Tesla SDU - Front and Rear Driveshaft Compatibility?

[elShankos]

Hi there,

I am planning on using a Tesla Model S Front SDU in an EV conversion of a Subaru Legacy which will involve putting the F SDU in the back and making the car RWD instead of AWD. The reasons for this (instead of using a rear SDU) are space and orientation of the SDU within the original subframe etc...
Q: For the Front SDUs, can one remove the half shaft arrangement a A) below and use a pair of Tesla Model S rear driveshafts at B) and C)? Could work if the splines are the same right?

Then I could use a pair of basic new aftermarket driveshafts such as these?

I have hunted all over the show for the answer to the above but haven't found anything definitive so apologies for the noob question if it is detailed somewhere where I have missed it!

[Peter]

Hi elShankos. I run an SDU in the same manner as you are contemplating. I removed the shaft (A) and used FRONT driveshafts as the cv joint bodies are smaller than the rear from what I have learned (and spent !!) The rear type (LDU ??) cv joint fouls the casing at (A) when I tried them but please check dimensions from what you are buyng.
Two things to note...
I am convinced my torque steer (with a heavy right foot) is due to the fact my driveshafts are now of different lengths, if I had located the SDU so the differential had been central to the car the torque steer may have been less I suspect.
Secondly I purchased new aftermarket driveshafts which from the ones I bought definitely do not have the same tolerances/quality as the OEM shafts, thus the play in the joints at the diff end is greater and you can feel the difference simply when twisting by hand.
I am looking for secondhand OEM shafts now ! Who said buy cheap, buy twice ! So right.
Hope that helps and hope all goes well for you.

[EV_Builder]

Yes; the A pieces is to equalize the length.
This is most important if the motor drives the front wheels.
On the rear the torque steer is more limited due to a stiffer wishbones setup.

I recommend too use the engines on their original place with A shaft.
The second advantage the A piece brings is that having the knockles closer to the wheel the shaft will make a steeper drop so less likely to fool somewhere in it's path.

[elShankos]

Hi elShankos. I run an SDU in the same manner as you are contemplating. I removed the shaft (A) and used FRONT driveshafts as the cv joint bodies are smaller than the rear from what I have learned (and spent !!) The rear type (LDU ??) cv joint fouls the casing at (A) when I tried them but please check dimensions from what you are buyng.
Two things to note...
I am convinced my torque steer (with a heavy right foot) is due to the fact my driveshafts are now of different lengths, if I had located the SDU so the differential had been central to the car the torque steer may have been less I suspect.
Secondly I purchased new aftermarket driveshafts which from the ones I bought definitely do not have the same tolerances/quality as the OEM shafts, thus the play in the joints at the diff end is greater and you can feel the difference simply when twisting by hand.
I am looking for secondhand OEM shafts now ! Who said buy cheap, buy twice ! So right.
Hope that helps and hope all goes well for you.

Hi Peter, thanks a lot for the reply and all the details - I will then definitely go straight to OEM driveshafts as you suggest. I am aiming to centre the differential part of the SDU to keep the driveshafts the same length - but the potential snagging as you and EV_Builder have mentioned is something to be cautious of. One thing I have been wondering about is : does anyone know if there would be any issues with mounting the drive unit in a vertical orientation as below?

I'm aiming to find a way to get the drive unit to fit into the space where the original rear diff is:

Nothing is impossible right ?

[elShankos]

Yes; the A pieces is to equalize the length.
This is most important if the motor drives the front wheels.
On the rear the torque steer is more limited due to a stiffer wishbones setup.

I recommend too use the engines on their original place with A shaft.
The second advantage the A piece brings is that having the knockles closer to the wheel the shaft will make a steeper drop so less likely to fool somewhere in it's path.

Thanks a lot for the heads up - if I end up needing to centre the drive unit instead of centering the diff then I will follow your suggestion. Appreciate the input!

[EV_Builder]

Vertical is no-go because you have an oil pump integrated. So at-least you should make it dry-sump then. What i did in my project is i removed the inverter from the motor and made a blanking plate.

If you measure between the 2 beams how much do you have there?

[catphish]

I hope the position of the SDU (in the rear) isn't a huge problem, mine's nowhere near centre!

[EV_Builder]

I hope the position of the SDU (in the rear) isn't a huge problem, mine's nowhere near centre!

In the rear it' s less of an issue. I have 100mm approx.
On the front since the wheels have extra degrees of freedom the x%difference will be corrected by steering wheel input, this will result in less favorable wheel positioning even while driving in a straight line.

[catphish]

This is what I have for the rear. I hope with CV joints all round, the length shouldn't matter there.

[EV_Builder]

Looks the typical offset like I got. Saying that it's better to say like I'm going since I have no practical experience...

[kellis91789]

Vertical is no-go because you have an oil pump integrated. So at-least you should make it dry-sump then. What i did in my project is i removed the inverter from the motor and made a blanking plate.

If you measure between the 2 beams how much do you have there?

Tilting the whole sdu forward to some degree must be ok or else lubrication would fail on a long downgrade. Yes, downgrades are rarely more than 6 or so degrees, but still ... How far can the motor be tilted to vertical before the oil pickup tube is sucking dry ? Obviously the entire case is not full of oil or it would be impossible for it to not find oil to pickup and pump, but I doubt there is only a puddle an inch deep in the nominal bottom of the case, right ? One concern I can see is that the oil fill level is probably one of those "on level ground fill until oil drips out the fill hole" instructions, and if the fill and drain holes aren't where they are supposed to be, then filling the oil while mounted in the car won't be possible. How often is the oil supposed to be changed, anyway ?

I am in the same spot as the OP, thinking a rwd car whose existing diff bolts on the rear to a large subframe leaving no space behind the axles for motor/inverter. A vertical or tilted slightly forward position I think would work and avoid the other option of running the motor in reverse and diddling the internal oil pump.

[EV_Builder]

Yeah so there is a difference between vertical and "to some degree" so long story short...7degrees? And what about if you do have a downgrade of 7 that means 14degree design spec?!?

The pump can be made to work in reverse at-least it's what some have done on this forum. For me the answer depends on the amount of angle necessary on each design.

[Peter]

You could fit a pipe to the filler port on the SDU and route that to a convenient place so filling is easier ? That would also allow you to put fluid in to a point above the filler port if SDU orientation is not as per Tesla.

[kellis91789]

Hi there,

I am planning on using a Tesla Model S Front SDU in an EV conversion of a Subaru Legacy which will involve putting the F SDU in the back and making the car RWD instead of AWD. The reasons for this (instead of using a rear SDU) are space and orientation of the SDU within the original subframe etc...
Q: For the Front SDUs, can one remove the half shaft arrangement a A) below and use a pair of Tesla Model S rear driveshafts at B) and C)? Could work if the splines are the same right?
T FDU Driveshafts.JPG
Then I could use a pair of basic new aftermarket driveshafts such as these?

T Rear Driveshafts.JPG
I have hunted all over the show for the answer to the above but haven't found anything definitive so apologies for the noob question if it is detailed somewhere where I have missed it!

Not to go too far off on a tangent, but in terms of difficulties fitting a Tesla motor that normally trails behind the axles, have you considered a Large Rear Drive unit mounted in the front driving your existing Legacy rear diff ? There is an interesting thread over on DIYElectriccar https://www.diyelectriccar.com/threads/ ... st-1071361
you might check out. He separates the inverter from the motor/differential so it isn't in the way, first. Basically, by removing the Tesla diff and replacing that gear case with a custom one, he uses no NEW gears, just eliminates the final drive gear from the Tesla gear box and mounting a Dana u-joint to what WAS the intermediate gear between the motor shaft and the OLD final drive gear. Instead of an output shaft turn 9.4x slower than the motor, this one turns only 3.5x slower. That means you can turn a drive shaft to the Legacy rear diff with a similar top end to what the old engine and trans would send it ~5700rpm redline. With the Legacy 3.9 (or 4.11 depending on your car) you should still have 1600 wheel revolutions and over 120mph top speed. If you really don't want the motor all the way up front under the hood, you can watch his video of the Tesla motor with his near output gearing directly coupled to his diff in a VW Rabbit he transplanted a Mustang rwd diff into.

How difficult it was for him to build this mod -- casting his own aluminum case and pressing bearing seats and seals into it -- he doesn't go into. Nor does he mention any plans to mfr or sell this conversion as a kit. Seems like if you had a good CAD rending for it you could have a new case milled out of billet aluminum by a shop that does CNC work like that.

Quaife DOES sell a kit to change the gear ratio within the existing LDU gear case, but their final ratio is 4.5 which when combined with your Legacy's ~4:1 final drive would give you only 1000 at the wheels. A 26" diamer tire turning at 1000rpm is only going to cover 6800 feet per minute, or 77mph as a top speed with the Tesla motor screaming at its redline. You could look into getting a smaller ratio for the Legacy rear end and/or change the ratio by using bigger tires to get higher top speed. You would have great low speed torque using the Quaife but start to notice torque fade at 30mph as the Tesla motor reaches 8K rpm and field weakening starts. Or so I've read. The Quaife is something like $2K as well, so there is that. But either of these solutions would have the Tesla motor turned 90 degrees and leave the existing Legacy rear diff in place, so trying to fit the LDU into that gap isn't a problem anymore.

[kellis91789]

You could fit a pipe to the filler port on the SDU and route that to a convenient place so filling is easier ? That would also allow you to put fluid in to a point above the filler port if SDU orientation is not as per Tesla.

On the SDU, where does the oil go besides the gear case ? I thought the actual windings of the motor were cooled by the same oil. I may be confused, but I've only seen pics of where the oil pump sprays the gears in the gear case. I don't recall how the oil is ever shared with the motor windings or how the oil returns to the gear case to be pumped around again -- or even through the heat exchanger which it obviously does. At any rate, tilting the sdu has to account for how the oil flows back and forth between the gear case and the motor windings for cooling as well as lubrication. I need to look for youtube videos of a teardown that might explain the path of the oil flow.

If the gear case was all we had to worry about, then flipping the motor completely upside down should be fine as long as we turn the vent hole into the new drain and the fill hole into the new vent on a snorkle or something as you say. Then just extend the pickup tube inside the case from the pump to where the oil will pool in the upside down position. Completely upside down might leave the motor and inverter case hanging to low since that would be about 8" below the centerline of the diff. Maybe effectively horizontal with axles trailing would be safest. Where would that put the fill, drain, and vents ?

[Peter]

Fluid does spray gears but also it is pumped through the heat exchanger and then to rear motor bearing and returns via motor to diff casing from what I have seen when I stripped mine down.
If this gives you a clue..... I use an external electric pump which draws fluid via a filter from the SDU drain port. Pump is fitted to a bespoke manifold which is also used to relocate the heat exchanger too as I needed to reduce height of the SDU. Output from the pump block (via a high pressure hydraulic hose) goes to another bespoke manifold which sits where the exchanger was, thus the pumped fluid enters at that point so is then diverted to the spray nozzle and bearing(s) as per Tesla set up. Hope that helps.

[kellis91789]

Fluid does spray gears but also it is pumped through the heat exchanger and then to rear motor bearing and returns via motor to diff casing from what I have seen when I stripped mine down.
If this gives you a clue..... I use an external electric pump which draws fluid via a filter from the SDU drain port. Pump is fitted to a bespoke manifold which is also used to relocate the heat exchanger too as I needed to reduce height of the SDU. Output from the pump block (via a high pressure hydraulic hose) goes to another bespoke manifold which sits where the exchanger was, thus the pumped fluid enters at that point so is then diverted to the spray nozzle and bearing(s) as per Tesla set up. Hope that helps.

Yes, that helps, thanks. I also found a youtube teardown of the front sdu that has a nicely cleaned up and labeled motor with all parts.


Here is a still frame where I have tilted the motor forward 90 degrees to see where the oil pickup and pump end up. Ignore the temptation to turn you head sideways and the man holding the motor casing. If the motor was mounted in this vertical position it looks like the oil pickup location is still good. Only flow return through the crescent shaped passage from the rotor housing is questionable. A new hole might be needed there. And apparently, the fill level for oil is a specific amount of 1.8qt and not all the way up to the normal filler hole, so I can't tell whether the filler hole would be submerged with the motor up in this position or not. The drain hole would obviously not allow gravity drain anymore.

[elShankos]

Hi All,

Thanks for all the replies and info - loads of useful stuff in this discussion. I did a fun exercise on the weekend: basically took an image of a rear SDU (not a front one) and scaled it in CAD, flipped the image to be "looking from below", plotted it out full size and stuck it to a piece of cardboard. I then went to my favourite friendly wheel alignment place down the road and put the car in the wheel alignment bay to have a look from below:

This gave me a good idea if I could make the rear SDU fit in its standard orientation as there seems to be a lot of complexity if one changes the orientation, reverse etc of the drive units - and if I can, I would like to try and avoid this. I'm quite happy chopping the subframe, welding, all of that stuff.
So it mostly fits! There will be some chopping of the subframe for sure and also some alterations to the 2 LCAs that adjust the suspension toe in at points B and C:

I think I will either alter the existing LCAs which are actually adjustable or just get some adjustable ones - maybe something like this:

So that would here at E and F with the motor sort of taking up the space at D:

It seems that there are very few cases where people can just drop in a motor! So I guess we all have to compromise / alter something on the donor vehicle to get it to work. I think the above is 1 way of doing it, certainly not the only way - will no doubt have its challenges but I recon its doable!

Would welcome comments / experience / suggestions

[EV_Builder]

You cant just move those rotation points. If you do the geometry during movement will be altered / wrong...or am I missing something?

You can try to drop the front motor from the top sacrificeing boot or what ever is there..

Other option is to go Nissan leaf motor and keep the gearbox and propshaft and diff etc.

Other option is a complete rear subframe from model S.

[Doig5710]

It's a lot of work redoing the suspension mounting points like that it will mess with how the wheels act with suspiension travel and may even fight other mounting points.
What about a front SDU that sits the other way and will hang into the old fuel tank area?

[elShankos]

It's a lot of work redoing the suspension mounting points like that it will mess with how the wheels act with suspiension travel and may even fight other mounting points.
What about a front SDU that sits the other way and will hang into the old fuel tank area?

I'm ok with re-locating the mounting points (welding, frabrication etc) and the alignment guys I talked to about the control arms didn't think it would be a big issue as these control arms just control the toe in - so I wouldn't be changing anything to do with the upper and lower control arms which control the roll etc.
So the toe angle of the rear wheels will be a bit affected when the ride height goes up and down but I'm aiming to adjust the ride height and then set the toe correctly and if there is some additional tire wear then that might have to be the sacrifice I guess.
I have never done the above before so whether or not it will work perfectly is a bit of an unknown I must admit.

I actually originally wanted to use a front SDU but there are 2 issues then:
1) if I put it in reverse as you suggest it compromises the rear seats which would be less than ideal
2) flipping it the other way, and as I originally posted it still clashes with the back of the subframe - so I was wondering about making it fully vertical to get it to fit etc (see above)
Option 3:
3) so no I'm at option 3 which is a rear SDU with some altered toe angle control arms which seems so be the least compromise without altering the SDU for reverse etc (I'd like to avoid this if I can).

Hope my thinking makes sense and I'd welcome more comments / suggestions!

[Doig5710]

Yea fair enough on keeping the rear seats that's what I'm loosing when I mount my motor one day. If your better with drawings and numbers than me you could probably draw the rear hub and work out what the rear toe will do with the shorter arms when at your new height and with the travel it'll have there.

[EV_Builder]

The front SDU will lean backwards in original position so the biggest material will be behind and higher then the diff drive point/centre.
So you are behind the seats in direction of the trunk i think...

I don't think its possible to easyly remount those toe arms.
In your case it would mean the arm gets longer atleast the influence of the radius is smaller then.

What i would do is just try it! : I would dismount that TOE-IN bar and make a plate bolt it to the original place (temporarly) and check if your idea or new length is feasible. You will soon discover the result. atleast its simple and easy to check and non disctructive for your subframe.
You only need some flat pieces with holes to test. I think you discover that staying in the original line of mounting hole is the best bet, so you can only open the arms seen from above so to speak.

Do check if these modifications are allowed in your country since axle loads / weights might become a discussion point.