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Rear subframe conversion ruminations

5137 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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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
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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
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
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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 😂
View attachment 135451
We used to close the center of town off for an annual cruise - two years we were joined by a steam traction engine - so the speeds were not as high as 30 mph
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.
When I put my device on the road I had 500 amps and 144 volts - 72 kW - at 820 kg - 1800 lbs - that was ... 40 Watts per pound !!!
And it was a very fun drive at that - would just fly up the steepest hills
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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:
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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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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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
This installation guide's "Adjusting Pressure Between the Front and Rear Brakes" for Summit's brake proportioning valve may be useful to its understanding:

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
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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My tuppence worth on the Panhard rod

I am building a Marlin kit car - the kit was made in 1995 - after several people faffed about and spent money but did not finish it I am finally getting it on the road

This Marlin has Morris Marina suspension - leaf springs and a live axle
One of the previous owners had fitted a Panhard Rod
That was one of the first things I threw away!!
As built and installed it would have severely restricted the rear suspension
I'm sticking with the live axle and leaf springs

Leaf springs
One of the great things about leaf springs is that you can add or subtract leafs to change the spring rate

Corrosion between the leafs and stiction can be a problem - I have used some Teflon sheet cut into strips between the leafs - it was sold for use in Barbeques
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I see the prototype had hard mounts for the power unit - and rubber mounts for the final solution
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