sustain a Jeep at 60mph.

Back of the napkin:

1) Leaf battery pack (though you may not need all of it) - $4,000
2) Charging/monitoring - $3,000
3) Two Leaf motors - $1,000
4) Two motor controllers - $1,000
4) Adapters, tools, driveshaft components - $bazillion

Your acceleration would be akin to 4th gear launches, but you're working with a Jeep putting 420ftlb of torque to four wheels. 4,000rpm is 70-100mph, depending on tires/gears...You can even get 7.17s in a Dana 60 if you're looking to go nuts...

I'm sure controlling two motors with the same pedal poses some challenges, but that napkin seems pretty good for a single-gear Jeep. Check my math?
 

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Discussion Starter #18
Yum yum, so much food for thought!

Yup, drive shafts are somewhere between no-problem and a little problem. No way I would go with dana 60's. Been there, done that. Maybe that works for somebody else's Jeep build.

Power, as you mentioned, is not a problem. With 2 motors, the HP to maintain highway speed is easy. In fact, that may be part of the problem. If you can get motors with enough torque to get off the line, you will have way more power than you need.

As brian said you have twice as much motor as you need.

I'm not sure how much HP it takes to get a jeep down the highway at 65mph, but I'm guessing it's in the neighborhood of 40HP.

I'm curious about the torque number for the leaf motor. That seems high for direct motor output and not the result of gear reduction.

If I compare a stock Jeep 3.31 rear to a 4.56 rear then it's like a stock 3rd gear launch. But if I take the Leaf numbers at face value and look at wheel torque, it would be somewhere between a 1st and 2nd gear launch (assuming the stock ICE equivalent is the 4.0L with about 230 lb-ft). That's not bad at all.

Quick google search shows you would need about 1000A total to get that torque out of 2 leaf motors, so you need a battery pack that can take that discharge rate.

SO basically, Amps = torque and Gear reduction = Torque. So the question is, is the elegance of a 2 motor system enough to justify the amps required?

I like the transwarp motors because they have a the yoke already built in and they are not very expensive. But you might need to put 2000A to the motors to get the same torque output.
 

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I'm curious about the torque number for the leaf motor. That seems high for direct motor output and not the result of gear reduction.
Oh, that's a good point! I'm quoting manufacturer specs, which I assume would include the 7.91 reduction gear bolted to the motor assembly.

210 ftlb / 7.91 = 26.5 ftlb

That's...painfully weak. Is that math right?

Edit: I think it is 280Nm (207 ftlb) to the output shaft:

https://www.marklines.com/en/report_all/rep1049_201202
 

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I'm quoting manufacturer specs, which I assume would include the 7.91 reduction gear bolted to the motor assembly.

210 ftlb / 7.91 = 26.5 ftlb

That's...painfully weak. Is that math right?

Edit: I think it is 280Nm (207 ftlb) to the output shaft:

https://www.marklines.com/en/report_all/rep1049_201202
While torque at the wheels would be a rational specification for comparison of complete powertrains with fixed-ratio transmissions, EV specs are usually for the motor, just as specs for cars with engines are for the engine; the 280N・m value is for the motor, so peak torque at the wheels is multiplied by the transaxle gearing to 280 N・m * 7.91 = 2215 N・m or 1634 lb-ft.

It's straightforward to confirm this: power is the product of torque and speed, so if the motor is producing 80 kW while producing 280 N・m, it must be turning about 2700 RPM... which is the speed at which the Leaf motor and controller transition from being current-limited (and so producing a constant torque) to being power-limited (to 80 kW).
 
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