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I was trying to keep the Make and Model under the radar, but it will be in a classic mid-70's GMC Motorhome.
Nice! :) For those who are not familiar with this classic, just do a web search for "GMC Motorhome".

That's a substantial vehicle. Packaging is a challenge, because it has a low floor which is great for getting in and out, and for overall height, but doesn't leave much space under the floor. There is the big engine space in front, of course, and with front wheel drive it does make sense to pile the weight up there.

Driving more than the front wheels would be problematic, because
  • the rear suspension and hubs are not designed to be driven,
  • the rear floor is (deliberately) low so it would probably be impractical to mount drive motors inboard,
  • there are tandem rear axles (that's a lot of motors to drive all of the wheels), and
  • the space between the frame rails at the leading rear axle is filled with a tank.
I suppose that if you're okay with radial mechanical changes, you could set up inverted portal drive axles at the rear.

Aerodynamically it is better than a typical class C, and roughly comparable to a Class B or other Class A's. Of course it isn't as good as a modern car, and has over double the frontal area of even a large car, so it's going to use perhaps two to three times the power (for a given highway speed) as large electric car.

It certainly is hard to find an alternative transmission for this unusual configuration, or at least one which is suitable for the 12,000 pound GVWR. That alone may justify, in a way, a series hybrid configuration, but on the other hand finding a suitable electric motor and transaxle or pair of motors with gearboxes (for separate left and right wheel drives) is a challenge, too. If you can find the right electric drive, and squeeze the battery in somewhere further back, there should be space for the engine+generator set ahead of the front axle.
 

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Discussion Starter · #23 ·
Nice! :) For those who are not familiar with this classic, just do a web search for "GMC Motorhome".

That's a substantial vehicle. Packaging is a challenge, because it has a low floor which is great for getting in and out, and for overall height, but doesn't leave much space under the floor. There is the big engine space in front, of course, and with front wheel drive it does make sense to pile the weight up there.

Driving more than the front wheels would be problematic, because
  • the rear suspension and hubs are not designed to be driven,
  • the rear floor is (deliberately) low so it would probably be impractical to mount drive motors inboard,
  • there are tandem rear axles (that's a lot of motors to drive all of the wheels), and
  • the space between the frame rails at the leading rear axle is filled with a tank.
I suppose that if you're okay with radial mechanical changes, you could set up inverted portal drive axles at the rear.

Aerodynamically it is better than a typical class C, and roughly comparable to a Class B or other Class A's. Of course it isn't as good as a modern car, and has over double the frontal area of even a large car, so it's going to use perhaps two to three times the power (for a given highway speed) as large electric car.

It certainly is hard to find an alternative transmission for this unusual configuration, or at least one which is suitable for the 12,000 pound GVWR. That alone may justify, in a way, a series hybrid configuration, but on the other hand finding a suitable electric motor and transaxle or pair of motors with gearboxes (for separate left and right wheel drives) is a challenge, too. If you can find the right electric drive, and squeeze the battery in somewhere further back, there should be space for the engine+generator set ahead of the front axle.
There is a little room under the floor between the frame rails for some of the battery. The rear of the coach under the bed area typically has the onboard generator and the propane tank, both of which I would toss. That leaves more room for more battery. I was hoping to keep the motors down low either driving the differential or ideally direct drive with the range extender mounted above. Theoretically, the range extender could be relocated to the back.
 

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Discussion Starter · #24 ·
Also, I should note that I am planning to do a pretty hefty restoration, so if it is not feasible right away, I can wait until technology improves or at least gets less expensive and I 'll just enjoy it with the ICE in the mean time. Maybe a Tesla Semi will be crashed in a junkyard by then :)
 

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I was hoping to keep the motors down low either driving the differential or ideally direct drive with the range extender mounted above.
I'm not sure what you mean by "direct drive", but you certainly need a reduction drive (gearbox) between the motor and the axle shaft, so that you can use a motor of a reasonable size. A motor which turns barely over 700 rpm at 60 mph (the rotational speed of the stock size of tire) would have to be massive - you want the motor turning about ten times that fast.

(The GMC Motorhome came with 8.75R16.5 tires, which are the same height as a more modern 225/75R16.)
 

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... I can wait until technology improves or at least gets less expensive ...
Maybe a Tesla Semi will be crashed in a junkyard by then :)
You're just kidding, but the Tesla Semi is planned to use two Tesla Model 3 motors per axle. You can similarly use two Tesla or other EV motors for the front axle, if you can sort out a pair of gearboxes for them.
 

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Discussion Starter · #27 ·
You're just kidding, but the Tesla Semi is planned to use two Tesla Model 3 motors per axle. You can similarly use two Tesla or other EV motors for the front axle, if you can sort out a pair of gearboxes for them.
Well, I could possibly use two motors end to end and adapt to the original "final drive" differential.
 

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Well, I could possibly use two motors end to end and adapt to the original "final drive" differential.
End-to-end is very long, even for the relatively wide motorhome. In the case of Tesla drive units, it would help to remove the inverter from the gear case (and move it, using extended cables). The Leaf, in contrast, has the inverter on top of the motor, so it is taller but not so wide.

The most compact arrangement of two typical drive units would be to rotate one 180 degrees and shift them sideways, so that the axle lines (through the differentials) are in line with each other, but one motor behind the axle (the stock Tesla position) and one motor ahead of the axle (the stock Leaf, etc position). The likely problem is that the rotated unit would need to drive in reverse, which is no problem for the motor and no problem for the electronics, but the gears probably wouldn't lubricate properly.

In any use of drive units with a differential (used to drive both wheels of one axle with one motor) to drive just one wheel, the internal gears of the differential would need to be locked or replaced with a solid part, which is called a spool. A full spool replaces the ring gear carrier and all internal differential parts; a mini-spool just replaces the internal gears. Then the output facing the wheel gets a normal axle shaft (of whatever length is needed to make it work) and the other output is just capped off.

I'll attach an image of two Tesla large (rear) drive units (taken from the thread in this forum Tesla Large Drive Unit Dimensions). For scale, see that thread for dimensions (the overall width of one unit is 33.53 inches). Since this combination is almost certainly too wide, I'll note that the outer parts are the inverters (which look almost like the motors). In this quickly pasted image, you can see small dark stubs where one axle shaft would connect to each (locked) differential.
 

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At that point, I was thinking non-Tesla motors end to end with a driveshaft coupled to the RV's original differential. The reduction is only 3.07:1
This scheme is often called "direct drive", but it is really using only the final drive unit for reduction. With such a tall (little reduction) final drive ratio the motor would still be turning only 2200 rpm. That speed is great for an engine (and with newer transmissions even lower engine speeds are used), but it would be using only a fraction of the speed range of a typical motor... although a lot better than 700 rpm!

For comparison, while this final drive has a 3.07:1 ratio (and similar units range from just under 3:1 to over 5:1), the overall reduction ratio of a Nissan Leaf (which has smaller tires and so doesn't need as much reduction) is 7.94:1, and a typical Tesla has a 9.73:1 ratio. Overall ratios mentioned for the proposed Tesla Semi (Class 8 truck) are 15:1 and 23:1 (although with tires about one-third taller than the tires that a pickup would use).

Although the final drive of this motorhome's THM425 transaxle can be removed, I don't think it's necessary to stick with this unit if not using the rest of transaxle. Much higher ratios are available in other final drives, so if taking this general approach it would make sense to look at other units used in independent suspensions. A challenge is to find something strong enough to haul six tons. For instance, whatever is under the front of a Chev/GMC 3500 4X4 pickup is a candidate (because they're all independent), and the current standard ratio with the gas engine is 4.10:1... but you really want more reduction than that. It might be reasonable to use a GMC unit, with an aftermarket high-ratio ring and pinion gear set.

It is also possible to connect two motors side-by-side to one shaft (a final drive input), but that generally means gears (which are usually not practical for DIY) or chains or belts (which are good to avoid). For an example, the old Solectria E-10 (a commercially produced conversion of the Chevrolet S-10) had two AC motors side-by-side, driving a common shaft with a toothed belt each. That extra step of gears or chains or belts is an opportunity to get the desired overall ratio.
 

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The best RV for conversion is the Vixen which came with a naturally aspirated 6 cylinder Mercedes diesel and 30mpg stock

CD of .3
The diesel Vixens had a BMW M21 engine (not Mercedes). While this engine was available naturally aspirated - unusual for modern diesels - I think the Vixen used the turbo... the model name is even Vixen TD. ;) The turbo was needed for adequate performance - the 63 kW (84 hp) of the other version wouldn't have been enough.

Of course the original engine doesn't matter. :) The Vixen is a low-profile design (needing a pop-up top or extension to allow standing up), and of moderate size (reportedly only 5,100 pounds curb weight). It should be great for aerodynamic efficiency, although it isn't very big.

The rear engine design of the Vixen might also be easier to work with for conversion than the GMC Motorhome. The manual-transmission version used a Renault 5-speed, in a longitudinal rear-engine configuration (like an air-cooled VW).
 

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Discussion Starter · #33 ·
Well I officially own the GMC so that is the ride if I do this. It weighed in at 10,500 with mostly full tanks.

The drag coefficient of the GMC is around .31 without the AC units I believe.
 

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TM4 makes a bunch of different units that are made for busses or small commercial vehicles, but they don't sell to the general public.
Somebody here was parting out hybrid E450 chassis perhaps in the classifieds.
At this time I would suggest electric city transit bus salvage yards.
A small transit bus or anything on an E450 chassis would be roughly appropriately sized.

I think it would be worth looking at the buses in a salvage yard, if only to see what they use for a transmission and final drive. They may be on the hybrid E450 chassis.

I assume that the hybrid E-450 is the Azure Dynamics adaptation of Ford's E-450 stripped commercial chassis to become a parallel hybrid, which they branded as "Balance Hybrid Electric". Apparently AZD mounted a 280 volt 130 hp AC (induction) motor in parallel with the stock transmission's output. From the diagram and some descriptions, it that is was a motor with a double-ended shaft, forming part of the propeller shaft (driveshaft) and so running at transmission output / final drive input speed. The final drive ratio (in at least some of them) was 4.56:1.

This illustration was taken from the manual (linked below), which also provides system weights and dimensioned drawings of the component layout, as well as many photographs of the system installed on the E-450 chassis.

In the Balance E-450, the electric motor only handled propulsion by itself at low speeds; above a set speed (20 to 35 mph depending on report), or when the accelerator is pressed enough, the engine ran as well. It's unlikely that this motor could handle a six-ton motorhome by itself. About a thousand of these were built, with over half being Purolator delivery vans.

References:
 

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Discussion Starter · #36 · (Edited)
Interesting testing here. I'm starting to think that a Model X motor could pull my GMC around just fine. It can pull it's own 5000 lbs plus a 5000 lb trailer. My GMC weighs in at 10,500, and when I'm done with it, it will weight considerably less. The amount of steel frame couches, and heavy wood cabinets plus the marine plywood floor that will be upgraded to much lighter materials will take out some weight. Removing the generator will take out some weight. I'm guessing I could get it down under 9000 lbs. Without the engine and transmission, it would be closer to 8000 lbs....maybe less. The shell itself is pretty light. It is all the stuff in it that weights it down.

edit: Forgot that the Model X is AWD. Definitely need two motors.

https://insideevs.com/tesla-model-x-energy-consumption-towing-various-trailers-video/
 

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edit: Forgot that the Model X is AWD. Definitely need two motors.
That's the conclusion we reached and I'm going to test the towing capabilities of my VW Bus Conversion to give us some benchmarks based on a single 'small' drive unit :cool:

Here's another cool Model X towing video :)

 

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Are Model X motors any different from Model S motors? I assume that the S and X are just the sedan and tall wagon (or "SUV") body variants of the same car, with the same drive units and batteries. My guess is that towing is discussed more for the Model X simply because that's an expected capability of an "SUV".
 
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