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How do I go about calculating what gear ratio I would need?
  1. From the tire outside diameter (or circumference, or rolling radius) and the highest road speed you ever want to be able to go, work out the axle speed (in RPM) at that road speed.
  2. Multiply the axle speed by the axle ratio to get the propeller shaft (driveshaft) speed.
  3. Divide the motor's maximum speed by that highest driveshaft speed.
  4. Use that ratio or less (taller gearing) so that the motor never goes too fast; the higher ratio (shorter gearing) that you use, the more torque you have to the wheels at low speed, and the lower the speed where you start into getting full motor power available.
For example, with 235/75R15 (29x9.25R15) tires, 130 km/h (81 MPH) top speed, 4.27:1 axle ratio and 10,500 RPM Leaf motor...
  1. speed divided by circumference is about 975 RPM at the wheel/axle
  2. axle speed times 4.27 is about 4160 RPM for the driveshaft
  3. max motor speed divided by driveshaft speed is about 2.5
  4. ... so you can use up to 2.5:1 reduction gearing.

The only down side is I do like the idea of being able to slam on the clutch as a safety measure
You can slam a big red STOP button to kill high voltage power for the same effect. You can even attach the switch to a clutch-style pedal if you like that. :)
 

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Keen to see your cruiser build too, all the rich kids parents had those when I was a teenager in Australia 馃槀

I thought about this but I talked myself out of it. I wasn't sure how to go about the gear reduction and didn't realize there were good options out there. How do I go about calculating what gear ratio I would need? Are there generic options that would do what I want? Would be amazing if I could fit the whole motor and gearbox assembly further back to make room for batteries. The only down side is I do like the idea of being able to slam on the clutch as a safety measure
You can make it complicated or not complicated. The Nissan LEAF has a total ratio of 8.2:1. My Land Cruiser has a final drive of 4.1:1 so if I want to match the LEAF's top speed of 95MPH at 10,000RPM then it's just a simple 2:1 reduction (4.1 x 2 = 8.2:1). But since it's a heavy truck and 95MPH is way faster than I need I decided on a 2.7:1 reduction which gives me 2.7 x 4.1 = 11.07:1 and a top speed of 70MPH with stock size tires.

You can also just cross multiply using the LEAF's numbers:
95MPH/8.2 x 11.07 = 70 MPH

You'll likely be limited by what is available to bolt on to your truck but anything from 2.5:1 and 3:1 additional reduction should work for you. Also depends on your truck's differential ratios.

I do like the safety aspect of a clutch, with a transfer case you can simply shift to neutral, also. Not as natural or fast but just as effective.

THIS is what I've been looking for. So what exactly would I need to connect this coupler to the gearbox/doubler?
You would need to have a custom driveshaft built. Also that coupler is only halfway made, it's designed to be drilled out to bolt on a specific drive coupling but instead of that my plan was to have a machine shop weld or bolt on a common sized driveshaft yoke.
 

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You can make it complicated or not complicated. The Nissan LEAF has a total ratio of 8.2:1. My Land Cruiser has a final drive of 4.1:1 so if I want to match the LEAF's top speed of 95MPH at 10,000RPM then it's just a simple 2:1 reduction (4.1 x 2 = 8.2:1). But since it's a heavy truck and 95MPH is way faster than I need I decided on a 2.7:1 reduction which gives me 2.7 x 4.1 = 11.07:1 and a top speed of 70MPH with stock size tires.
That works, but by the time you've considered the target speed adjustment your calculation is just as complex as doing it from the fundamentals, and you still haven't accounted for any possible difference in tire size between the Leaf and the vehicle being converted.
 

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Discussion Starter · #24 ·
Ah this is helpful, thanks folks. I appreciate the fundamentals because I'm pretty new to this aspect (I mean, and all aspects of how cars actually work). So, I need to get underneath and confirm exactly what I have on this truck but say the rear axle is a Dana 44 with a gear ratio of 3.71, and I'm using 235/75R15 tires with a 130km/hr top speed as in Brian's example:

Tire circumference is 2304mm, so 1 rotation is 2304mm.
Speed in m/min: 130 * 1000 / 60 = 2167m/min
RPM at the axle going 130km/hr: 2167 / 2.304 = 940RPM (this is slightly off from your example Brian, did I do this wrong?)

Driveshaft RPM: 940 * 3.71 = 3487

Maximum ratio: 10500 / 3487 = 3.0

So 3:1 gear reduction. Does that seem about right?

Use that ratio or less (taller gearing) so that the motor never goes too fast; the higher ratio (shorter gearing) that you use, the more torque you have to the wheels at low speed, and the lower the speed where you start into getting full motor power available.
Reading this it sounds like ideally I want to be as close to that maximum ratio as possible, without going over. Is that right?
 

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Discussion Starter · #25 ·
You would need to have a custom driveshaft built. Also that coupler is only halfway made, it's designed to be drilled out to bolt on a specific drive coupling but instead of that my plan was to have a machine shop weld or bolt on a common sized driveshaft yoke.
Gotcha, so I'd need a custom driveshaft with this coupler bolted/welded to one side, that can bolt onto the input shaft of the doubler on the other side.
 

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Tire circumference is 2304mm, so 1 rotation is 2304mm.
Speed in m/min: 130 * 1000 / 60 = 2167m/min
RPM at the axle going 130km/hr: 2167 / 2.304 = 940RPM (this is slightly off from your example Brian, did I do this wrong?)
Nothing wrong :), the exact tire size depends on brand, and the tire squishes a bit under load so it effectively has a smaller diameter than it does without load so you get different numbers depending on specifically what is measured... the difference isn't important.

...
Maximum ratio: 10500 / 3487 = 3.0

So 3:1 gear reduction. Does that seem about right?
Yes, that seems reasonable.

Reading this it sounds like ideally I want to be as close to that maximum ratio as possible, without going over. Is that right?
Yes, for best performance.
 

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Discussion Starter · #27 ·
What are the considerations for a minimum reduction ratio? Amount of torque available from the motor at the low RPM end?

I'm wondering if I could just skip the doubler and mate the motor directly with the Dana 20 t-case already in the Scout and run it in its 2:1 mode. Obviously wouldn't provide any higher reduction if I wanted to actually use it for crawling or anything, but that's not a primary goal. Thoughts?
 

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What are the considerations for a minimum reduction ratio? Amount of torque available from the motor at the low RPM end?
Yes, torque to the wheels, which depends on gearing and motor torque. And that "low end" extends well up into the motor's possible speed range.

I'm wondering if I could just skip the doubler and mate the motor directly with the Dana 20 t-case already in the Scout and run it in its 2:1 mode. Obviously wouldn't provide any higher reduction if I wanted to actually use it for crawling or anything, but that's not a primary goal. Thoughts?
Yes, but then
  • you wouldn't have an even lower gearing option for crawling off-road (as you realize),
  • the input speed to the transfer case is limited,
  • low range in some transfer cases use straight-cut gears which are noisier than the helical-cut gears (or no gears, depending on design) used for high range, and
  • most transfer cases don't easily shift between ranges when moving a significant speed.
I haven't taken the time to review the Dana 20 design to see how many of these apply.
 

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I should have mentioned that most transfer cases do not allow the use of low range in 2WD, so unless it has a centre differential the low range would not be usable on pavement. In the case of the Dana 20, according to Novak:
"There is an interlock pin between the shifter rails. Removal of this interlock allows for the added function of 2wd/Low and front-wheel-drive High or Low to the 20 - if you have the "twin-stick" version (see below) of the 20."​
If leaving the transfer permanently in low range, this could presumably be arranged even if you don't have the ideal version. If you defeat the interlocks, be careful, since it become possible to engage one output in high and the other in low at the same time, binding all of the shafts.

The Dana 20 (and 18) is an all-gear design, described in a Ford manual for the Dana 20 on a Bronco enthusiast site.
In the high range, power goes directly through from input shaft to rear output shaft (no transmission through gears), and via helical parallel gears including the main drive gear, an idler shaft drive gear, and high speed gear from input shaft to front output shaft - no chain, and only helical gears, so that's good.
In low range, power flows through the first pair of those helical gears (main drive gear and idler shaft drive gear) to the intermediate ("idler") shaft, then by straight-cut gears (idler shaft low-speed gear and the sliding gears) to rear and front output shafts - that's going to be noisy.

With any luck you can follow the power flow in this image from that manual, and the manual also includes a longer version of the power flow descriptions:


To get 2WD low range, it appears that you would need to engage the rear output sliding gear with the idler shaft low-speed gear (using one lever), but leave the front output sliding gear in neutral (not back to engage with the idler shaft low-speed gear, and not forward to couple the front output shaft to the high-speed gear). The sliding gears actually slide in and out of mesh with the idler shaft low-speed gear; there are no synchros anywhere, so this is not practical to shift on the fly... okay if you want to leave it in low range all of the time, but that will be loud on the highway due to the straight-cut gears.

I think the Dana 20 is a brilliant design, but it is not intended to be an auxiliary transmission to reduce the speed of a high-speed drive motor.

Other discussions in this forum have considered this sort of "leave it in low range" approach, and some discussions have included the Dana 20, but in a few minutes of searching I haven't found anyone who has gone ahead and left a Dana 20 in low range.
 

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Discussion Starter · #31 ·
Thanks for the detailed analysis, Brian. Sounds like leaving the Dana 20 in low range isn't a great plan. It seems like the best path forward is either finding/building a 2.5-3:1ish doubler or just proceeding with the original plan of retaining the transmission and clutch. I've read a bit about building doublers from other transfer cases and I'll do some more investigation. I'm not sure it's worth the weight reduction if I have to spend multiple thousands on a special purpose doubler, but if there's the possibility of building something out of a common transfer case then it might be worthwhile.
 

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Discussion Starter · #32 ·
123253


Quick update: Got the 152 running well, done the shocks, brakes, shoes and put some Corbeau seats in. Decided to go the Leaf swap route. Coupler for the Leaf em57 motor is en route courtesy of user Bratitude, and I鈥檓 on the hunt for a salvage Leaf. Hopefully will have a breaking down the Leaf update soon!
 

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Discussion Starter · #33 ·
Hey folks, I鈥檇 like to get some feedback on my current plan. As I mentioned above I鈥檝e got my Scout up and running and I鈥檓 ready to buy a Leaf to cannibalize.

I鈥檓 planning to buy a whole salvage Nissan Leaf >2014, preferably with the Plus trim for the 62kWh battery. I鈥檒l use the following components from the Leaf in my build: motor, inverter, battery pack, BMS, contactor/precharge circuit, PDM (slow and fast chargers, DC-DC), with an aftermarket controller.

After some discussion with the folks from Thunderstruck and Resolve-EV I think Resolve is the better option for the controller (though I鈥檓 open to input here). Their focus seems to be more on full vehicle swaps rather than just running the motor and inverter. They support comms managing the PDM, and say it should work with the gen3 battery pack with a little additional reverse engineering.

I鈥檝e got a coupler on the way from user Bratitude. After chatting to them I think a clutchless design is going to be the way to go. My desire to retain the clutch was primarily to have a simple mechanical way to disengage the motor in an emergency (I work on software security for a living - I don鈥檛 trust software), but Bratitude pointed out that I can just pull the stick into neutral in that case. I鈥檓 not too concerned about shifting as I should be able to get close to top speed in 2nd (tho I need to do the accurate math on the gear ratios here), and it鈥檚 not expected to be a speed demon anyway. I鈥檓 sure I鈥檒l figure out clutchless shifting. Bratitude also suggested an electrical cutoff on the PWM input to the motor controller (similar to Brian鈥檚 suggestion upthread). So I think between these I鈥檓 pretty satisfied with the non-software safety aspects with a clutchless design.

The motor and transmission will be joined directly by the coupler, with an aluminium adapter plate mating the transmission bellhousing to the motor (I'll likely buy one from user: Electric Land Cruiser over here once I figure out if that will have enough metal to mate with the bellhousing on the Scout's trans, otherwise I鈥檒l have one fabbed).

The coupler has internal splines for the Leaf output shaft on one side, and blank metal on the other. My options are:
  • Find a machine shop that can mill internal splines matching the Scout鈥檚 transmission
  • Cut down a clutch disc and weld it onto the end (this seems super janky and tough to get right)
  • Find some other hub that matches the scout trans spline and attach it somehow?
For my initial proof-of-concept the whole Leaf battery pack will sit in the tray of the Scout, and I鈥檒l figure out where they can go once I get everything going. I鈥檇 like to have a rear seat, so there鈥檚 a few options for batteries:
  1. Under the hood
  2. On top of the long, flat wheel wells that span the entire length of the tray
  3. Behind the rear seat
  4. Under the rear seat
Between 2, 3 and maybe 4 I can possibly fit the whole broken down pack without needing multiple BMSes. We鈥檒l see when I get there.

Concerns, challenges and opens:
  1. Best way to get the Scout trans connected to the coupler
  2. Unknowns with using the 62kWh battery
  3. If I get a gen3 Leaf, the Resolve folks haven't had much experience there yet so I might be doing a bit of reverse engineering of the BMS comms. I'm comfortable with that, but it's something that's going to add some time. From chatting with Isak I think I'll probably have some support getting it to work, and I'll gladly feed back information from my own reversing. Alternatively I can focus on getting a gen2 Leaf with low miles, but it鈥檚 somewhat dependent on what I can find.
  4. I believe the Leaf inverter and PDM require liquid cooling
Any feedback? I鈥檓 particularly interested in any huge flaws in my reasoning, or big things I might have missed that I need to make a decision on before I commit to this plan, and definitely experienced input on the best way to approach the coupler situation -- bladesmithing experience has not prepared me for designing things involving real torque.

The next steps for me are:
  1. Buy a leaf and strip it down
  2. Decide on and buy a controller
  3. Get the motor/inverter/batteries/basic charging running on the bench with the new controller
  4. Pull the transmission out of the Scout (the point of no return [not really, but the Scout will sit in the carport until it's drivable from here, unless I buy another transmission or something])
  5. Get the coupler machined to fit the Scout trans
  6. Buy or get an adapter plate made
  7. Get the motor and trans mated on the bench
  8. Put it all together
Then it's phase 2: polish. Stashing batteries and figuring out whether I need another BMS, getting fast charging working, instrument panel, etc. At some point in there when it makes sense I'm probably gonna have the Scout painted (frame off), clean up and POR-15 the axles, etc. I'm loath to strip it all back down once it's built, but I also don't wanna get sidetracked with general restoration stuff and focus on the EV conversion. I also don't wanna have a nicely painted and restored Scout body that I then have to cut up to fit stuff into, so we'll see. There's some other stuff I wanna do like disc brake conversion, maybe improve the steering, etc. But that can probably come later. Anyway, lmk what you reckon! Thanks, as always for the collective wisdom!
 

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I think one big line item should be figuring out how to accommodate the battery. 62kWh is not a joke, and this vehicle doesn't seem to offer a lot of room without having to break the thing down and rewire. Maybe do some CAD modelling first. You called it out as an "unknown", but it should be specifically itemized in terms of questions/problems to solve.
 

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Good advice -- I'll do some more research and check the dimensions before I go with the 62kWh battery.
Gen2 battery is significantly smaller, but I don't think it will get you to the desired range with the boxy aerodynamics of this vehicle. Leaf itself can barely make the range. Though of course I don't think you specifically stated at what speeds you're planning to commute for that distance.
 

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Discussion Starter · #37 ·
Gen2 battery is significantly smaller, but I don't think it will get you to the desired range with the boxy aerodynamics of this vehicle. Leaf itself can barely make the range. Though of course I don't think you specifically stated at what speeds you're planning to commute for that distance.
Yeah I didn't really factor the increased pack size in. The 24kWh pack will (just) fit unmodified in the back, but I'll have to find some dimensions on the 62kWh one.
 

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Discussion Starter · #38 ·
Re: the range, 55-65mph top speed but it's hilly so probably not gonna get that range. I can charge at both ends. Honestly I'm less concerned now about commuting in it - it'll probably be a fun local runabout in the end. I forgot how rough old trucks are 馃槀
 
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