Drive-train Question

Drive-train Question

From the video at, the motor is on one side, the gearbox is in the middle, and the power inverter is on the opposite side.

My question is simple, but might not be so easily answered. In ICE vehicles, a differential would take the output of the gearbox and connect it to the wheels (very simply stated). With the Model S I would assume that the differential would be built into the gearbox, but how would they get that output to each wheel considering that the motor sits on one side of it and the power inverter on the other side of it?

Perhaps nobody here knows and that's just part of the magic of their shrink-wrapped drivetrain, but that one has been bugging me for a while.

The one thought I had was that perhaps instead of taking one input and creating two outputs it takes one input and creates one output. The wheel closest to the motor gets a more direct output from the motor and the gearbox handles differentiating it only to the other wheel while the power inverter is just kind of wrapped around that axle.

While I'm not a mechanical engineer, the design they chose fascinates me.

Daxz | November 14, 2011

At 1:26 in the video you linked, the motor and the PEM are slightly behind the rear axle. It also shows the gearbox between the PEM/Motor which extends forward into the line of the rear axle.

also see images at: without car body.

longhorn | November 14, 2011

As Daxz says, the motor is not on the axle (it is behind it), so the motor will drive the gearbox, then onto the differential, just like an ICE.

jackhub | November 14, 2011

There is no differential in the Model S, only a single reduction gear reducing the speed of the rotor in the AC motor (several thousand RPM) to the desired speed of the axle. The purpose of a differential is to take the rotation of the drive shaft- parallel to the length of the car- and change it 90 degrees to parallel with the axle. The motor and the axle are already parallel in the Model S.

Zelaza | November 14, 2011

Sorry jackhub, but the purpose of a differential is to allow for different (differential) speeds on the drive wheels of a car when it's going around a curve; the outer wheel has to turn faster than the inner wheel. If you don't do tis you'll wear out tires, and other stuff, really fast. I've seen pictures of that rear end and assume that the large pancake disk between motor and controller somehow contains a gear set up that performs the functions of the differential.

mwu | November 14, 2011

ah, I totally missed that in the video because of how dark it is under there. My eyes were attracted to the visible parts and I totally missed the axle back there -- thanks for the pictures, that clears it up a lot.

Looking at it now, it seems obvious that the line the motor, gearbox, and PEM don't line up with the rotors... *facepalm*

jackhub | November 14, 2011

'Roadster Innovations' has a good discussion of this issue.