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Rear subframe conversion ruminations

5135 Views 83 Replies 10 Participants Last post by  Duncan
Cars with live rear axles make for frustrating conversions, mainly because you have to keep a manual transmission up front. Adapter is a grand or two, and the 2:1 TorqueBox is $4k (plus custom driveshaft). If you ditch the axle, you have to reengineer the suspension, which involves much more than cutting and welding.

Enter the rear subframe swap. Many conversions will take a Tesla subframe and stuff it in the rear. The difficulty here is that Teslas are wide, and many of the cars I'd like to convert are narrow. It also makes using different wheels tricky.

Has anyone successfully gotten an electric motor and gearbox into something like a Miata rear subframe? E30? What are some other rear subframes that are narrow with pickup points that a Leaf/Tesla motor could bolt into without moving the suspension pickup points?

What are the implications of changing the rear suspension geometry and travel while keeping the front stock? For a sportscar it would make me nervous, but for a daily driver or cruiser...not so much—just get over these bumps!

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E30 has another spacious trailing arm setup...Track width is 56" (though I think I really want the hub face to hub face measurement):

Tremelune: Are you thinking of a specific vehicle, or is your post about general applications? I agree in principal with your comments, and have thought along the same lines. Saw a guy last year throwing Subaru rear units under all sort of odd vehicles, but they were pretty narrow. I bought a Ford 9" rear end several years ago for a project in order to get better brakes, and a beefier rear axle, but it turned out to not be a positraction unit as advertised when it arrived. I sent it back. Since then, I've looked into various suspension upgrades, and have come to the conclusion that it is probably easier to just swap something in like you are referring to. Add in the EV conversion issue, and it might make sense.

Last summer I measured a 5 series BMW rear that was within 3/8" of inside tire to inside tire dimension of that same car, which happens to be a Unibody. I'm not confident about hacking up something like that, and I don't weld. Looking at various specifications, I've noticed that published track widths don't tell the whole story, and are only a guideline. It appears different specs are used, such as center of wheel, back of disc mounting surface, etc. A tape measure seems to be the right tool for the job.

I'd like to see some projects where a Leaf unit, or small Tesla motor is used with a rear end swap like this. Very interesting.
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"Track width" is not ambiguous: it is the lateral distance between tire centres on the same axle. Hub face is also clear, but is not the same thing: it is the track width plus the wheel offset on each side.

Some amateurs and incompetent professionals may measure hub face width and report that as track width, causing confusion.

Wheel offset is not bad. There is an ideal wheel offset for each hub design, and to fit required components within the wheel volume essentially all modern vehicles are designed to work with moderate (15 mm to 50 mm) positive (hub face outboard of wheel centre plane) offset.

The same vehicle is sometimes available with wheels of different offsets, resulting in different track widths even with identical hub face spacing. Too much change in track width by this method results in poor suspension and steering geometry, and inappropriate bearing loads. This is especially important with steering axles (not the usual swap situation considered in this forum), and with independent suspension.

Due to the suspension, steering, and bearing issues, the important dimension really is the intended track width, but hub face width is what can be readily measured when there are no wheels mounted, and the offset required to result in that track width must be known to select the correct wheels.
Brian: Thanks for that explanation. I did have a pretty good handle on offset, but had the wrong terminology.
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"Track width" does seem to be an ambiguous term, and the real measurement we would want (to avoid wheel offsets) would be hub face to hub face.

Many of the cars I've thought about converting are old RWD cars with solid axles, and this decision comes up every time...Right now, I'm thinking mostly about a 1951 Studebaker Champion, which is body-on-frame.
Tremelune: I really like the old Studebakers. My favorite steel one is the GT Hawks, from around 1962-1965, if I'm recalling properly. My favorite is the Avanti, '63, '64. Of course, they are fiberglass. Big fan of the 50's trucks too.

Have you ever been to the Studebaker museum in Indiana? If not, you should go. It is very interesting, and well done.
Tremelune: I really like the old Studebakers. My favorite steel one is the GT Hawks, from around 1962-1965, if I'm recalling properly. My favorite is the Avanti, '63, '64. Of course, they are fiberglass. Big fan of the 50's trucks too.

Have you ever been to the Studebaker museum in Indiana? If not, you should go. It is very interesting, and well done.
Tremelune and others:

I'd like to learn more about these various suspension options, and how well they work with options like the Leaf motors, and the Tesla small motor. There are so many old rwd vehicles that could benefit from an upgrade, without having to go crazy on power. Personally, I don't know how to quantify what you could do with either of those options, trying to use OEM parts, and keeping it reliable and simple.

I don't think I've seen any posts about how reliable a Tesla small motor is, or comments about how you would repair the inverter if it fried. With Toyotas, Lexus, Nissan, Volt, etc there are plenty of options, but does that apply to Tesla?

I've seen posts where people talk about horsepower factors, sizing batteries, etc, but still have no way to judge what the overall experience would be like "in the real world" where most people rarely use much of the power that a typical ICE motor can provide. Is there a rule of thumb to determine if a Leaf (for example) could adequately power a Camaro, or a '51 Stude? I'm too old to be interested in little cars, so I'm much more interested in things I can actually get in and drive, load up some tools for a project, or just go cruising.
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All depends on what you mean by "adequately".

"Cruising" is usually a bunch of bald guys or stoners going down a road at 30mph 😂
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Remy: Well, I might be getting old, but I'm not bald, and have never been a stoner. Even when I was young, and had very fast cars and motorcycles, I just never saw the appeal of doing burn-outs, or screaming down the road...stop light to stop light. I did spend a year in the Army in Monterey, riding my KZ-900 up and down Highway 1 as fast as I could.

I'm really serious about this "adequate" thing. Don't know how to quantify it. I look at some of the cars I have (ugly ones) and can do a rough comparison of the available power from a V8, but since I don't run them at their "optimum" level, I don't know how much of the available power I'm really using. That means I cannot decide if, for example, a Tesla small motor would be enough to replace the V8 in a pony car. Most of the cars I own weigh in at about 3200# in stock form. I doubt the factory specs would include a full tank of gas in those calculations.

I can certainly see why a lot of people go for smaller, lighter cars. Of course, smaller means less room for batteries, but that situation might improve as battery chemistries improve. But I'm not interested in the little cars any more. I like the pony cars, and big old lead sleds with hoods and trunks a mile long. Right now, I don't own any vehicles in that latter category.

I find myself going back to the idea of a GS450h a lot. But I'm worried about that oil pump issue. Been keeping an eye on Damiens projects regarding those.
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These are all helpful comments.
The Bolt EV weighs a couple hundred pounds more than a 60's/70's muscle car and has plenty of fun city and highway scoot at 150kW.

That said, it starts to fall off after about 70-75mph (that's why you need more horsepower), but who cares?

I'd say it's pretty much similar to the 350cu. in. 1970 Chevelle auto trans I used to drive.

So, a fun driver is ~40Watts/lb.
Remy/Duncan/Electric Land Cruiser: thanks for those comments. They help to frame some thoughts. I've got enough BMW modules sitting here on the shelf to reach that level, assuming a conversion weight around 3500# all in. Those modules are pretty heavy for their capacity, so I've been thinking more along the lines of using them in a project where the whole I3 battery would fit in the vehicle as is, with a 2011 Leaf setup. Heating and cooling that battery might be an issue, although it would be strictly for around town errands, no hills, range not a big deal.

I've been reading up on the Open Inverter forums to get a handle on the latest with the small Tesla (front) motor, and the gs450h for a pony car. Both of those are available to me right now, within driving distance. I watch Damiens videos for updates, and it appears that parts are starting to become available again, and various people are working out bugs. The gs450h would by far be the easiest, and it is actually smaller than my existing automatic. So the decision is which project to do. I don't have time to tackle 2. The Leaf project would be more work than the pony car, and in the end, would be worth squat. But fun, and usable, and I have most of the stuff now. The pony car, especially with a GS450h, would look stock except when you lift the hood. Lots of room in that engine compartment for batteries and inverters. The pony car would not be driven in winter.

A quick comment on the Tesla sdu...putting it in a rear IRS cradle would allow me to do that work outside the pony car, test it, and make minimal changes to the car. It would weigh more than the existing live axle, but would actually match or exceed the original V8 power. It would get rid of the drum brakes, but likely extend up into the trunk area. I like what I read this past week about splitting the inverter off that unit, and mounting it separately.

Fun stuff!
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ELC: that is a great example. Thanks for sending that. That car is very close to the overall size and weight of mine, in stock form. His horsepower goals, and range exceed anything I'd attempt, but the example is still valid for consideration. I had never heard of a "push rod " suspension until about 2 years ago, when reading about a car done on this forum. Very ingenious, and adaptable.

The Leaf motor could also be finessed into a rear cradle. There is a project on the web where a guy modified an old Jag IRS with a Leaf, or maybe it was a Spark. Can't find that link right now.

Thanks again for the link!
Why not use the Leaf front subframe in the rear of your car - complete with motor unit

Bit like the old days when people would put a mini front subframe in the back of their kit sports car
Duncan: I'm having a hard time wrapping my head around that concept. Admittedly, I get easily confused about how front suspensions work. If you have time, could you elaborate?
Hey, all of you that answered my plea for knowledge..thanks! I get it now. That concept would work well with my crash test 2011 Leaf. The front is unharmed, and complete. The 2011-2012 can easily remote the inverter, so that helps too. In another post, Remy commented that the guy could use a proportioning valve to balance out the powerful Leaf front brakes, if used in the rear. I really like the idea of factory parts, components bolted in factory positions, and then minimal adaptations as needed.
I would prefer using a racing type twin master cylinder brake pedal - that gives you the ability to adjust the front/rear brake balance - not sure how the proportioning valves work - the old ones just limited the brakes - so you could only apply a set amount to the back
I see, said the blind man to the deaf dog, as he picked up his hammer and saw....:)
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