[brian_]

I'm attempting something very similar with a Bronco, but I'm removing the engine, transmission, and transfer case and replacing them with a Tesla large drive unit mounted at an angle to drive the front and rear axles....

The angled shaft made necessary by the Broco's centred rear propeller shaft (and the off-to-the-left drive unit output) is a concern, but I recently realized that the Lada Niva has an offset like that in standard form (due to their choice of transfer case and their centred rear axle pinion) so it apparently can work. In this case, even if the offset is okay, the angle is still a concern: if the rear output of the drive unit is not parallel to the pinion shaft of either axle, then the propeller shafts need constant velocity joints (instead of the usual universal joints) to run smoothly... perhaps that's the plan.

The other way to handle this offset issue is to have a rear axle made up with the differential off-centre to match the front. This is the stock configuration of some vehicles (notably the original Land Rover), but is generally not used in vehicles that come in both RWD and 4WD versions. It would require custom-length axle tubes and axle shafts, with the tubes welded into the centre section and end housings welded on to them. Custom-width axle housings are commonly made for custom vehicles, and having different left and right lengths is not a problem. This would be expensive, and would require ensuring that there is room for the rear shaft in that position.

 

[brian_]

So if I used the components from a Leaf I'd have to use the battery setup in pretty much its exact existing configuration if I wanted to use the Leaf BMS right?

If by "existing configuration" you mean 96 sets of cells in series, then likely yes. On the other hand, other than the length of wiring between BMS components and battery modules, the physical arrangement how the modules are stacked up and packaged) doesn't matter to the BMS.

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

Gen2 battery is significantly smaller...

What do you mean by "Gen2"? If you mean the 40 kWh packs... yes, like the 24 kWh and 30 kWh packs, they are smaller in physical volume and weight than the 62 kWh pack. The 24 kWh, 30 kWh, and 40 kWh packs and modules are all the same size and about the same weight.

Looks like the new packs are actually about the same size. Slightly taller, some lumps and bumps (e.g. BMS on top) and a lot less free space inside the case.

Yes, the 62 kWh pack is basically the same on top (fits against the same vehicle floor), but the case is deeper. The closer packaging and deeper case accommodate the almost 50% increase in module volume resulting from 50% more cells of essentially the same size.

The other thing is weight. I think Gen 2 was about 440lb, Gen 3 62kWh is 900lb.

There shouldn't be nearly that much difference. The 62 kWh pack has 50% more cells of essentially the same size and weight each. The current (40 kWh and 62 kWh) packs use essentially (or maybe exactly) the same cells, so 50% more of them means 50% more capacity. The weight increase should be less than 50%, because the 62 kWh pack's modules are more efficiently packaged (they don't have end covers for every eight cells).

For the purpose of minimizing weight for the same energy storage, modules from either the 40 kWh or 62 kWh packs should be used (because they have the same cells, each containing much more energy than the older cells).

This project used, and showed in detail, both the older (24 kWh, 30 kWh, 40 kWh) and newer (62 kWh) styles of modules:
Toyota 4Runner 4x4 Race truck Leaf conversion

 

[brian_]

I was comparing 24kWh to 62kWh weight wise.

The 24 kWh, 30 kWh, and 40 kWh should all weigh roughly the same, because they have the same cell volume of similar materials (about 914 g/cell). Apparently the cells of the 40 kWh pack are a bit thicker or denser, but packaging changes made the later modules slightly lighter than the cells would suggest; Marklines reports:

24 kWh pack: 225 kg (a bit more than 440 lb) {3.8 kg per module, as per GreenTecAuto}​

40 kWh pack: 303 kg {8.7 kg per bonded pair of modules, or 4.35 kg each}​

This is more difference between the 24 kWh and 40 kWh packs than I expected.
With the 40 kWh pack at 303 kg and 50% more of the same cells in lighter packaging, the 62 kWh should be less than 450 kg (less than 900 lb) - PushEVs estimated 410 kg.

 

[brian_]

Also note that among the DIYers the "gen X" terminology refers to the battery module design. So like sardine cans is gen1, then the symmetrical design of the same capacity (newer) is gen2, and the final one is gen3. Not a reference to the vehicle itself.

Or the single modules in cans are gen1, the doubles are gen2, and the 62 kWh type are gen3. And what would you call the 62 kWh, since they don't replace the 40 kWh? Since Nissan doesn't define the generations, the generation naming is unclear. Of course, if Nissan has authoritatively defined them somewhere, I'll happily follow and refer to that.