[Tremelune]

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!

 

[Tremelune]

A remarkably spacious RX7 rear subframe:

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

 

[Tremelune]

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

 

[remy_martian]

Just buy the Cascadia longitudinal unit that @D&VsEVJeep are using.

 

[Tremelune]

How would that be superior to a Leaf with a TorqueBox?

 

[remy_martian]

Bigger motor, no motor coupling & plate design/fab needed.

 

[Duncan]

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

 

[remy_martian]

...that would include the steering knuckles and spindles, for those not familiar (the Pontiac Fiero did this to create a "mid-engine" solution - there are other OEM cars as well that I don't recall, so not merely the kit car crowd doing party tricks).

Tie rods get connected to the chassis/subframe.

 

[brian_]

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.

 

[brian_]

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.

 

[brian_]

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).

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.

 

[brian_]

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.

 

[brian_]

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.

 

[Duncan]

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.

Roll centers are bollocks - the important characteristics are corner stiffness the camber angle change with suspension movement and bump steer

 

[remy_martian]

Camber is old school bias-ply tire compensation stuff that gave us the swing-axle suspensions of the Bug and early Corvettes.

Also "bollocks", lol...and ricer-rubbish:

Wasn't Jaguar the revolutionary that went against that thinking? A much-coveted suspension setup by hotrodders back in my early days. About the lowest unsprung weight you could get with those inboard brake rotors.

 

[Electric Land Cruiser]

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. You can do the same with a Tesla DU.

 

[remy_martian]

A Tesla LDU won't fit inside the cradle framework due to the inverter cylinder.

That 300ZX setup also appears to require shock towers for the struts, which is major structural design & surgery when going from a beam axled rear suspension setup.

 

[brian_]

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.

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.

 

[Duncan]

While a MacPherson strut top does have to take lateral forces those forces act through the "lever" of the strut and are normally only about a fifth of the forces at the tyres so they do not need to be as strong/rigid as the wishbone mounts

 

[remy_martian]

True, but the vertical loads from springs still needs to be transferred from up high if, for example, a leafspring setup was there originally.