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Discussion Starter · #1 ·
First post, nice to meet ya'll! I have a 1995 Geo tracker (5spd RWD) with an engine that needs a rebuild, so I am seriously considering an EV conversion instead; based on a small rear Tesla drive unit. ~300HP in a <2500lb light truck chassis sure seems like a good time, doesn't it??

Planning to use a 40-50kwh tesla-module battery pack (this is a weekend romper car so long range is not critical. ~180miles would be more than enough.)

This will be my first EV conversion, but I have worked on cars before and have a broad engineering background with several "intense" projects under my belt, including HV solar battery systems. I have the resources and motivation to successfully execute this project.

Right now, I am in the research and planning phase, and I would love to be pointed in the right direction regarding light truck conversions. Specifically, I'd like to know the best practices regarding RWD fixed axle retrofits. As I see it, there are a few options:
  • Keep existing rear axle/diff, mount tesla drive unit transversely to the existing driveshaft
  • Dispose of the existing rear axle and driveshaft, convert to an independent rear suspension. Find or fabricate suitable halfshafts of the correct length.
  • Dispose of the existing rear axle and driveshaft, try to retrofit a complete tesla rear sub-assembly (including suspension, brakes, and hubs)

Benefits of keeping the motor at the rear are:
  1. it's a truck frame, so there's plenty of space in the chassis to work with
  2. frees up engine bay for battery modules & cooling
  3. less modification of the drive unit itself since don't need to worry about differential gearing
  4. fewer drivetrain losses
  5. opporunity to improve the ride quality (existing rear suspension is very stiff and harsh)

What do y'all think? Any prior examples of a light truck being converted to rear suspension to support a rear motor mount?

Thanks,
and I look forward to the journey!
 

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Lots of light trucks have been converted, but they're almost all pickup trucks so they have much more room available for battery than in a Tracker. That's going to be a challenge.

  • Keep existing rear axle/diff, mount tesla drive unit transversely to the existing driveshaft
  • ...
I assume that you mean longitudinally - the orientation is specified according to the motor shaft, which is transverse is a stock Tesla (and almost every other EV), but the drive unit would need to be rotated 90 degrees to longitudinal to connect an axle output to the existing propeller shaft (driveshaft).

There have been a few forum discussions of this possibility, with two issues:
  1. the drive unit would be off-centre, so it won't fit in the transmission tunnel, and
  2. the reduction ratio of the Tesla gearbox multiplied by the rear axle's final drive ratio is way too much.

Any prior examples of a light truck being converted to rear suspension to support a rear motor mount?
Yes and no. The Ford Ranger EV had a rear-mounted motor, but it used a de Dion suspension, not independent. There have been many conversions of pickup trucks to independent rear suspension, but I've never heard of one with an electric drive unit.

The frame makes an IRS conversion easier than tying into a unibody, but this is still a substantial project. With an electric drive unit packaging is difficult, because the drive unit is larger than a typical final drive (diff in housing). Specifically for the Tesla Model S/X drive unit, the bulk of the motor and inverter are behind the axle line, and that's usually a structural crossmember and control arms in modern IRS designs so fit it a problem - most projects putting a Tesla drive unit in the rear of a converted vehicle have simple semi-trailing arm suspensions, although it has been done with other designs including Corvette (C5).

One approach is to use the entire suspension and subframe from the Tesla, not just the drive unit. The Tracker is too narrow for this to be practical.
 

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Discussion Starter · #3 ·
Thanks Brian,
perhaps a Tesla drive unit is not the way to go here. It seems more straightforward to get a stand-alone motor such as the siemens: https://www.evwest.com/catalog/prod...ucts_id=257&osCsid=2s331brvnandcakoi4nb77lcc6

Once you get a suitable stand-alone controller, the cost is about the same.

The tracker has a rear diff ratio of 5:1, so that seems like a great match to the siemens motor, which would sustain ~80mph speeds at around 5500rpm on stock 25" diameter tires.

I could mount this motor in place of the existing trans and then determine a way to spline the motor shaft into the driveshaft, likely by welding on an adapter coupling to the existing shaft.

I think the batteries will not be too hard but I will do some measurements. There is a lot of space between the frame rails, so filling that in plus a few in the engine bay should be do-able.

Currently gravitating toward nissan leaf pouch cells, since I can get to the required voltage with less capacity than with tesla modules. 48 cells in series would give a 365V nominal pack at 24kwh capacity and a cell weight of ~340lbs, which seems fine to start with. If I can find room in the future, it could be doubled to 48kwh.

Some quick measurements:
- 24 cells will fit between the frame rails in the rear half of the vehicle on either side of the driveshaft.
- 20 cells will fit where the existing gas tank is, below the tailgate (might be able to squeeze in 22 or 24 with further thought)
That basically gets me to the 48 needed for the V1 pack. Since it's all in the back half of the vehicle, what I may do instead is relocate the 24 cells between the frame rails into the engine bay, so that there is equal loading at the front and rear of the vehicle. I can utilize the under-frame space in the future if I want to expend the pack, but for V1, looks like splitting the pack between the engine bay and the fuel tank area will be a good start.

Thanks!
 

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The tracker has a rear diff ratio of 5:1, so that seems like a great match to the siemens motor, which would sustain ~80mph speeds at around 5500rpm on stock 25" diameter tires.
Wow, that's extreme. With a bit of searching, it looks like while some are not that short, a few are even more extreme:
4.30X:1 Non overdrive 2 dr auto 1.6L 16v
4.625:1 2dr auto 1.6L 8v
4.88X:1 Listed elsewhere in the Vitara section
5.125:1 Most all 4dr's (Non Vitara) Most all 5-speed Track Kick's Most all Autos with overdrive
5.38X:1 Early model 1.6L 8v 4dr 5-spd Track Kick's (RARE)
5.63X:1 The early 1988-1989 1.3L powered Track Kick (RARE)
(this is from a forum post, and uses "Track Kick" to describe Geo Trackers and Suzuki Sidekicks)

I agree that this makes the vehicle a better candidate than most for using the final drive (ring and pinion gears) as the only gear reduction.
 

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Currently gravitating toward nissan leaf pouch cells, since I can get to the required voltage with less capacity than with tesla modules. 48 cells in series would give a 365V nominal pack at 24kwh capacity and a cell weight of ~340lbs, which seems fine to start with.
Given the nominal capacity for 48 of these, it looks like you are talking about Leaf modules, not individual cells (which is good, because in most cases it doesn't make much sense to break open these modules).

The modules are 2S 2P, so they have a stack of four pouch cells in them. Original Leaf modules are 303 x 223 x 35 mm (11.93 x 8.78 x 1.38 in) and weigh about 3.8 kg (8.4 lb); later modules (other than the new Leaf Plus/e+) are at least approximately the same size, but have more capacity and may vary in weight.
 

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Given that "cells" should be "modules"...
- 20 cells will fit where the existing gas tank is, below the tailgate (might be able to squeeze in 22 or 24 with further thought)
Isn't the Tracker fuel tank only 11 US gallons? That's 42 litres, and a single Leaf module occupies about 2.4 litres, so that's only 17 modules without even accounting mounting structure, enclosure, and space for wiring... is there that much extra space around the fuel tank, or do I have a number wrong somewhere?
 

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... perhaps a Tesla drive unit is not the way to go here. It seems more straightforward to get a stand-alone motor such as the siemens: https://www.evwest.com/catalog/prod...ucts_id=257&osCsid=2s331brvnandcakoi4nb77lcc6

Once you get a suitable stand-alone controller, the cost is about the same.

The tracker has a rear diff ratio of 5:1, so that seems like a great match to the siemens motor, which would sustain ~80mph speeds at around 5500rpm on stock 25" diameter tires
That makes some sense to me. This gives you a liquid-cooled induction motor in a good case and a controller/inverter which you can manage, instead of taking the Tesla drive unit apart to get a liquid-cooled induction motor without a proper case and a controller/inverter designed to work only in a Tesla.

Even with the short final drive ratio, you would be using only the lower half of the motor's operating speed range, so it would be limited to constant torque (rather than power) all the way up to 60 mph. Applications of this motor tend to include an additional stage of reduction gearing, either in the two-stage transaxle, or using an add-on gearbox such as the ev-TorqueBox before the final drive.
 

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Discussion Starter · #8 · (Edited)
Thanks for your replies!

Isn't the Tracker fuel tank only 11 US gallons? That's 42 litres, and a single Leaf module occupies about 2.4 litres, so that's only 17 modules without even accounting mounting structure, enclosure, and space for wiring... is there that much extra space around the fuel tank, or do I have a number wrong somewhere?
The fuel tank is sloped on all sides, so there is more space if you imagine a solid rectangle of the same footprint. The fuel tank is also not the full width of the available space - there are about 5" extra on each side which is what I would consider using to squeeze the extra couple modules there. The tank area is about 15" front to back, so I would stack the modules 5 high into signatures with the 12" dimension of the battery aligned to the 15" dimension of the cavity, with the remaining few inches reserved for wiring (i'll likely fabricate some bus bars for this purpose). Five signatures across the width for a total of 25 modules, like this:



Even with the short final drive ratio, you would be using only the lower half of the motor's operating speed range, so it would be limited to constant torque (rather than power) all the way up to 60 mph.
What does this mean, practically?
 

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Even with the short final drive ratio, you would be using only the lower half of the motor's operating speed range, so it would be limited to constant torque (rather than power) all the way up to 60 mph.
What does this mean, practically?
Consider the situation in a conventional car with a manual transmission when you know the car has more power, but you need to shift down a gear to get the engine up to the speed where it can produce it. With the tall gearing, this electric motor would be in that situation all of the time, up to 60 mph. That's normal for low speed in a single-ratio electric drive, but only up to a low enough speed that it isn't an issue.

Applications of this motor tend to include an additional stage of reduction gearing, either in the two-stage transaxle, or using an add-on gearbox such as the ev-TorqueBox before the final drive.
This extra reduction gearing gives you that downshift effect, all of the time. Because it is always there you can't overdo it, or your top speed is limited by how fast the motor can turn.
 

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Discussion Starter · #10 ·
The model S P85 actually has a very similar power curve; peak power is achieved only at 50mph. I'll look into this a bit further and determine if the added expense of an additional 2:1 gearbox would be worth it.
 

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Have you considered keeping your transmission? It solves all of your gearing, torque, and top speed concerns at the expense of some weight, space, and complexity. It would also let you use much cheaper motors that can't rev as high as the Tesla, and installation difficulty then becomes mating the motor and output shaft to the transmission, and figuring out where to put everything else.
 

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Discussion Starter · #12 ·
Have you considered keeping your transmission?
Perhaps, but there is a weight, efficiency, and space penalty to doing so. I think the siemens motor with the existing 5.13:1 final drive diff will likely be suitable on its own, given that the power/torque vs speed curve is quite similar to the model S 85.
 

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Discussion Starter · #13 ·
Current thinking:

Motor: Remy HVH 250-90-SOM
Connection: Direct to driveshaft with final drive of 5.13:1
Operating voltage: 600V
Battery pack: 80x Nissan leaf modules in series, 40kwh 608V
Cell weight: 560lbs
Desired range: 120mi (333wh/mi)
Peak motor current: 300A
Peak motor power: ~150kw @4800rpm (70mph)
Peak torque: 330nm from 0-4400rpm
Continuous power: 115KW between 4500 and 6000rpm (65-87mph)
Drive Inverter: Scott Drive SD300
Inverter maximums: 900V, 600A
Inverter continuous rating: 800V, 450A
 
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