# AWD Caravan conversion to RWD Electric



## frodus (Apr 12, 2008)

first, what are your range and speed requirements?

Budget?


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## HyperactiveSloth (Jun 19, 2011)

In order to avoid excessive weight (not to mention cost) that completely eats all the efficiency, a short range of say, 30 miles (babysitter and back) should be sufficient. For now the Escort will be used for all the long-range driving where packing the whole family is not required. As for speed, it must be highway capable so 60-70mph is required.

Honestly I'd like to able to complete this project for around 8k, but I'd rather extend the budget if necessary than make too many compromises.


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## frodus (Apr 12, 2008)

what is the fully loaded (everyone in the family) weight? 

I have doubts about this vehicle being that great for carting around people.... but 30 miles is doable, I just think it'l be expensive. You'll be using much more Wh/mile than a car, so something more like a truck, maybe more.

60-70 might be the harder part, since you're direct drive. Usually people have transmissions of some sort. Maybe you could put the transmission of something else in there, and attach the shaft of that to the rear gearbox.

8k is gonna be hard....


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## DavidDymaxion (Dec 1, 2008)

Vehicles that are primarily front wheel drive usually have the AWD just tacked on. It is generally designed only to help out briefly if the front wheels start spinning. They'll have a woefully tiny, wimpy rear differential that I doubt would be up to full loads all the time. You are also then likely to put more stress through the rear suspension than it was designed for.

I think you should look for other options:


Figure out how to use the auto trans
Do an older station wagon, with a real rear differential designed for the power and torque (better aero this way, too)
Do a truck based SUV or van, can handle the weight and has a tough rear axle (bad aero will drive up the cost, though)
Adapt a tough rear end from another car (tough job!)
The successful direct drives generally use 2 motors or one huge motor, stout batteries, and a huge controller. The gearing down a tranny allows lets you get away with one average motor and controller, and still take off fast enough and climb steep hills. If you are trying to save money keeping the tranny helps this goal.

A final thought: You'll generally want to gear down (higher numerically) the rear diff for direct drive. That's easy to do with a Ford 9 inch, going to be tougher with anything else.

Chrysler did make an electric Van for a while, as did GM. Your best bet is probably to find one of these used, and put in a new battery pack.

If you are rich, the Tesla Model S has an option to seat 7 people.


HyperactiveSloth said:


> Hello all! I am still in the planning stages of a build that I have mentioned in another topic. The plan is to use an AWD Caravan (or similar) as a base for building an EV RWD Minivan for use as a family hauler. Here's why...
> 
> The current family Escort Wagon is soon to be insufficient to haul said family. A Minivan is pretty much the only option for driving the complete blend family (her kids + my kids). Looking at what is available on the market in good enough shape to be worth converting, common enough to not be a collector's item, and have better aerodynamics and density than your average brick... there are few options, and all of them are Automatics (AT). Without a Manual, the other option in my mind is to go without a tranny and drive through the Diff directly using it as a single speed gearbox (appropriately ratio'd). Problem is that there are no minivans with a front Diff, they are all built into the AT Transaxle. Building an entirely new custom front end was just slightly beyond what I had hoped for as far as complexity for this project.
> 
> ...


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## HyperactiveSloth (Jun 19, 2011)

OK, then, looks like the worst option is now the best... try to find one of those factory-optioned oddities: a Manual Caravan.

I was about to ask about the cost/availability of a transmission that would fit into the space allotted (not much) and as light as possible, having a vague recollection of some discussion about 2 speed Powerglides, and a couple of purpose built electric 2-3 speeds out there, but if the drivetrain won't be able to handle it without more than a replacement Diff then it's probably not worth it.


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## Duncan (Dec 8, 2008)

_Vehicles that are primarily front wheel drive usually have the AWD just tacked on. It is generally designed only to help out briefly if the front wheels start spinning. They'll have a woefully tiny, wimpy rear differential that I doubt would be up to full loads all the time. You are also then likely to put more stress through the rear suspension than it was designed for._

I disagree, the only ones I have looked at have been pretty beefy - Subaru, Audi 

4WD tends to be offered on the top end vehicles with the biggest engine packages and their back ends need to be strong

You will probably (almost certainly) need a gearbox - look for something with rear wheel drive and a fore-aft engine 
Something like an old Ford would do - and is actually quite small
You may need to bash the floor a little to obtain clearance

A Caravan is a huge vehicle - aren't there any smaller 7 seaters available?


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## DavidDymaxion (Dec 1, 2008)

The Subaru and Audi are designed for a 50/50 power split, so you are right their rear diffs are likely tougher, but I'm sure most others are really meant just for getting you unstuck, not really full time high torque loads. Unlike the Sub and Audi, most don't get any real torque until the front wheels are spinning faster than the rear wheels -- a rare condition. Also, look at the diff on a RWD car vs. a primarily FWD with AWD tacked on -- the diff is physically much larger.

Hyperactivesloth: Finding an old van with a manual tranny is another trick you could do, that's an idea. Another idea is to see if any Audi wagons had rear jump seats -- you could get most Audis with manual trannies. Another thought is the Chevy Astro Van -- maybe you could swap a manual into one, and if you went direct drive they were RWD and would have a rear axle designed to move the vehicle full time.


Duncan said:


> _Vehicles that are primarily front wheel drive usually have the AWD just tacked on. It is generally designed only to help out briefly if the front wheels start spinning. They'll have a woefully tiny, wimpy rear differential that I doubt would be up to full loads all the time. You are also then likely to put more stress through the rear suspension than it was designed for._
> 
> I disagree, the only ones I have looked at have been pretty beefy - Subaru, Audi
> 
> ...


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## HyperactiveSloth (Jun 19, 2011)

Well, while at work today I did some checking on the components of the Caravan AWD's rear end and didn't find much. I checked the bearings, which are not shabby, the suspension components were not even listed in the aftermarket catalogs, and a Getrag 640 rear diff. The diff is rated at 700nm according the to manufacturer. God I wish I could think in metric. lol I did learn the AWD rear end has HUGEly upgraded brakes and such.

Hmmm, brain tired right now.... will do the math later and get a fresh view of all this. Learned a hell of a lot about the AWD drivetrain of these things, some encouraging, others bring up more questions. (has visions of viscous couplings, overunning clutches and synchronize dog clutches dancing in my head)


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## DavidDymaxion (Dec 1, 2008)

Getting an actual spec, great idea! 700 newton meters is about 500 foot pounds.

Remember, if you use a tranny that multiplies the torque, so take your motor torque and multiply that by the tranny gear ratios to see how much torque you'd be feeding the rear end.

The other thing you'll want to do is see if 500 ft*lb would be enough force for adequate acceleration. Here's a rough calculation:

Figure a 3:1 rear end ratio.

500 ft*lbs into the diff would give 1500 ft*lbs twisting the real axle. Suppose the rear wheels have a radius of 1 foot, that's 1500 lbs of force pushing the van forward. If the van weighed 4000 lbs, that's 0.38 g of acceleration (actually not too bad), and a max hill of 38% (again, not bad) (that's about like a very powerful car in 4th gear). Of course, this presumes your electrical system can dish out 500 ft*lb of torque. Now, if you can get a 6:1 Ford 9 inch under there, now it's 0.76 g, which would be quite impressive.

BTW my gasser Camaro puts about twice that much torque into the rear end. It's not too hard to get more torque with an electric than a gas motor, especially with a tranny multiplying torque, so you'll have to show some caution with the Caravan rear end. Also, the heavier the vehicle the more it stresses the rear end.


HyperactiveSloth said:


> Well, while at work today I did some checking on the components of the Caravan AWD's rear end and didn't find much. I checked the bearings, which are not shabby, the suspension components were not even listed in the aftermarket catalogs, and a Getrag 640 rear diff. The diff is rated at 700nm according the to manufacturer. God I wish I could think in metric. lol I did learn the AWD rear end has HUGEly upgraded brakes and such.
> 
> Hmmm, brain tired right now.... will do the math later and get a fresh view of all this. Learned a hell of a lot about the AWD drivetrain of these things, some encouraging, others bring up more questions. (has visions of viscous couplings, overunning clutches and synchronize dog clutches dancing in my head)


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## HyperactiveSloth (Jun 19, 2011)

The stock diff is 3.455, so with some sort of simple 2-speed it would be completely workable, and with the stout stub axles and truck type bearings it uses I may just leave the diff in instead of swapping... at least until/if it has a problem.

My new speed bump is the mid-drivetrain itself. Being AWD it incorporates a viscous coupling which would have to be removed, and a setup of over-running/dog clutches that would serve no purpose in this design... so I am now considering moving the motor to the rear and eliminating yet another chunk of unnecessary weight called the driveshaft. Now, I really need to actually get a hold of an AWD Caravan or some photos/breakdowns from someone who's dealt with them and decide where there would be good room and support for the motor. I may end up going beetle style with the rear placing the motor/trans behind the axle (have to check if the diff can be flipped or not, crap, there's another problem) and moving the batteries to the "engine" bay. Hmmm, maybe I could 180 the diff instead of flipping it due to the mounting requirements.... but then the diff would be running in reverse direction. Good or bad.... have to check.

Well, off to work with me, gonna be doing a lot more research when I get home. Time to sell some auto parts. (I work at a parts store)


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