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MS Differential?

I was told there is a reduction gear between the motor and the driven wheels, but I was asked how the difference in rotation between the driven wheels is handled and realized I had no idea. Embarresed! Help!

I assume yes. Unlike Chevy and the Volt Tesla is not very forthcoming with technical information. I also want to know is the reduction done? Reduction gears with a chain or just gears? I wish someone would take the car apart and post videos on youtube.

It is simple: Reduction gear is a fixed gear with 9.73:1 reduction ratio and of course there is a differential to make up the difference in rotation in curves build into the drive unit (no car would work without it).

Thanks. Makes sense.

There is an open differential between the axles, just like on any car that doesn't have hub motors. Tesla achieves limited slip by selectively applying the rear breaks to transfer torque to the the non-slipping wheel (the same as Porsche and some others do). This is tied into stability management which also needs to brake the rear wheels independently, and is probably why SM can't be turned off.

Another case of replacing complex (and non-adjustable) hardware with simple hardware and complex software.

Help! Something I don't understand. Reading this, the first images that come to mind are Ben Hur racing his chariot, the conestoga wagons crossing the prairie, and motorcycles with sidecars- all turning sharply without differentials- not to mention that little red wagon I once pulled around. How and Why?

@jackhub - those aren't powered wheels, so they turn freely. If you have a straight axle where the inside wheel turns the same as the outside wheel, one or both are going to slip. Some vehicles can get away with that like go-carts or skid-steer construction equipment, but you don't want that in a car on paved surfaces.

@jackhub. Your examples are where the two wheel are not connected in any way. Add a motor between them and there needs to be a way to ensure the motor is not turning the two wheels at the same rate when turning because the one on the outer edge would need to be turned faster

Hmm. But if both wheels are connected to the same axle, and the axle turns both wheels when the axle turns, why does it matter what turns the axle- a motor or a horse?

@jackhub - it is really hard to believe you aren't a troll, but for now I will assume you aren't.

The horse doesn't turn the wheel -- the horse pulls the cart, and the wheel turns because of that motion. Each wheel is free to turn at its own rate.

If you really are having difficulty understanding this, go read wikipedia.


On a wagon or a chariot, both wheels are NOT connected to the same axle. It's more like a pine derby racer, or a red wagon. The axle does not spin. Each wheel spins at the end of the axle.

This has nothing to do with Tesla. Just red wagons, roman chariots and ships of the prairie.
“Tesla achieves limited slip by selectively applying the rear breaks to transfer torque to the the non-slipping wheel “

I do not think this is what Tesla does. I seen you post this a few times and I also wonder how TM does control the slip.

I have a 2012 MS 85kw with over 10,000 miles. I put the car up on a lift and ran it. I notice the brakes do not come on at all. What I notice is the power is cut way down and the wheels turn very slowly. You also see this on the KW meter on your dash and the TC warning is on also. Not sure if the power is cut down to both wheels or just the one that is slipping. Where did you get the info that TM uses the brakes for TC? From my testing it looks like it’s all electrical and no mechanical brakes involve at all.

Almost all manufacturers use the brakes and control throttle to limit wheel spin. I am sure Tesla is the same. Someone who has the car pull off to the side of the road where one wheel is on the pavement the other is in the dirt and punch it and see what happens.

For traction control, the immediate response is independent braking as needed, then power is reduced to limit additional wheel spin, which is much smoother than jamming the brakes on over and over again against 400+ HP. This is the same on most new cars, including the Tesla.

+1 It would great to get a source on this.

Those accusations are uncalled for. Not everybody is as technical as some of us are.

@DickB - when I was having the car pull to the right under acceleration (and to the left under regen), I was talking with the service manager at the ATL service center. When we were having trouble identifying the problem, I asked if it could be something in the limited slip differential, and he said the diff on the Tesla was an open diff and the limited slip functionality is provided by braking the spinning wheel. I haven't disassembled it or done any further investigation, but I assume he would know what he is talking about. The problem I was having turned out to be some bolts that needed to be tightened on the suspension.

@jat, that was probably the most uncalled for "troll accusation" that I have ever seen. The chariot question was very valid, because at face value it appears to be two wheels attached to a single axle. Jbunn's answer (not yours) nailed it - the axle doesn't rotate on a chariot, the wheels rotate independently around the fixed axle.

@pebell - I had no problem with the first question, but persisting after two explanations rather than just reading up on it seemed hard to believe he was really having trouble understanding it. Despite that, I still assumed the question was genuine and explained further.

I also said in the chariot example each wheel is free to turn at its own rate, so I am not sure why you think that was insufficient anyway.

I also talk to service mechanic but he wasn’t clear how the TC work. He also told me that the MS has a parking pawl which I don’t think it has. The roadster has a parking pawl which I seen in one of the videos. I haven’t seen a picture of the MS differential apart yet. Elon did say a while back that the TC was all electrical since the mechanical was to slow to react. That’s what got my interest on how this differential work. So far I haven’t found the answer.

I will put my MS back on the lift next time I get chance to make sure the brakes are not going on. Last time the car was on the lift you could hear the slight drag from the pads as the rotor was turning. Gave it more throttle and no change in sound from the pads, but the TC came on preventing the wheel to go faster. I still heard the sound and felt no pressure in the rubber brake hose. Putting slight pressure on the brake pedal, then you feel the pressure in the brake hose and the sound from the pads will change.

@jat, i didn't say your answer was wrong, I said Jbunn's answer _nailed it_ because instead of calling troll or refering to Wikipedia, he took the time to try and understand what caused the confusion, and addressed and resolved it.

I know you went on to (correctly) answer the question, but what's the point of calling troll on someone _you_ feel takes a little bit to long to "get it"?? If you don't feel l like answering any more, just go to the next thread and leave it to someone with more patience..

Typo: ... If you don't feel like ..

Pebell: Agree. Negativity reflects badly upon this forum. Friendly does it.

Beyond all the varied replies, I am hearing that of course there is a differential in the rear end. I hadn't thought to ask about limited slip. Sometimes we get more than we bargained for.
Thanks for the fire hydrant of explanations. Bottom line for me is, of course there is a differential. We are just not entirely clear how all functionality is provided. Seems to work well on my MS.

Here's the blog about the roadster, I imagine it is very similar. It sounds like it electronically limits power. It seemed to work extremely well in the test drive I did, in the model s.

I'm not much of a fan of the brake based systems, but they do work, although counter intuitively. Why fight against the energy usage with the brakes?
I have a trutrack in my range rover and it works very well (not that the rover could ever spin the tires on pavement!) until the wheel comes off the ground, then it basically behaves like an open diff. Right when you need it most!
I think tesla nailed it (as with the rest of the car!), with room for improvement with a torque vectoring system

@Bikezion - to do torque vectoring, you need multiple motors, and probably hub motors which have their own negatives (unsprung weight, primarily). If you are building a money-is-no object performance car, then that is fine (see the SLS eDrive and the aforementioned Rimac), but it isn't really practical for what the Model S is aimed at.

I think the model S is a far better car than either of those 2, although the torque vectoring has some huge advantages, with some downsides, that may not be worth the trade offs. As I said I think Tesla nailed it perfectly with the S. Its not a perfect car, but a perfect first attempt!

@jbunn. Tks. got it!

DickB, when you ran your car on the lift, did you hold one of the rear wheels to try to prevent it from turning? That's the only time the car would need to apply the brakes (to the other wheel). If both rear wheels are able to turn easily, the car can simply reduce power to the motor, to prevent them from turning much faster than the front wheels.

@ye – What I’m trying to find out is if the TC uses the brakes. I don’t think MS does. It seems to be all electrical.

With the car up on the lift, TC off, push down on the throttle and both wheels were going about 80 mph. The speedometer does not work since the front wheels are not turning. Let off the throttle and the regen goes up to 60kw. Push down on brake and the wheels stop and you feel the pressure in the brake hose going to the caliper.

With TC on, the wheels will only go about 10 mph. The power is being cut down and the brakes are NOT coming on. This TC does not use the brakes like some other cars.

Last winter I stop on an icy road and step down on the throttle and the car just crawl down the road. You could see that the TC was working but very little power going to the wheels. I think TM did a very good job on TC and the regen.

I thought we were talking about whether the Model S achieves the same effect as a limited-slip differential, and if so, how it does it.

A regular differential lets the rear wheels turn at different speeds. If one wheel is much easier to turn than the other, e.g., because it's on ice and the other one is on pavement, the wheel that's on ice will turn twice as fast as usual and the wheel that's on pavement won't turn at all. And you won't get anywhere.

A limited-slip differential doesn't absolutely force the two wheels to turn at the same speed, but it makes it somewhat difficult for them to turn at different speeds. That way, the wheels will turn at different speeds when you drive around curves, but both wheels will still get power even if one is on ice.

A car with a regular differential can simulate a limited-slip one by applying the brakes gently on the faster wheel if it notices that the two wheels are turning at very different speeds. This will cause the slower wheel to speed up. said that this is what he thinks the Model S does. You (DickB) cited the behavior of your car on the lift as evidence against his opinion. But it isn't really, because even if he were right, your car would still have behaved exactly as it did behave.

@ye – Sorry if I didn’t explain this correctly, but I own a transmission shop for over 40 years. We do repair the complete driveline so I do know how a differential works. My question was on how the TC (traction control) works. Does it use the brakes to stop the spinning wheel or is it all electrical? After running my car on the lift it seems like it’s electrical and the brakes are NOT use.

One of the main reason I bought the Model S was how the driveline works. Love driving this car.

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