Drive wheels question

The MS is a single-motor RWD car. There are not independent motors driving each wheel if that is what you are asking. HOWEVER, in the AWD MS/MX, there will be 2 independent motors, one driving the forward 2 wheels and the other driving the rear 2 wheels.

invisik said:

Hi!

Sorry if this is basic, I didn't see any posts about it via search.

Does the electric motor drive both rear wheels? My 1992 Mercedes drives the right-rear wheel (easy to tell when you're stuck in deep snow).

Thanks.

-m

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Never heard of a car that only drives one wheel. Your car may have an open diff, so that if one drive wheel loses traction, all the power goes to that wheel. The Model S also has an open diff, but the traction control/stability control will apply brake to the wheel that is spinning freely, so the power goes to the wheel that still has traction.

You are asking if the Model S has a limited slip differential, ie, a differential that mechanically limits the percentage of drive torque that can be sent to the wheel with lower traction.

The answer is no, it does not. It has an "open" differential (like your 1992 Mercedes). In a situation like you describe, the Model S differential will also route all of the torque from the motor to the wheel without traction.

But that's not the whole story, as someone mentioned above. The Model S (like many if not all cars these days) has an electronic system that uses the brakes to mitigate this problem. When you get into a situation where one wheel is spinning substantially faster than the other, the electronics will apply the brake on the faster-spinning wheel. The resistance provided by the brake pads mimics the resistance provided by traction from the tire, and effectively causes the differential to route power back to the "unstuck" wheel.

This works reasonably well. The downside is that the extra friction from the brake application reduces brake life, though in non-track driving it's not that big an issue.

JST said:

This works reasonably well. The downside is that the extra friction from the brake application reduces brake life, though in non-track driving it's not that big an issue.

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It could be worse, it could be clutch plates inside a limited-slip differential. Out of sight, out of mind. Many LSDs don't LSD after 100,000 miles.

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invisik said:

Does the electric motor drive both rear wheels? My 1992 Mercedes drives the right-rear wheel (easy to tell when you're stuck in deep snow).

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As has already been pointing out your Mercedes-Benz does not drive just the right-rear wheel. You could lift either rear wheel off the ground and the engine would spin that wheel and not push on the other. If you are observant you will notice the spinning wheel is going twice as fast as the speedometer indicates. If you lift both wheels off the ground with the transmission in park then if you turn one wheel forward the other will turn in reverse.

This sounds counterintuitive but if your Mercedes has drum brakes on the rear, or even just drums for parking brakes, then when the wheel is spinning on snow apply the brake (the one with drums). This doesn't work with disc brakes because a rotating drum will try to rotate the brake shoes with it, wedging the shoe a bit, applying the brake harder. In the heyday of drum brakes this non-linearity was considered a feature, very early form of power brakes. The spinning wheel will brake more than the stationary wheel which serves to transfer torque to the wheel with traction.

JST said:

The Model S (like many if not all cars these days) has an electronic system that uses the brakes to mitigate this problem. When you get into a situation where one wheel is spinning substantially faster than the other, the electronics will apply the brake on the faster-spinning wheel. The resistance provided by the brake pads mimics the resistance provided by traction from the tire, and effectively causes the differential to route power back to the "unstuck" wheel.

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I got stuck in a snow bank at the end of my driveway a couple of days ago. Only one wheel would spin and it took me about 20 minutes to dig and rock it out. I sure wish it had Positraction that day!

mknox said:

I got stuck in a snow bank at the end of my driveway a couple of days ago. Only one wheel would spin and it took me about 20 minutes to dig and rock it out. I sure wish it had Positraction that day!

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If you were spinning then you had TC off right? I guess they need to make a snow mode for the traction control for situations like these...

Sounds like mknox was definitely in 'stuck pig' mode: no amount of technology is going to succeed. Time to get out the shovel and clear out both tracks plus extra on both ends. And if the belly is up unto packed ice you will need to pull that out as well. Then . . return of the Sublime.
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mnx said:

If you were spinning then you had TC off right? I guess they need to make a snow mode for the traction control for situations like these...

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Yep. TC Off. One of the annoying things about that is every time I got out to shovel a bit further, the TC would be back on again when the car re-started. I would then have to go through all the steps, including the "are you sure?" dialog to turn it back off again.

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wycolo said:

Sounds like mknox was definitely in 'stuck pig' mode: no amount of technology is going to succeed. Time to get out the shovel and clear out both tracks plus extra on both ends. And if the belly is up unto packed ice you will need to pull that out as well. Then . . return of the Sublime.
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That's exactly what I did. Putting the suspension in "Extra High" also helped push the wheels down because I was floating on the belly a bit too.

mnx said:

If you were spinning then you had TC off right? I guess they need to make a snow mode for the traction control for situations like these...

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Yes, the traction control seems to completely cut power in certain situations. I'd prefer to have less traction control at low forward speeds (when the accelerator isn't mashed).