Dual vs Quad motor pros and cons?

[SoCal Rob]

You can simulate a locking diff with brake pressure on the spinning wheel.

Yes, sorry, that’s what I was referring to when I wrote about open diffs front and rear with traction control.

We had a 2001 Land Rover Discovery 2 that used a setup like that, except with 1 engine there was also a locking center diff. The traction control on ours worked really well. That was with primitive tech compared to what Rivian will be using if they choose to go that route so I imagine Rivian’s implementation will be even better.

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[thrill]

I don't know what the actual power curve is, but usually at very low speeds you are restricted on the total power you can put into a motor since the eddy currents build up pretty quickly.

As I remember from long ago studies, the eddy currents are not going to be a problem until you reach high RPMs. There is efficiency loss at zero-to-low RPM die to I^2R high-current and low-voltage but the large size of the motors and the active cooling is how to manage that. There's no loss of precision in the control that I know about.

 

[crashmtb]

Yes, sorry, that’s what I was referring to when I wrote about open diffs front and rear with traction control.

We had a 2001 Land Rover Discovery 2 that used a setup like that, except with 1 engine there was also a locking center diff. The traction control on ours worked really well. That was with primitive tech compared to what Rivian will be using if they choose to go that route so I imagine Rivian’s implementation will be even better.

I wish the original owners of my Defender had optioned ABS; that includes the same traction control system as your disco. Also anti-stall.

ABS-based systems like that are made even better with a helical LSD in the mix, as the brake torque will automatically move power to the wheel with grip.

 

[Bee]

I don't understand this thinking. Any *single* wheel on the Rivian can deliver 200+ HP and so has enough power to lift the entire vehicle 15 feet per-second (10mph straight up). A quad motor design with a control system can put *exactly* the required power to each wheel to the point of slippage. If all four wheels are slipping then it's not a question of power but one of technique in delivering continuously rotating force to capture what traction each grain of sand can deliver, rather than modulating it such as is normal on a firm(er) surface. That's a question of the quality of the "sand mode" software. Software can be improved to the limits of the hardware, and the only limits are the tire design and size and the frequency of the sensors of the weight on and rotational speed of each wheel. I think they're still a ways from maximizing the best application of force to each wheel, but you need individually controlled wheels to do that in the first place. With a locking system you are limited to a transfer of power from one wheel to another, and as stated, the amount of power is not a limitation. The only "thing" a locking system gives you is the ability to manually spin the wheels by applying too much power, which is basically what the sand mode gives you (not that I've driven it).

Spend a couple hours watching R1Ts off-road and you'll see them stuck without wheels spinning and general complaints that if the driver had a mechanical locked differential they would know what was going on at the wheels with confidence while here you're kind of just guessing.

Every YouTuber that has offroading chops has expressed the lack of connection to what wheels are spinning and is mentioned immediately when trucks were first starting to get into their hands. Kyle from Out of Spec says it's improved, I'm sure others here can chime in and explain how it's changed over time and how it feels now vs. then but from my understanding, it's not quite there yet still.

I'm personally watching it closely, to be frank, if they don't sort this and it's just a fat heavy SUV with no great options for diffs then I'll actually not be getting an R1S and R1T and instead will be getting a 4xE. I'll sign up for tradeoffs and being apart of new and exciting but if it's just not good, then it's just not good.

 

[SoCal Rob]

Spend a couple hours watching R1Ts off-road and you'll see them stuck without wheels spinning and general complaints that if the driver had a mechanical locked differential they would know what was going on at the wheels with confidence while here you're kind of just guessing.

Every YouTuber that has offroading chops has expressed the lack of connection to what wheels are spinning and is mentioned immediately when trucks were first starting to get into their hands. Kyle from Out of Spec says it's improved, I'm sure others here can chime in and explain how it's changed over time and how it feels now vs. then but from my understanding, it's not quite there yet still.

I'm personally watching it closely, to be frank, if they don't sort this and it's just a fat heavy SUV with no great options for diffs then I'll actually not be getting an R1S and R1T and instead will be getting a 4xE. I'll sign up for tradeoffs and being apart of new and exciting but if it's just not good, then it's just not good.

I haven’t gotten to drive an R1 off road yet, but I wonder if a lot of this comes down to technique.

Our Land Rover has computer controlled locking diffs center and rear plus traction control in front. It seems to work best with light to moderate throttle which remains steady. If there is no progress then gently ramping up the throttle gets things moving. I think this gives the computer time to determine where there is traction and where there isn’t, then adjust the diffs and front brakes to optimize power delivery. Newer Land Rovers have all terrain progress control (ATPC) which is like very low speed cruise control where you can set a speed and let the vehicle apply as much throttle as is required to maintain that speed.

I would expect a Rivian to adapt faster, but I suspect there is still a delay. Plus if people see no progress and let off the throttle then they’re working against the vehicle by not giving it enough power to the appropriate wheel(s). Something like Land Rover’s ATPC would help folks who aren’t used to this driving style. Because there is no engine noise, it might be nice if a Rivian system like this displayed the percentage of throttle being used to help us understand what it takes to get results.

 

[mkg3]

The thing most people don't take into an account is that there are infant mortality issues with any new design.

The current Bosch motor has been in the field for many months and even years running inside the Rivian testbeds as well as vehicles delivered. At this point, it has actual field use millions of miles.

The new Rivian designed motor has little to no real world milage other than test mule that the motor must be in now. It will take couple of years of being installed and used by dual motor buyers before it can catch up to anywhere near the miles Bosch motor has. During that time, things will break and design will have be modified and so on to build more reliable motor.

So, my take is that the dual motor is really in its infancy and I would not want to be an early adopter of it. By the time R2 series are rolling off the assembly line, it should be fine but for now, no thanks. Just take a look at the early R1T (and even R1S) delivered vehicle issues. Similar thing can be expected with the motor (not the same problems but problems due to its newness).

 

[lefkonj]

Since we know nothing except the dual motor will be cheaper it is impossible to know. When they release details, I am sure the discussions will happen but until then it is pure guess work by anyone stating 'Dual motor does XXXX'

 

[Madsen203]

Dual motor should be better off road as they can put 300hp to a single wheel vs 200 they can with individual motor when one wheel doesn’t have traction or is in the air. Similar benefit of Heep with solid axle.

If they can manage traction control system, dual will be a better set up than what 4 motors currently are.

 

[BillArnett]

Dual motor should be better off road as they can put 300hp to a single wheel vs 200 they can with individual motor when one wheel doesn’t have traction or is in the air. Similar benefit of Heep with solid axle.

If they can manage traction control system, dual will be a better set up than what 4 motors currently are.

It's extremely rare that 300hp vs 200hp to one wheel is actually useful. But given the possibility the U-joints and axles must be beefier and the possibility of breakage goes up.

 

[MountainBikeDude]

Torque vectoring would take a hit, as would the life of your brakes compared to a QM. The positive is potential for a fully locking differential with the DM.

 

[BillArnett]

It seems to me that it should be easy enough to implement a "software locker": just control each wheel so that they all turn at the same rate. Better yet, not quite the same rate based on the steering angle. (If you've ever driven locked up on slick rock or pavement you know how awful that can be.)

 

[Madsen203]

It seems to me that it should be easy enough to implement a "software locker": just control each wheel so that they all turn at the same rate. Better yet, not quite the same rate based on the steering angle. (If you've ever driven locked up on slick rock or pavement you know how awful that can be.)

The locker means more power to one wheel. Software locker doesn’t provide same multiplying effect.

 

[SoCal Rob]

It's extremely rare that 300hp vs 200hp to one wheel is actually useful. But given the possibility the U-joints and axles must be beefier and the possibility of breakage goes up.

Yeah, I think a lot of us are going to have to learn some new habits and techniques, especially if coming from an ICE-powered vehicle with manually locking diffs, solid axles, and fixed height suspension.

Just typing this makes me think that Rivian or one of the content providers should make a video showing how to setup the vehicle (air down or not, regen setting, suspension height, and soft or firm ride) for different situations (road surface, weather, grade, smoothness) and how to drive once properly configured.

 

[AlexF]

Braking one wheel with an open differential is not the same as a locker. You’ll still only get a maximum of 50% torque to the wheel with traction. It may not be intuitive but this article does a decent job explaining it.
With a torquey electric motor it may not be much of an issue, but it’s still not as good as a mechanically locked differential.

 

[BillArnett]

Again, with 200hp per wheel there's no need for more.

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