Rear subframe conversion ruminations

5111 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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UglyCarFan

· #41 · 4 mo ago

Electric Land Cruiser said:
This is like the coolest car ever if you want some inspiration: The Story of Kevin Erickson’s 1972 Plymouth “Electrolite”, a Tesla Swap Done Right

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!

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

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.

olegil

· #44 · 4 mo ago

remy_martian said:
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.

Since this live axle problem keeps popping up: Today I received a picture of a conversion that apparently made it work, with seemingly a Tesla drive unit mounted to the body of a car, and the live axle replaced with a U-shaped dead beam axle, and using the Tesla hubs and axles. I had actually thought this should be possible, but seeing that someone has already done it is very encouraging. I'm hoping to extract a bit more information from the person who provided the image, but at least here's the image:
https://www.reddit.com/r/AwesomeCarMods/comments/1109xg3/_/j8dw33d
disregard the thread title, it's about not the car in question

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Duncan

· #45 · 4 mo ago

olegil said:
Since this live axle problem keeps popping up: Today I received a picture of a conversion that apparently made it work, with seemingly a Tesla drive unit mounted to the body of a car, and the live axle replaced with a U-shaped dead beam axle, and using the Tesla hubs and axles. I had actually thought this should be possible, but seeing that someone has already done it is very encouraging. I'm hoping to extract a bit more information from the person who provided the image, but at least here's the image:
https://www.reddit.com/r/AwesomeCarMods/comments/1109xg3/_/j8dw33d
disregard the thread title, it's about not the car in question

Its called a De Dion rear axle - and its a damn good idea - you get all of the benefits of a live axle without the unsprung weight problem
The old Rover 3500 used a De Dion back end

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UglyCarFan

· #46 · 3 mo ago

Duncan said:
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?

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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Duncan

· #48 · 3 mo ago

UglyCarFan said:
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?

Some - not all - front subframes have the mountings for the bottom wishbones as part of the subframe
So mounting the front subframe gives you all of the power unit mountings and the mounting places for the bottom wishbones

That leaves the mountings for the strut tops - you will have to fabricate them

Finally the steering rack mountings - assuming that you don't want a rear steering rack
This is very important - and can lead to very dubious handling

If the rack is mounted to the front subframe THEN you will need to make yourself a "rack substitute" (with no steering)

If the rack is not mounted to the front subframe then you will need to make up your "rack substitute" and the mountings for it

The steering rod mounts are very important - if you have them in the wrong position then you will get "Bump Steer" - which is horrible

The whole steering rod bit is actually very easy to do - as long as you are aware of the bump steer problem and ensure that the wheels do NOT "steer" as the suspension goes up and down

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dddrewski

· #49 · 3 mo ago

remy_martian said:
...(or if you're super brave, put an electric steering rack back there for 4WS)

If you're using the Leaf subframe with steering rack, don't toss out the power assist steering. It looks like it's capable of steering control.

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.

UglyCarFan

· #51 · 3 mo ago

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.

Duncan

· #52 · 3 mo ago

UglyCarFan said:
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

UglyCarFan

· #53 · 3 mo ago

Duncan said:
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....

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

Duncan

· #55 · 3 mo ago

remy_martian said:
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

Yep - just as I thought - operates by limiting the pressure in the rear circuit

Would be a BAD BAD idea with my car when I have a more rearward weight distribution and less weight transfer (low center of mass)

Good idea if the changes you have made mean the back locks up - not good if your problem is the front locking up

A proper brake balance bar is a much better idea - ONLY you then lose the brake booster - which is OK with a light car and a strong leg

Evbeddy

· #56 · 3 mo ago

Can I just go back to the de dion suspension for a moment.

For all tbe clever suspension people. Can the soild beam which replaces the live axle go ahead of the drive unit or does it have to go behind?

Duncan

· #57 · 3 mo ago

Evbeddy said:
Can I just go back to the de dion suspension for a moment.

For all tbe clever suspension people. Can the soild beam which replaces the live axle go ahead of the drive unit or does it have to go behind?

Either - or - does not matter
It was normally behind - because of the prop shaft -

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olegil

· #58 · 3 mo ago

Evbeddy said:
Can I just go back to the de dion suspension for a moment.

For all tbe clever suspension people. Can the soild beam which replaces the live axle go ahead of the drive unit or does it have to go behind?

As has been answered, either is fine. If the gearbox you're mounting normally sits behind the front axle, just place it behind the De Dion axle. But something that hasn't been mentioned enough in my opinion is that a De Dion used with leaf springs should be centered with Watts linkage. Or a panhard bar. But who wants different handling in left vs right hand turns on anything but an oval track?

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.

Tremelune

Discussion Starter · #60 · 3 mo ago (Edited)

2

It's not clear to me how you can keep suspension points on the solid axle when moving to a dedion setup...Cut the axle inward of the spring mounts, and weld the dedion bit? I think the axles would hit the tube at certain angles if the motor wasn't bolted to the dedion tube...I guess it depends on how much clearance there is inside the part of the axle...

Recreation Lego Font Nonbuilding structure Pipe

It just seems harder than getting a subframe in.

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