Rear Differential? Positraction?

Rear Differential? Positraction?

Does the MS have a solid rear axle with simultaneous power to both rear wheels, or does it have some sort of Positraction to direct more torque to which ever wheel has better traction? And is there a differential? If not, how does it allow for the fact that the outside wheel travels farther on a turn and inside wheel travels less. I do not see this explained on the website (at least in terms that I understand). I know MS has Traction Control and Stability Control, but I am not sure how this all fits together. Can anyone explain this simply?

jat | August 28, 2013

It is an open differential, and the car emulates a limited slip differential by using the brakes separately in each rear wheel. Ie, if one wheel is spinning while the other isn't, it will apply brakes lightly to the spinning wheel to make sure some torque reaches the other wheel. So, it is like an advanced computer-controlled LSD without requiring extra components in the diff -- quite clever, and I'm not sure why others haven't done it as you already have separate speed sensors and brake control on the rear wheels for TC/ABS/ASM.

chrisdl | August 29, 2013

Interesting. Where can I find more information on the LSD emulation in an MS? How is that different from the Traction Control System?

Note that just about every other brand has a system which applies brakes on the spinning wheel to provide torque to the other. It's a poor man's choice as LSD, however, because it wastes energy (= it applies the brakes on one wheel), but it's cheap and software only (and integrated in just about all braking-modules nowadays).

cloroxbb | August 29, 2013

Your thread title instantly made me think of "My Cousin Vinny." I love the movie!

jat | August 29, 2013

@chrisdl - I don't know - I talked to a guy at the service center while they had a car up on the rack with it disassembled.

TC is about controlling the power to keep the wheels from spinning -- LSD is about transferring some of the power from a slipping wheel to the wheel with traction.

Earl and Nagin ... | August 29, 2013

I can't exactly speak to how the Model S TC works, however, in the Roadster, it works by reducing the torque from the motor when it detects speed differential between the 2 rear wheels.
The incredible control of torque that an electric motor has enables this to work as it can apply very close to the optimal torque that both tires can grip through. It doesn't waste any energy like it would by braking the slipping wheel.
Just another benefit of the electric motor.

jat | August 29, 2013

@Earl - that doesn't help you if one is on pavement and the other is on wet grass or in the air, for example. In the old days with open diffs, you could jack up someone's car and put the right rear wheel (so they wouldn't see it) in a watermelon rind. They would accelerate, and the car wouldn't move, because the friction was so much less than the tire on the ground and it would just spin at 2x speed in the watermelon rind. Not that I have ever done that, of course :).

In this example, simply reducing the power would never help you move. Braking the wheel that is spinning will transfer torque to the non-spinning wheel, which is exactly what a LSD does -- it has some way of transferring torque to the non-spinning wheel, by resisting turning of the spinning wheel. There are lots of ways to do that, such as clutches that are spring loaded or compressed by pressure rings or hydraulics or electronic control, cones rubbing together, friction on helical gears, or viscous fluid. All of these mechanisms convert some power to heat -- the advantage of using a software controlled brake is that it is only costing you energy when you have to use it.

chrisdl | August 30, 2013

All I could find using Google was that the reduction box of the Model S has a built-in open differential. Computer Controlled LSD or Emulated LSD is mostly marketing slang for Traction Control System ;-)

Hans (Amsterdam) | August 30, 2013
Earl and Nagin ... | August 30, 2013

I'm pretty sure the Roadster can't handle your watermelon example or having one wheel in the air. I kind of didn't expect Tesla to make the Roadster handle driving on anything except a flat road. I plan to keep my Xterra around until there's a good PHEV offroad 4X4 on the market. Ideally that would have a suspended electric motor driving each wheel so it could have optimal traction control all the time.
With the Roadster, I suppose you might be able to use the hand brake for a "poor man's LSD" in that case. I don't know what you'd do with the Model S though since it doesn't offer as much control over the e brake.
Clearly, there's room for Tesla to grow.

chrisdl | August 30, 2013

Yes, the Porsche ABD is exactly like the system that jat describes in his first post. It's poor man's LSD or it's an advanced TCS, depending where you're coming from. Still, I'm happy to hear that the Model S has it. It would be good to find some reference to it somewhere, though, although I don't doubt jat at all.