Redesigning the rear suspension; making the half shafts NOT a control element?
 

This is for a c3 electric conversion, with aspirations of being a canyon carver and autocross machine.
Part of that is shoving a Tesla large drive unit where the differential would go, a 200 pound nugget that shits out close to 700 horsepower and about as much torque
however, the stock rear suspension design and even a lot of the aftermarket suspension designs use the half shafts as a suspension component, they press into the diff, or use the diff casing as a structural component....

With the Tesla LDU you can't do either of those. Pressure from the half shafts would lunch the gearing and the ldu causing is NOT designed for those kinds of dynamic lateral loads.

So, what are my options? The decalink system would probably work pretty well but $11,000 is.... incredibly steephttps://www.detroitspeed.com/detroit-speed-decalink-rear-suspension-kit-1963-82-corvette-double-adjustable-remote-shocks-30-spline-axles-600hp-041741-dr
the racevette 6 link rear looks potentially promising but I'd need to design some kind of bracing which goes around the LDU without anchoring to it
https://www.racevette.com/store/p10/Dragvette_6-Link_Suspension_System_1963-79.html

The obvious solution that carries a lot of side effects is to fabricate subframe mount points to accept the entire rear subframe from the model S, a lot of other conversions do this but Mike breeme of EV west warned me against it saying that suspension design has VERY little adjustment and it designed for a 5000 get luxury sedan and not a lithe sports car....
this will also extend the track width by a lot, like 4 inches on each side, bringing its own host of issues.Not to mention the fabrication needed to mount this is...... extensive....the now pretty famous Electrolite converted Plymouth satellite had to have a cantilever shock setup beco the shocks are too tall and in the wrong place for the satellite rear.
https://www.whichcar.com.au/news/tesla-swapped-plymouth-satellite
(Pic of the rear suspension is about halfway down the page)

So what options am I missing? Am I massively overestimating the work and risk of customizing the suspension a little?

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Try a Camaro SS rear cradle with CV style axles & new aluminum hubs. You need the CV axles to take the pressure off the diff. The race vette version only allows la small amount of movement and it requires the splines to slide. Not ideal. One of the Aussie builders has already put one of these in. ZL1 rear IIIRC.

Adapting a different modern rear end....I had not considered that.

Although looking at it, it would be tough to get the model s rear drive to fit inside that relatively compact rear cradle without cutting the hell out of it and thus impacting its strength, I'd also need to be fabricating mounts for the motor into that cradle
Maybe I could cheat by replacing the nice shaped aluminum cradle parts with tubular beams going across and around the Tesla motor?

Have you looked at C5/C6 suspension. Probably would have to get the cradle narrowed but it just holds the lower arms, springs and sway bar so there might be room above it to fit that drive. You'd have to fabricate the upper arm and shock mounts onto the existing frame.

If the half-shaft was not to be a 'structural' member of the suspension, the one end or the other of it would have to be "floating", as with a splined member, instead of a U-joint.
What would be the point?

I'm not sure what you mean. The idea is to change the suspension design so the half shafts don't destroy the Tesla motor due to plunging

If it were me, I would go with the entire Tesla rear sub-frame.
The extra width can be dealt with by installing fender flares.

Another cool ev swap example, the Electrolite (Plymouth satellite), a really clean build


The guy also has been documenting the project for a while in YouTube videos, up to 15 now
https://youtu.be/QVT7Iq4PvTM

Floating means that one end of each half-shaft would have to have the ability to move in and out as the trailing arm goes through its range of movement. This assumes that the TA is not a constant distance from the differential. Similar to the connection of the driveshaft at the transmission.

seem to recall the drive shafts of some british cars that were fixed on each end but were splined so that the shaft could expand and contract as the differential went up and down. wonder if something like that could be adapted for half shafts.

FWIW
NOT IRS ... for decades, STRAIGHT-LIVE AXLE racers have/do use a floating half-axle shaft that's splined on both ends. This allows the straight housing tube to be cambered at outboard ends. as there's an outboard splined drive HUB the outboard spline end of axle fits into. That junction requires Very high pressure lube such as CMD # 3. Very hi-horsepower 410 sprints use this axle method as does Cup and even local CT racers running 9" Ford floater.

Straight axles in Cup are soon going the way of buggy whips; as IRS + transaxle is mandated for 2022. It'll be interesting to see that final IRS design; and how it works out for both engineers & drivers. S'pose the good ol' days of local racers getting hi-$ takeout R&P from Nascar teams for small $ will soon be in the rear view.

I'm surprised P-Ayr hasn't come up with a plastic mock-up of the Tesla motor...


https://mockupmotors.com/

An accurate Plastic model is a cool idea but I can do something very similar by 3d scanning the car and using CAD
that said I wish I could get a big block+transmission full dimensions so I could make a rough size approximation using foam board, as the basis for my front battery box

I have a small block mock up, no big block.

adapting an existing modern rear suspension might be a fool's errand.

the guys doing this f100 conversion realized only after doing some fab work that they'll have to cut too much away from the IRS unit they want to use, to mount a tesla drive, so they gave up and are making a subframe from scratch which sounds...involved


this has me reconsidering the mount-model-s-rear-subframe approach, since it solves the drive unit mounting and IRS problems, i just don't know how good that rear design is for spirited driving

a guy on the diy ev forums is hitting the same wall
https://www.diyelectriccar.com/threa...d-help.203958/

ah, if only the taycan's rear suspension sans-motor was available from ebay or something by now...

https://www.autoblog.com/2020/02/05/...ion-deep-dive/

This is commonly called "back halfing" a frame.
It's done a lot in the Pro Street world.
It is fairly involved, but doable.

Google Link

Custom CV to U-joint shafts and some links could keep the C3 rear suspension. It really might be the easiest way to do it. The electric motor would be ahead (or behind?) the diff area and that should give some room to put structure for the upper and lower links to replace the half shafts being used as links.

Time to revisit this
A fellow on the diy EV forums is going through the same shit I am, his solution is using the dragvette 6 link and then also a coilover trailing arm setup.
Combined with a little frame around the motor to hold take the forces of the crosslinks
https://www.diyelectriccar.com/threa...2#post-1081336

https://cimg4.ibsrv.net/gimg/www.cor...6ab45b853.jpeg

This isn't all that expensive and solves most of the problems but I imagine this wouldn't be particular nimble on the street
I'd personally prefer the ridetech setup but I don't think it'll be possible, I have to fab up another set of top mounted links to totally remove the axel shafts from plunging, and it seems like there just ain't enough room above where the motor will go
not to mention the shocks mount at an angle....possibly too close in, and they'd hit the motor's sides! Fellow on the diy EV forums is already running into potential problems with ground clearance, and this ridetech setup could be even worse
What are we missing?

Just out of curiosity, where are you going to put the batteries? Most electric cars I’ve seen they’ve mounted them in underneath and in the center of the car. Is this possible with a C3? If you put them either in the front or rear, how is this going to change the balance of the car especially when your plans are to autocross? Just curious. I’ve been tempted once or twice to convert mine to electric since it’s a ‘79.