Will Model X have all-wheel steering?

[Johan]

As I mentioned in my original post, the motor can not control torque split between the left and right wheel while it is connected to both via the reduction gear/differential. It can be done only by applying brakes to one of the wheels, which forces the open differential to transfer the torque to another wheel. This is conventional mechanical system that is employed on many cars and really has nothing to do with the motor (2004 Toyota Sienna AWD which I owned a while back had this system).

The advantage of the electric motor combined with the above described mechanical system is in response time of the total torque that produced by the motor, it is almost two orders of magnitude faster than for an ICE power train. Elon mentioned several times that response of the MS/MX power train is in milliseconds.

Ok makes sense.

 

[Chrisr400cc]

I was reading the conversation and I can't understand why does the Model X have only two motors and the SLS has 4? Is it more efficient to have 2 big motors vs. 4 smaller ones maybe?

 

[InTheShadows]

The same way Model S does now on the rears with traction control? If my rear right tyre is on ice and the left is on asphalt TC will direct torque to only the left? And it's not mechanically braking on the right? My experience in winter is that if there is no grip on any of the rears I can floor the pedal but the kW gauge barely moves.

I agree. The speed at which torque is transferred back and forth between the left and right rears is not done with braking. Using braking would reduce the cars acceleration potential. There has to be a way the car is controlling exactly how much torque is going to each wheel. Perhaps the dif has electronics in it which can control how much torque is given to each wheel.

 

[InTheShadows]

Elon mentioned several times that response of the MS/MX power train is in milliseconds.

Just to put some numbers on the Elon's milliseconds comment, if response time is two milliseconds, the electric drive train of a car traveling at 55 mph can respond to the detected slip condition after moving only 2 inches after detecting it.

This is another reason I do not think it is the brakes. I'm not a mechanical or fluid expert but I don't think the hydraulic braking system could react that precisely and quickly.

IMO there is a distinct difference between the feel of traction control, stability control, and stability control during hard acceleration. The stability is obviously brakes straightening the car.

 

[vgrinshpun]

I was reading the conversation and I can't understand why does the Model X have only two motors and the SLS has 4? Is it more efficient to have 2 big motors vs. 4 smaller ones maybe?

The reason lies in the fact that these two vehicles serve vastly different purposes.

Merc. SLS electric is a result of an R&D project, a half a million dollar car designed to show-off the concept.

Model S is a car designed to be mass produced, and sold in quantities that will allow the idea of compelling EV to take root in the conscience of mass consumers, while allowing well-to-do and early adopters owning an exceptionally compelling EV . At a small fraction of the cost of Merc. SLS electric, it is doing just that.

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This is another reason I do not think it is the brakes. I'm not a mechanical or fluid expert but I don't think the hydraulic braking system could react that precisely and quickly.

IMO there is a distinct difference between the feel of traction control, stability control, and stability control during hard acceleration. The stability is obviously brakes straightening the car.

Doing basic independent research on the subject would help you understand that there is actually no way that modulating one motor connected to two wheels via a reduction gear/differential (the setup in Model S) can allow to control torque split to each individual wheel that is different than roughly 50% -50%. Using Google search for an "open differential" might help...

 

[Nubo]

This is another reason I do not think it is the brakes. I'm not a mechanical or fluid expert but I don't think the hydraulic braking system could react that precisely and quickly.

IMO there is a distinct difference between the feel of traction control, stability control, and stability control during hard acceleration. The stability is obviously brakes straightening the car.

Hydraulics can be very fast and precise thanks to the incompressibility of the fluid. Many years ago I worked with a machine that read hand-written gas-meter reading cards. This required a rapid feed and stop/start motion to freeze the card for the image reader. It took a considerable amount of power to move the transport in such sudden starts and stops.

When it required service I was surprised to see that the transport was driven by hydraulics. The engineer explained that electric motors (at least at that time) weren't capable of accurately providing the rapid acceleration/deceleration precisely enough for the machine to efficiently do its job whereas powerful hydraulic pressure could be rapidly and accurately pulsed by small valve actuators.

 

[stopcrazypp]

Doing basic independent research on the subject would help you understand that there is actually no way that modulating one motor connected to two wheels via a reduction gear/differential (the setup in Model S) can allow to control torque split to each individual wheel that is different than roughly 50% -50%. Using Google search for an "open differential" might help...

It's possible if the differential used is not a simple open diff. The Honda SH-AWD system has a rear differential that can split rear wheel torque from the left and right at 100%/0% or 0%/100%. It uses electro-mechanical clutches in the differential to do this.

But you are right in the case of an open diff, the only way to accomplish this is with the brakes.

 

[vgrinshpun]

It's possible if the differential used is not a simple open diff. The Honda SH-AWD system has a rear differential that can split rear wheel torque from the left and right at 100%/0% or 0%/100%. It uses electro-mechanical clutches in the differential to do this.
But you are right in the case of an open diff, the only way to accomplish this is with the brakes.

I agree that there are several systems on the market that allow torque split, some of them rely on brakes, others on different types of clutches. The point of my post, though, was to show that for a particular MS/MX platform:

1. Motor is connected to both wheels via a passive reduction gear/differential, so there is no way that modulating motor output would allow torque split other then approximately 50% - 50%
2. The only way to control the left - right torque split in MS/MX is to apply brakes to one wheel forcing open differential to shift more torque to the other wheel.

 

[vgrinshpun]

Just to put the issue of open differential in Model S to rest, here is information on the subject from Edmunds.com:

The 2012 Tesla Model S is powered by a transverse rear-mounted liquid-cooled AC induction motor that is capable of 16,000 rpm. It drives the rear wheels through a single-speed transaxle with a 9.73-to-1 reduction that feeds an open differential.

2012 Tesla Model S Road Test

 

[AlMc]

I have nothing to add to the discussion but have learned a lot based on the 'back and forth' and some links supplied. Thanks!

 

[Macgaver]

Waking up that old thread. We still don't have any idea today how the Model-X will have a better turn radius than a mini.

Known numbers:
- Current Model S: 37 ft.
- Audi Q7 : 39.4 ft.
- Audi Q5: 38.1 ft.
- Mini Cooper : 35.4 ft.

We are missing something here (I am for sure). Looking at the latest prototype video, I don't see back wheels moving strangly. My guess is that it's not going to happen and we will have what Model-S have.

Anyone spotted something related to this ?

 

[Bangor Bob]

Actually, I was at the reveal event and rode in the prototype. One of the young design engineers was our driver. He said that the tight turning circle (tighter than Model S, despite longer wheelbase) was indeed due to the ability to control the torque delivered to each of four wheels independently. Apparently, even without actually steering the rear wheels, this "torque-steer" is sufficient to twist the car faster than otherwise possible!

All-wheel torque vectoring is one of the holy grails for curve handling. If they're doing it with a fancy diff on each end of the car?... Wowza!

And just so we're clear where I'm coming from when I say "fancy diff":

Put a sun and planet arrangement in one output leg of the diff. The sun gear also being the stator for a a high-torque/low-power motor. Modulating the torque on that motor will vary the diff's left/right torque split. There are a variety of other ways to implement it. Traditionally, as noted above Honda uses an electromechanical clutch. Subaru uses something similar as their center diff to vary the front-rear split in their simpler auto transmissions (Not the VTD models).

 

[trils0n]

All-wheel torque vectoring is one of the holy grails for curve handling. If they're doing it with a fancy diff on each end of the car?... Wowza!

They'll probably do it with the brakes, like the Model S.

 

[Bangor Bob]

They'll probably do it with the brakes, like the Model S.

That's really the slow, imprecise and energy-robbing way to do it. But it's certainly the most inexpensive solution.

 

[trils0n]

That's really the slow, imprecise and energy-robbing way to do it. But it's certainly the most inexpensive solution.

McLaren P1 uses brakes to do torque vectoring with an open diff. They said it was better (faster, more precise, efficient and lighter weight) than a locking or limited slip diff.

 

[BerTX]

Waking up that old thread. We still don't have any idea today how the Model-X will have a better turn radius than a mini.

Known numbers:
- Current Model S: 37 ft.
- Audi Q7 : 39.4 ft.
- Audi Q5: 38.1 ft.
- Mini Cooper : 35.4 ft.

We are missing something here (I am for sure). Looking at the latest prototype video, I don't see back wheels moving strangly. My guess is that it's not going to happen and we will have what Model-S have.

Anyone spotted something related to this ?

Hilarious. This points out that the Mini just has a crappy turning radius -- that's why it was low-hanging fruit to say the MX would be as good. The MS is almost as good.

I love that the engineering discussion picked right back up, ignoring this.

 

[Bangor Bob]

McLaren P1 uses brakes to do torque vectoring with an open diff. They said it was better (faster, more precise, efficient and lighter weight) than a locking or limited slip diff.

And they're right. Locking and limited-slip diffs are crude, passive and purely mechanical devices. The fun active handling stuff was being developed in the World Rally Championship before they banned active differentials. (Anti-lock brakes are also banned, so they were back to pure passive mechanical diffs last time I checked a couple years back. Series was trying to contain costs.)

 

[de704]

I was watching a bunch of Model X videos from 2012 when it was revealed and in one interview Elon Musk mentioned that the Model X has an amazing turning radius, comparable to a MINI.

Everything I've learned about Tesla leads me to believe that they wont make the car more mechanically complicated than it needs to be... well except for Falcon Wing :biggrin: doors but that's not drive train. I remember the video your talking about. I believe the turn radius will be amazing because there's no engine in the way. You can shape the inside of the wheel well how ever you want.

But a word of caution the prototype & production Model X's two different things all together.

 

[AnxietyRanger]

4wheel steering goes as far back 1987, via Honda. Concept was good, yet played out so subtle, that consumer demand never took hold. And has been dropped by nearly (if not) all manf.

Steering - Wikipedia, the free encyclopedia

This is definitely not true globally, although I guess it is possible this feature has been discontinued in America?

Four-wheel steering (turning opposite in slow speeds ands slightly with the fronts in higher speeds) has been making a big comeback on many European premium brands in recent years, at least in Europe.

For example the new Audi Q7 has it (previous Q7 was named as a competitor to the Model X by Tesla) - also in the U.S. it seems:

http://www.audiusa.com/newsroom/news/press-releases/2014/12/the-new-audi-q7-sportiness-efficiency-premium-comfort

 

[hobbes]

Hasn´t Tesla always said that the MX would be built on the same platform as the MS? IMHO that would rule out any changes but some kind of torque vectoring (no matter how it is achieved) which is basically software. That would mean small improvements to handling under speed but not a decrease in turning radius at low speeds. Unlikely to me that they put the dual motor already into the S to have the technology proven when the X comes out and then change something like making the rear wheels steer. Also, none the prototypes that were sighted showed that feature.