Rear subframe conversion ruminations

5136 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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remy_martian

· #4 · 4 mo ago

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

remy_martian

· #6 · 4 mo ago

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

remy_martian

· #8 · 4 mo ago

...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.

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remy_martian

· #15 · 4 mo ago (Edited)

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:

Car Vehicle Wheel Tire Hood

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.

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remy_martian

· #17 · 4 mo ago

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.

remy_martian

· #20 · 4 mo ago

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

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remy_martian

· #23 · 4 mo ago

It's not as straight forward as you'd think to remote the LDU inverter.

remy_martian

· #25 · 4 mo ago (Edited)

The LDU does not use cables between motor and inverter.

It's a contorted internal 3 phase bus bar connection with two of the phases having current sensors integrated with the busbars.

Each phase leg of the inverter has its own coolant inlet and outlet from the gearcase.

It has been done a few times (I know of maybe four), yes, but it is far from a trivial endeavor.

remy_martian

· #32 · 4 mo ago

All depends on what you mean by "adequately".

"Cruising" is usually a bunch of bald guys or stoners going down a road at 30mph 😂

Automotive parking light Vehicle registration plate Vehicle Automotive lighting Hood

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remy_martian

· #36 · 4 mo ago

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.

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remy_martian

· #38 · 4 mo ago

To clarify, the 40W is a minimum, but more is not necessarily better. More will give you top speed, but more torque than tires can hold is useless and loses stoplight races (the stoplights being offroad, of course).

Low rolling resistance tires, for example, spin to easily from a standing start and can be outright dangerous pulling out in traffic. Dragsters lose races if they wheelspin.

Why?

Because there are two kinds of friction. Static friction, what we call "grip", then as soon as a tire breaks loose, the amount of friction available drops dramatically.

This is why your ABS stops the wheel (relative to the pavement speed under the car) - it resets the slipped friction level back to the higher static value.

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remy_martian

· #43 · 4 mo ago

You shouldn't like what you read about splitting off the inverter - it is far from straight forward....on the LDU.

Is the SDU easier and why would you want to....it's not a giant symmetrical can to the motor?

@428RC was the one who most recently looked into the split LDU minefield in great detail.

remy_martian

· #47 · 3 mo ago (Edited)

Take the front subframe from a front wheel drive car with suspension mounted to it and shove it straight back. Bolt the inner tie rods to the subframe or frame so the car permanently steers straight (or if you're super brave, put an electric steering rack back there for 4WS)

The Pontiac Fiero and a few other (MR2?) "mid engined" cars did this trick by reusing almost everything from a front drive car.

Leaf, Bolt, and others might lend themselves to this, though they'd obviously need their inverters decapitated and remoted. The first gen Leaf had a remote inverter...

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remy_martian

· #50 · 3 mo ago

You'd need a CANbus rack from an ADAS car. Using the assist, which he's doing here, is not servoed and will not fail safe.

remy_martian

· #54 · 3 mo ago

This installation guide's "Adjusting Pressure Between the Front and Rear Brakes" for Summit's brake proportioning valve may be useful to its understanding:

https://static.summitracing.com/global/images/instructions/sum-g3906_lg.pdf

remy_martian

· #59 · 3 mo ago

Why do you want to center leafs with a panhard bar when the shackles and front spring mounts hold the car laterally? I'd think it would bind up.

The entire point of this setup is not to do any suspension mods/fab to the originals in the car. It's also arguable you could mount the drive unit to the Dedion tube - not that great for unsprung weight, but zero mods to the car, in theory.

remy_martian

· #62 · 3 mo ago

EV West Guy bolted his in...leaf springs is key to simplicity, imo.

You can't reuse anything from the old solid axle, unfortunately. You need splined hubs out there and outer CV joints.

Yes, it doesn't come for free. Still a lot of fab. I like it because no butchery of the car to speak of. For an old car, I would mount the DU to the Dedion tube and attach it all to existing leafs....anyone see any gotchas other than unsprung weight?

remy_martian

· #63 · 3 mo ago

^^ I'm not sure what we're looking for in that video.

The operator drives like a girl is all I see so far 😂 and the camera mount creates an illusion there's transverse wobble in the rear axle.

remy_martian

· #65 · 3 mo ago (Edited)

The bar goes through an arc. The old leaf spring cars never had them.

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remy_martian

· #68 · 3 mo ago (Edited)

The idea is not to fabricate anything you don't have to. One principle on restomodding is reversion - putting the car back to its original condition, which some people want for whatever reason. Not one new hole drilled is their rule.

To Brian's point, if you're building a race car that whips into curves at the edge of traction cutting loose, yes squirm could break tires loose. For road cars, it's unnecessary....even pickup trucks that do towing don't have them....not even my 4 wheel steer trucks have them.

Hard bushings can accomplish the same goal, btw.

Coil spring cars have to have them, though.

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