Rear subframe conversion ruminations

5107 Views 83 Replies 10 Participants Last post by Duncan, 3 mo ago

Tremelune

Discussion Starter · 4 mo ago

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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brian_

· #9 · 4 mo ago

Tremelune said:
A remarkably spacious RX7 rear subframe:

Sure seems like you could just bolt this thing into a reinforced rear and roll along...

That looks like the FC RX-7, with its weird tweaked semi-trailing arm design. The subframe is out of the way because it's really only half of the subframe, with the final drive case and mount completing it. Any swap of an electric drive unit into this would need to use the drive unit case the same way, or provide additional framework.

Some semi-trailing arm setups - such as in BMWs - work the same way. If the final drive portion is not included or replaced by something else, the crossmember which this calls the subframe will rotate about the lateral axis.

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brian_

· #10 · 4 mo ago

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

That's one of the BMW systems that uses the final drive housing as part of the subframe structure, as I noted in my previous post.

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brian_

· #11 · 4 mo ago

Duncan said:
Why not use the Leaf front subframe in the rear of your car - complete with motor unit

That works, as long as the vehicle has the space to accommodate the MacPherson struts, and the structure to take the strut loads (both vertically and horizontally).

Duncan said:
Bit like the old days when people would put a mini front subframe in the back of their kit sports car

Similar, but the Mini subframe includes the spring and shock mounts; the Leaf subframe does not.

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brian_

· #12 · 4 mo ago

Tremelune said:
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!

If the rear suspension swap changes the roll centre height, without changing the front suspension geometry, understeer/oversteer characteristics will change.
If the rear suspension swap changes the roll stiffness, without changing the front roll stiffness, understeer/oversteer characteristics will change... but that's not geometry, and just fixable with just spring and anti-sway bar changes.

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brian_

· #13 · 4 mo ago

Tremelune said:
Has anyone successfully gotten an electric motor and gearbox into something like a Miata rear subframe? ...

For those not familiar with Miatas, that's the recent NC or ND generation Miata/MX-5 (they're very similar); it's very different from the earlier NA or NB generation. The NC Miata is nearly the same as the RX-8... but the RX-8 is a much less common vehicle if looking for salvage parts, and they have about the same track width.

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brian_

· #18 · 4 mo ago (Edited)

Electric Land Cruiser said:
Check out this 300ZX build: 300ZX conversion for sale

Uses a LEAF transaxle inside the subframe. This is how I would do just about any RWD car. ...

View attachment 135388

I was going to link that one... thanks for catching it first.

This is the thread for the build: 300ZX Electric Conversion
The rear suspension is from the S platform, presumably the S13, so if the Leaf unit fits in this 300ZX it will also fit in the subframe (but perhaps requiring body modification) of the same-generation 240SX (for example), and a subframe from any of these models could also work for a Leaf drive unit in another vehicle.

Like most suspensions, the shock extends well above the top of the subframe, and mounts to the vehicle structure rather than the subframe. That's certainly a feature to consider in the potential use of any subframe in any vehicle. Fortunately, it's just a shock-mounted spring, not a MacPherson strut, so it doesn't serve a lateral locating function and takes only vertical force.

Electric Land Cruiser said:
You can do the same with a Tesla DU.

Most Tesla drive units place the motor(s) behind the axle line, not ahead of it like the Leaf unit - that will certainly affect fit and required subframe modifications. The rear unit of the Model 3 and Model Y is the notable exception, with the motor ahead of the axle line.

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brian_

· #24 · 4 mo ago

Tremelune said:
Welding up shock mounts to the frame or subframe seems straightforward as long as the angle is reasonably close to stock and there is no interference through travel. Probably involves cutting into a trunk, but that seems like a given for the rear-subframe approach.

It should be straightforward, unless the placement of something (such as vehicle structure) makes it not straightforward.

Tremelune said:
Doesn't the Tesla inverter bolt off, or does that let the fluid out? It wouldn't be hard to rig up some wires for a "remote" inverter.

The Tesla large drive unit uses cables between the inverter and motor anyway, so I agree that longer ones shouldn't be a problem. Fluids are probably a bigger issue, but it's obviously possible (although not necessarily easy) since it has been done many times.

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brian_

· #28 · 4 mo ago

UglyCarFan said:
... 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.

Tremelune said:
"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.

"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.

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brian_

· #42 · 4 mo ago

UglyCarFan said:
The Leaf motor could also be finessed into a rear cradle.

Nuts&Volts put a Leaf drive unit into a rear subframe of an unrelated Nissan model:
300ZX Electric Conversion

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brian_

· #67 · 3 mo ago

Panhard rods are sometimes used with leaf springs, normally for higher lateral stiffness, and only when suspension travel is relatively short. This is the sort of thing usually found on race-prepared production cars from decades ago (such as the early 1970's Datsun a friend of mine had, which was race-prepared around 1980), not anything in street use and not anything recent at all.

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brian_

· #70 · 3 mo ago

olegil said:
...
But I don't understand your fear of adding some centering. It's standard on all modern cars with live or dead rear axles and leaf springs...

There are no modern cars with beam axles and leaf springs. There are many trucks (light and medium duty) with basic beam axle and leaf spring suspensions, but they don't have Panhard/track rods or Watts linkages... just look at any pickup truck or van (such as the Tacoma).

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brian_

· #73 · 3 mo ago (Edited)

Electric Land Cruiser said:
That's not true. My Land Cruiser has a panhard bar and it has tons of suspension travel. A brand new 2023 Land Cruiser also has them. Most all 4x4s with solid axles have them from the factory.

Also Camaros and Mustangs from the 2000s. Crown Vic. Etc. Tons of cars have them and modern cars too.

Electric Land Cruiser said:
Tacoma, 4Runner, Sequoia, etc all had panhard bars at least until 2016. Ford Bronco, Chevys, Ram. Practically every truck has them.

Those vehicles have coil springs and control arms, not leaf springs, except the Tacoma (which doesn't have a Panhard rod). Here's your 80-series, with visible rear coil springs, no leaf springs... and yes, a Panhard rod. Did you just not notice that we're talking specifically about the use of a Panhard rod with leaf springs?

That includes GM full-size SUVs with beam axles (Yukon, Tahoe, etc) and even Ram light-duty pickups. "Practically every truck" would be only Ram trucks and the 2023 Tundra, because they're the only ones with coil springs. While a Tahoe had (until it went IRS) a rear beam axle with coil springs and links including a Panhard rod, but the Silverado of the same platform series has leaf springs... and no Panhard rod.

Of course a lateral location function (Panhard/track rod, or Watts linkage, or angled upper arms, or upper A-arm) is required with coil springs.

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brian_

· #78 · 3 mo ago

Tremelune said:
I think we've gone a bit too far into the weeds here—the point of this thread is specifically to avoid re-engineering suspensions. Let's stay focused on subframes that would be advantageous for EV swaps into narrow RWD cars.

Fair enough - the original subject is clearly using a complete subframe-mounted suspension. That will be independent, not any kind of beam (including a de Dion beam).

The beam axle - and then de Dion - sidetrack started with a post by olegil. The specific issue of a Panhard rod with leaf springs was also raised by olegil in another post, and compounded by his comments in another post and yet another post and the apparent inability of Electric Land Cruiser to understand the difference between leaf and coil spring suspensions. With any luck, we're done with that, and everyone understands that a beam or de Dion system may be a sensible alternative for some vehicles, but is not intended to be the subject of this thread. The Comet project build thread (1962 Mercury Comet + 2013 Nissan Leaf) is a good place for that discussion.

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