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Discussion Starter #1
Good morning new friends. I've been kicking this idea around for awhile and now I'm here to ask the experts if my idea is feasible. Keep in mind, I'm a decent shade tree mechanic, have built a couple engines and even an automatic transmission but still consider myself pretty green in the overall scheme. I have ZERO knowledge of electric motors and electricity in general but I'm willing to learn. Here's the skinny-


I have a 1981 VW Rabbit 1.6 n/a diesel I've been driving for the past few years. The whole time I've been using a 50/50 blend of diesel and WVO. I average 45 mpg. I have about a 20 mile trip to town, 90% highway.


My plan isn't to totally convert to electric but rather make a hybrid. I would use the diesel engine to drive to town, then use electric in town. I would be happy with a top speed of 40mph but faster is always better.


Obviously the Rabbit is front wheel drive, the rear wheels are just back there, doing nothing, just rolling. My idea is to install an electric motor to drive one or both, OR a pair of motors to drive both wheels, depending on your guy's recommendations. Seems to me a (insert correct term here) motor, with a shaft going all the way through it, mounted in the center rear of the Rabbit with a couple of cv shafts going to each wheel would be the cleanest install.


The car is a real piece of crap and I have a parts car as well so I'm not too worried about cutting stuff up or whether or not I'll ruin it in the process!


I'll admit, I have done very little research, but it appears with the right motor selection I probably won't need a transmission, especially with my goal of 40mph. I was also planning to use 4-6 12V batteries to power the motor.


A couple things I'm worried about-


With the motor(s) connected, what will I see when I'm using the diesel engine going down the highway? Will it create a major drag? Will I over-speed the motor(s)? If so, I can always create a way to disconnect the cv shafts if I have to, not that I want to.


Actually I guess that's all I'm worried about right now! :D


I have manual steering, manual.. everything really so I'm not worried about driving around in town with the engine not running, transmission in neutral.


Go easy on me, but please dissect my plan and give me some pointers. If it sounds like it may work, this will probably be my build thread.


Thanks guys.
 

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My plan isn't to totally convert to electric but rather make a hybrid. I would use the diesel engine to drive to town, then use electric in town. I would be happy with a top speed of 40mph but faster is always better.


Obviously the Rabbit is front wheel drive, the rear wheels are just back there, doing nothing, just rolling. My idea is to install an electric motor to drive one or both, OR a pair of motors to drive both wheels, depending on your guy's recommendations. Seems to me a (insert correct term here) motor, with a shaft going all the way through it, mounted in the center rear of the Rabbit with a couple of cv shafts going to each wheel would be the cleanest install.
This configuration is frequently proposed in this forum, although of course you're unlikely to stumble across the discussions so you wouldn't be aware of them. I just spent a few minutes looking for them, and haven't found one yet.

This scheme has lots of problems, and no one actually completes a project with this design. For a start, do you understand how to add a drive axle to a hub which isn't normally driven? After you install the rear motor and reduction gearing and motor controller and batteries, do you still have space for the fuel tank?

On a more fundamental level of what this hybrid is supposed to be, how does the battery get charged? Is it just a very heavy way to get an extremely short electric-only range with energy that you get by plugging it into a charger while parked?
 

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Here's a previous discussion, one that I recall from my participation in it:
Add EV motor to drive rear wheels???


I did a Google search for suitable threads in this forum, and found some older stuff which I had not seen before, plus this one which I had forgotten about (it's at the opposite end of the car, but otherwise similar):
Planning 2000 BMW 328i to hybrid conversion

Here's an old discussion, which I haven't even read completely, but not surprisingly went nowhere:
FWD car & engine with RWD diff + elec. motor = DIY Plug in hybrid?

This one actually looked mechanically feasible, but still didn't go anywhere:
DIY diesel-electric hybrid AWD

This one is mostly general musing, with no real plan:
Homebrew conversion to hybrid?

I'm sure there are more, but that was enough search time for me.
 

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Discussion Starter #5 (Edited)
[/QUOTE=brian_;1022523]
This configuration is frequently proposed in this forum, although of course you're unlikely to stumble across the discussions so you wouldn't be aware of them. I just spent a few minutes looking for them, and haven't found one yet.

This scheme has lots of problems, and no one actually completes a project with this design. For a start, do you understand how to add a drive axle to a hub which isn't normally driven? After you install the rear motor and reduction gearing and motor controller and batteries, do you still have space for the fuel tank?

On a more fundamental level of what this hybrid is supposed to be, how does the battery get charged? Is it just a very heavy way to get an extremely short electric-only range with energy that you get by plugging it into a charger while parked?[/QUOTE]

I do realize I'll have to make the hubs drive hubs, was going to look at the front hubs to see about adapting, if possible. My plan was to try to fit the motor in the spare tire compartment, batteries on top/side of the motor. Haven't looked at where the fuel tank is... My plan was to charge the batteries with an outlet, at least at first but I'm going to assume you're right about the short range.

Thank you for the links, I have some reading to do and you told me exactly what I wanted to know, doesn't sound... practical. Thank you very much for the insight.
 

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I do realize I'll have to make the hubs drive hubs, was going to look at the front hubs to see about adapting, if possible.
The chances that the front hubs use the same bearing configuration and the same size bearings as the rear is nearly zero, so that's unlikely to work.

In some front wheel drive vehicles there is an AWD variant (as discussed in the CRX thread), and so driven hubs from the AWD might possibly be adapted; however, I don't think there was ever an AWD version of that first-generation Golf/Rabbit, or any other VW/Audi vehicle using the same rear suspension with RWD or AWD.
 

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Kelly has marketed these car hubs for along time: http://kellycontroller.com/car-hub-motor-72v-7kw-p-711.html . Although, I've never heard of anyone using them. They are sized for 15" wheels from a Buick - a very popular brand in China. With ~14kW of power for a pair of them and 1300 RPM they might be good for 35-40 MPH city driving if you played with the wheel sizes. They look like they would be easy to adapt to a simple beam rear axle with welded on tubes with keys and keyways to counter the reaction torque.

A pair of these hub motors with their controllers cost ~$2500 plus shipping. With the cost of a battery and the time and expense of installing everything, you could buy a lot of diesel for your 50MPG VW. Your fuel economy in the ICE mode would suffer somewhat from the slight amount of extra drag from the motors and the extra weight being dragged around. Also, you should check the safe upper RPM limit of the motors when the car is being driven at freeway speeds in ICE mode.

This hybrid stuff is not so easy as you might initially think.


CORRECTION: See Brian's correction, in the next post
 

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Kelly has marketed these car hubs for along time: http://kellycontroller.com/car-hub-motor-72v-7kw-p-711.html . Although, I've never heard of anyone using them. They are sized for 15" wheels from a Buick - a very popular brand in China. With ~14kW of power for a pair of them and 1300 RPM they might be good for 35-40 MPH city driving if you played with the wheel sizes. They look like they would be easy to adapt to a simple beam rear axle with welded on tubes with keys and keyways to counter the reaction torque.
It wouldn't be necessary to convert the car to a beam axle, since the VW trailing arms could be modified... by someone who is up to designing and building critical suspension components for a car.

1300 RPM wheel speed with Rabbit-sized tires corresponds to 136 km/h (85 mph), which is about as fast as the Rabbit can run on its gas engine.

So speed isn't an issue, but
  • what is the load capacity of this thing, which replaces the original hub and hearing?
  • how would the scooter-sized brake disk be replaced with a real car's disk?
As for performance, the data published by Kelly is from a crude friction brake device, and only covers momentary load above the peak torque point, which is 1108 rpm or 116 km/h (72 mph). At the speeds at which the car will actually be driven:
  • torque will the no more than than the listed 78 Nm (56 lb-ft)
  • power will be less than the listed 12.2 hp (9 kW), and reduced in proportion to speed so that at (for instance) 30 mph it will be less than 3.7 kW
  • efficiency will be no more than the listed 82%
Hub motors seem like a great idea, until real numbers are examined. There are reasons that no one uses them in cars!
 

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Thanks for the correction, Brian. In writing my late night post, when I was tired, I mixed up the formulas for calculating a circle's circumference and its area to figure the hub motor RPM.

Here's a recent video of someone using slightly more powerful hub motors with maybe better torque characteristics for a school project: https://www.youtube.com/watch?v=ospqKPp4rbo
Still doesn't look very practical. But, it might have some limited applications.
 

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Discussion Starter #10
Thanks you guys. While this project doesn't look like it will go much further, I am going to hang around for awhile and maybe learn some stuff! :D
 

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Discussion Starter #11
Okay don't take me seriously, BUT! Looking at something like a Chevette rear end to fit under the Rabbit. Might take quite a bit of fabrication but it would take care of the biggest issue.. Has the same bolt pattern too! Just kicking the idea around still, no real plans.
 

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Okay don't take me seriously, BUT! Looking at something like a Chevette rear end to fit under the Rabbit. Might take quite a bit of fabrication but it would take care of the biggest issue.. Has the same bolt pattern too! Just kicking the idea around still, no real plans.
With this suspension, the motor would need to be in the conventional location of a transmission or further back (roughly between the front seats); the Rabbit doesn't even have a tunnel in the floor there. The Chevette rear suspension is better in some ways than a basic leaf spring suspension for a beam axle, but it doesn't package well with anything else under the floor. I guess the best feature might be that the springs might end up in the same place as with the original Rabbit rear suspension.

Then there's getting this stuff - the Chevette went out of production 30 years ago, and it's likely that most usable parts have been used up or melted down as scrap. Availability of parts will depend on location; here, they were used for a single-make ice racing series, but I think that ended because they had used up the supply.

The stock Rabbit rear suspension won't work directly, but if you're completely changing the rear suspension, I think it would be nice to use something independent rather than downgrading the vehicle to a beam axle. But hey, it's not mine...
 

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Discussion Starter #13
Can I first say, thank you so much for challenging my ideas without being a total jerk about it. That's really a breath of fresh air compared to most people on the interwebs.

What I was thinking was mounting the motor above the axle and driving it with a chain or something. Unfortunately the Chevette rear axle is a little different having like torque tube thing that's a couple feet long looks like. Hopefully I can do away with it somehow.

And to think I scrapped one and sold another one for a derby car a few years ago..
 

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Unfortunately the Chevette rear axle is a little different having like torque tube thing that's a couple feet long looks like. Hopefully I can do away with it somehow.
That tube extending forward from the Chevette's axle housing is a critical part of the suspension.

Any beam axle suspension needs some way to control rotation of the axle, in reaction to drive or braking torque.
  • In the simplest leaf-spring setup, the leaves resist bending to control this.
  • Some coil-spring designs have long arms which are attached to the axle housing so the housing can't rotate relative to the arm; that requires either soft bushings between the arm and housing (Range Rover Classic front, Mercedes G-Wagen) or a bearing in the axle housing (3rd gen Honda Civic) so that the vehicle can lean without binding (since leaning would mean different rotation between the ends of the axle).
  • The usual approach is to have both upper and lower control arms, acting together to stop the rotation.
  • The Chevette uses that tube as a lever (called a torque arm in other vehicles such as the 1982-2002 Camaro) to control rotation, while the rest of the location work is done by the other arms; this means that the whole setup is very flat, with no upper arms.
You can't get rid of the Chevette's tube and still have a working suspension unless you replace it with other parts which change the design. I suppose that you could remove the tube and replace it functionally with a different torque arm, but by the time you've done that (or converted it to upper and lower arms or any other design) you could have started with any live (with driven axle shafts in it) beam axle.
 

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What I was thinking was mounting the motor above the axle and driving it with a chain or something.
Aside from the problem of the Chevette's specific issue with that tube, there's the idea of mounting a motor directly on the axle beam, so it rides with the beam...

This is successfully done at the small/slow and large extremes of the vehicle spectrum, demonstrating that it's a potentially workable idea:
  • golf cars and similar low-speed utility vehicles commonly have the motor on the axle beam (although parallel to the axle, not longitudinal), and
  • vehicles from electric buses to 100-ton off-highway dump trucks with locomotive-style drive systems have one motor on each end of a drive axle.
In between, this arrangement is usually not considered acceptable due to the adverse effect on ride and handling of so much mass on the "unsprung" side of the suspension. A beam axle suspension used with jointed shafts to a motor or final drive unit mounted to the vehicle structure can fix this; this is called a de Dion axle, and it was used for the Ranger EV. The other approach is to just use an independent suspension.
 

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Suspension swap possibilities

I think that many people underestimate the difficult of putting a suspension into a car which is not designed to work with it, and all of the problems (fitting components in, mounting components to the vehicle structure at places where the structure was not designed to take loads) are there whether you use a beam axle or an independent suspension. It seems much more practical for most people to use a production suspension than to design and custom-fabricate one. If you're going to adapt a suspension from another vehicle, it might as be one that has the features that you want: it needs to be driven (from a rear-wheel-drive or all-wheel-drive car), it should be independent (especially for a car that had IRS to start with), it needs to fit reasonably in the car (compatible track width, not too bulky in the wrong places, space for a motor, reasonable to mount in the car...). I don't think the Chevette axle and suspension are a good match.

Many independent rear suspensions are mounted in the car with a subframe; that can simplify installation because you need to structurally mount the subframe to the car (usually at four places) and perhaps mount the top of the springs to the car, instead of correctly positioning every control arm pivot point to the car.

One type of source to consider is a car which has a rear engine or (even better) rear electric motor. Unfortunately, the only rear-drive with electric motor designs that I can think of are too narrow (Smart ForTwo ED) or way too wide (Tesla Model S/X). There are electrically driven rear axles for AWD hybrid vehicles, such as the Toyota Highlander and RAV4, and Acura MDX and RLX; one of them might be usable, complete with motor.

Another option in complete suspensions with subframe is to use a front suspension in the rear of your car. Most current production EVs are adapted from front-wheel-drive gasoline-engined models, and so are potential donors if you want to use the motor, transaxle, suspension, and subframe as a complete assembly. Fixed track rods would replace the steering linkage. If you start with the front suspension of a front-wheel-drive car with an engine, then you're only taking the suspension and subframe. Either way, these are mostly MacPherson strut designs, so they have big and tall struts with springs around them sticking way up into the interior, and needing to be mounted in a tower which structurally supports the vehicle. Many cars don't use a front subframe, so it's not as if every front-wheel-drive car is a potential donor.
 
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