Quad Motor Better Than Lockers...?

[Blakejakely]

The new Rivian quad motor page has a section where they mention their new "Quad-motors offer substantially better torque control than locking differentials while also being instantly adjustable for on-road performance". As a gen1 owner, I appreciate if they fixed this, however I really would like them to actually show us. Its pretty clear from current owners that gen1 had a lot of promise that it underdelivered on in terms of offroad torque vectoring. Don't get me wrong, my gen1 quad gets me everywhere I need it to, but there is still a ton of wheel sleep even on small rocks. When do we get quad motor offroad videos actually demonstrating this against current offroad competition with lockers? Its quite a bold claim to make without evidence

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[TripleB]

More control the better

 

[Dark-Fx]

I'd totally agree that they have better torque control than a locking differential. Locking solves nothing that can't be solved with individual motor control, especially if your control loop encourages the motors to spin at the same rate irrespective of tire slip. As long as your torque available is enough to propel the vehicle if only a single wheel has traction. I'd guess the updated quad does.

 

[Blakejakely]

I'd totally agree that they have better torque control than a locking differential. Locking solves nothing that can't be solved with individual motor control, especially if your control loop encourages the motors to spin at the same rate irrespective of tire slip.

This isn't the case with gen1 though, the torque vectoring is too slow to handle this in practice. You have to gun it with the Bosch motors, which defeats the whole purpose of trying to have control offroad. Going offroad with 4x4 ICE truck friends with lockers its clear they have more control at slower speeds than gen1 quad motor Rivians do. In theory this is correct, but in practice its not

 

[Dark-Fx]

This isn't the case with gen1 though, the torque vectoring is too slow to handle this in practice. You have to gun it with the Bosch motors, which defeats the whole purpose of trying to have control offroad. Going offroad with 4x4 ICE truck friends with lockers its clear they have more control at slower speeds than gen1 quad motor Rivians do. In theory this is correct, but in practice its not

Rivian's engineers know where their shortcomings are in the Bosch system. If they haven't considerably reduced them with their in-house system, I will be surprised.

 

[Davethadog]

Rivian seems more focused on higher speed rally type stuff than on wheeling and slower off-road stuff.

The other part of why your buddy’s have better control has to do with low-range gear selection. Torque multiplying gives way more granular control such that even if Rivian were able to match the final output of torque compared to a traditional 4:10 or whatever gearbox and low range gear set, they would have a tougher time modulating it. It’s why stellantis has been developing a two speed EV gearbox.

IMO the dual performance motor with brake controlled wheel spin modulation is the sweet spot for these trucks right now. It also has the best hope of potentially adding a locker at a later date but given the low volume of these trucks and the complex software I won’t hold my breath.

 

[Blakejakely]

Rivian seems more focused on higher speed rally type stuff than on wheeling and slower off-road stuff.

The other part of why your buddy’s have better control has to do with low-range gear selection. Torque multiplying gives way more granular control such that even if Rivian were able to match the final output of torque compared to a traditional 4:10 or whatever gearbox and low range gear set, they would have a tougher time modulating it. It’s why stellantis has been developing a two speed EV gearbox.

IMO the dual performance motor with brake controlled wheel spin modulation is the sweet spot for these trucks right now. It also has the best hope of potentially adding a locker at a later date but given the low volume of these trucks and the complex software I won’t hold my breath.

They did have the patent for a low range gear box which was one of my hopes to avoid the wheel spin issue. Maybe they figured it out without that though if they're making claims of this magnitude

 

[Davethadog]

Yeah I hope so too but there’s also a difference between torque control and performance. I don’t care how well you can control the torque if it’s insufficient to put the vehicle where I want to put it.

Individual motors will obviously offer better control than a locking differential where the ground has more influence than you do, that’s not a hard claim to make. But will it help actually control the vehicle precisely on difficult terrain? Jury remains out.

 

[freshpow]

The new Rivian quad motor page has a section where they mention their new "Quad-motors offer substantially better torque control than locking differentials while also being instantly adjustable for on-road performance". As a gen1 owner, I appreciate if they fixed this, however I really would like them to actually show us. Its pretty clear from current owners that gen1 had a lot of promise that it underdelivered on in terms of offroad torque vectoring. Don't get me wrong, my gen1 quad gets me everywhere I need it to, but there is still a ton of wheel sleep even on small rocks. When do we get quad motor offroad videos actually demonstrating this against current offroad competition with lockers? Its quite a bold claim to make without evidence

Paging @R.I.P.

 

[AlexF]

Here we go again!
https://www.rivianforums.com/forum/threads/quad-motor-vs-locking-diff.12010/

 

[zefram47]

The Gen2 QM will be better than the Gen1 QM off-road because they improved the stall-torque and thermals of the new motors. But at least the way that they calibrated the drive system on the Gen1, it's absolutely *not* better than locking diffs. If you're in terrain with different grip levels (split-mu) the vehicle has a hard time figuring out where to put power down. I have been in situations where two wheels aren't in contact with the ground and the other two wheels have split-mu and the truck just wouldn't move at all even though my foot was to the floor. With locking diffs and a standard 4x4 setup that is just not a situation that would happen. You'd either start moving or all the wheels would rotate at least.

 

[R.I.P.]

Paging @R.I.P.

It would be pretty cool if they "fixed it".

But even the verbiage of their claim leaves me with significant doubt... ?

 

[Glembi2]

The comment from someone a while back that changed my mind (possibly @R.I.P. ) was that each wheel of the QM is fixed on the max torque it can apply while the DM shares that torque between the pairs of wheels.

So with three wheels slipping, the one wheel with grip in the QM can only apply the torque from that motor = roughly 900/4 ft lbs.

The DM in the same situation brakes the spinning wheel on the axle with the wheel with grip (the virtual locker situation) and all torque from that motor goes to the wheel with grip = roughly 900/2 ft lbs (or whatever the DM puts out).

I hope I’m never in a situation with only one wheel with grip and yes this is an edge case but at least here, the DM would go while QM would be squirrelly. Can the QM be tuned to provide a virtual limited slip diff? I would think so.

 

[Dark-Fx]

The Gen2 QM will be better than the Gen1 QM off-road because they improved the stall-torque and thermals of the new motors. But at least the way that they calibrated the drive system on the Gen1, it's absolutely *not* better than locking diffs. If you're in terrain with different grip levels (split-mu) the vehicle has a hard time figuring out where to put power down. I have been in situations where two wheels aren't in contact with the ground and the other two wheels have split-mu and the truck just wouldn't move at all even though my foot was to the floor. With locking diffs and a standard 4x4 setup that is just not a situation that would happen. You'd either start moving or all the wheels would rotate at least.

Yeah I'd suggest anyone doubtful here read this Rivian patent. The parts about "target wheel speed" is why the quad could be better than the dual motor. It doesn't matter much if 3 of your wheels are spinning and one has traction, if you can react quick enough to the wheel speeds not being as expected, and you have enough torque at any single wheel to propel the vehicle forward.

https://ppubs.uspto.gov/dirsearch-public/print/downloadPdf/11932117

And if Rivian is using this same inverter tech in the dual motor, it could contribute to why others might find the open diff dual motor superior in certain situations.

 

[IGR]

The comment from someone a while back that changed my mind (possibly @R.I.P. ) was that each wheel of the QM is fixed on the max torque it can apply while the DM shares that torque between the pairs of wheels.

So with three wheels slipping, the one wheel with grip in the QM can only apply the torque from that motor = roughly 900/4 ft lbs.

The DM in the same situation brakes the spinning wheel on the axle with the wheel with grip (the virtual locker situation) and all torque from that motor goes to the wheel with grip = roughly 900/2 ft lbs (or whatever the DM puts out).

I hope I’m never in a situation with only one wheel with grip and yes this is an edge case but at least here, the DM would go while QM would be squirrelly. Can the QM be tuned to provide a virtual limited slip diff? I would think so.

I recall in one of the video interview during gen 2 launch, it was said that for QM individual combined power for each motor is greater than vehicles total power, which makes me believe in those off road scenarios you would putdown more than just 1/4 of vehicles total power.
If that's the fix, they are just making it slightly better imo.

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