Rear spindle selection for a 450HP 818 build

[67 Car Guy]

I'm planning an 818 build which will use a Tesla Model S drive unit in an 818. I know it's been done, and should fit. My question is on the rear spindles. I know I'll need custom axles to go from the Tesla transaxle to the hubs. I have to find out the diameter of the normal Tesla drive axle. I'm not going to use a donor car, I plan to just buy the parts I need, so I have freedom to select whatever rear spindles I want. Are the 2007 WRX STI rear spindles compatible with the FF rear suspension setup? Will they take this kind of torque? Are there better choices that would be stronger? Total weight should be around 2600 lbs and I'm thinking of sticking with the FF wheel & tire recommendations. Any advice would be greatly appreciated.

[Hobby Racer]

The WRX STI spindles are different from the standard WRX units. They have a different wheel bolt pattern so the FFR wheels will not fit. They also have slightly thicker mounting areas so you need to modify the FFR brackets or machine the spindles to fit. They also have a different spline count so the FFR axles won't work, but you need custom ones anyways. The axles seem to be the torque limiting factor so you would be OK using standard WRX hubs with heavy duty axles.

Its been done many times here, if you look you can find threads about it. Maybe some of the builders that have used STI hubs can chime in with their experiences.

[67 Car Guy]

Thanks: that's very helpful info.

I haven't started accumulating hardware yet so I have all options open. There is a reference in the EV Controls blurb that says they used the rear spindles and brakes from a JDM Subaru STI. I can't find any information on whether the JDM STI spindles are the same as the US STI spindles. I also don't know if the JDM STI spindles are the best choice, or whether they were just convenient. When you say the mounting areas are thicker, I assume you mean the points where the suspension attaches? That would make sense, since the extra power would put more stress all around. I would assume I could use factory STI brake setups. I'm not worried about sticking with the FF wheels. I've already read a number of threads on brakes and tires (there are MANY!) and I may go with slightly wider rims anyway.

I've broken an axle in the past, and it snapped at the end, not in the middle, so I'm looking to get the largest hub spline in the rear spindle that I can possibly get. I'm assuming Tesla designed the splines in the transaxle to handle the torque of the electric motor. I'm going to have to find out what that diameter is.

[STiPWRD]

I'm curious, are you planning to use the Tesla transaxle or the sti 6 speed trans?

Here's some info on spindles:
From 02-04 the STIs came with 5x100 lug patterns. 05 and newer started using the 5x114 lug pattern. Since STIs came out in the US in 2004, JDM 02-03 STI spindles will be 5x100 lug pattern but the spindles are made for beefier axle shafts. In particular, the WRXs came with 24 spline and STIs came with 27 spline axles.

The front spindle brackets needing to be modified: I believe this only applies to USDM/JDM STI spindles that are 2005 and newer (ie 5X114). The brackets do not need to be modified for STI front spindles that are 2004 and older (5X100). I'm running JDM 02 STI spindles up front with zero mods to the brackets.

Check out Andrew's (LYTW8) build thread on running 450hp power levels with STI running gear, him and maybe some others have shown it can be done.

[67 Car Guy]

Thanks for the terrific detailed information. This explains a lot. Sounds like I really want to make sure I get the 27 spline STI rear hubs.

Funny you should ask about the transmission. I've been thinking about it over the weekend and I realized I may not have as much of an issue with axle torque as I'm thinking. I want to use the entire Tesla Model S rear transaxle as a complete unit. So I won't be using the STI transmission. But that got me thinking, those axles will see maximum torque with a manual transmission in first gear. I'm thinking that will be more torque than the Tesla Drive, since overall gearing in first gear is much higher (numerically) than the Tesla transaxle. The Tesla transaxle has 9.73:1 overall gearing, I just checked, the STI overall gearing is 14.18:1 in first gear. So I'll have to find out the STI engine torque and compare that to the Tesla Motor torque to see what kind of torque I could see on the axles. The trouble is that Tesla data is hard to come by. I did find a Dyno plot showing that the Tesla drive runs constant torque up to around 35mph if I remember, and then constant horsepower above that. Motor speed tops out around 16000rpm I think, maybe a bit higher. Which is what you'd expect from an AC motor drive. My fear (and hence reason I want to use the heaviest possible axles & hubs) is that the transmission guys blew it on the original 2 speed transmission design for the Tesla roadster. The transmission would explode at the shift point. Allegedly the electric motor characteristics were sufficiently "foreign" to the designers, so the transmission shifting had to be disabled for initial shipments.

[x.l.r.8]

If you want I can measure the model S shafts. I had the same set up in my 924 porsche. I don't know if you are looking to increase the power of the drive unit, I have a LDU P designation with just over 1200 amps running through it. The 924 spindles are from the original 2.0 and even when I ran a conventional engine (V8 up front) I would only snap 1/2 shafts up to the 944 turbo ones. I would suggest the 1/2 shafts are the weak point, fortunately all the model S and X had the same 1/2 shafts. Despite that I still opted for some from a P designated car. Right now everything stripped down waiting for the border to open so I can pick up my kit but I know where the 1/2 shafts are.

[67 Car Guy]

Hi;

You sound like another EV fanatic. This will be my third EV build. I just got my heads up: My new 818 kit is supposed to be done next week. I have been prepping for it, but I haven't been not monitoring this forum so I missed your comment. My understanding is that the Model S half shafts should be strong enough to handle a 1200 amp input. You might have had half shaft problems with a V8 and a gearbox, because you get a lot of torque multiplication in 1st gear. The tesla drive is more like being locked into 2nd gear (9.73 overall ratio) so I'm hoping the half shafts survive. It's been done before, and I haven't heard of any broken half shafts. Fingers crossed.