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Quad motor vs locking diff

Biturbowned

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I’m reserving judgement until I see what Mercedes does with the EQG.
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Again, we agree, the LC “currently” has an advantage. I believe the rivian quad motor technology will catch up and surpass differentials in the very near future. time will tell
So, I have to admit, I thought so too. But, like you, I did not think it through enough. Now what I have on-the-ground experience with it, I will walk you through some of the scenarios that are happening.

Located at 7777 Galice Road in Merlin, Oregon is a 4x4 park where all the "forum wisdom" is washed away and real 4x4s gather to negotiate real obstacles on the ground. These obstacles are a mix of real-world overlanding experiences, and some man-made obstacles that test a vehicle's ability in a variety of situations. We have had a number of R1Ts out there now, including mine, and they do not fare well. Some of that is weight; soft surfaces are not their friend. But it came as a genuine surprise to me at how often it is the quad motor design that stops them.

For purposes of this discussion, lets call your standard Jeep/Bronco/Yoda old-school 4x4s the "classics". The classics typically have 3 differentials that are controllable (lockable) and a power source. It can be gas, diesel or electric, this is not a EV vs. the rest thing. What turns out to be the overreaching advantage of the classics can be summed up with three areas of control;
  1. Torque to the wheels
  2. Rotational speed control of the wheels
  3. Torque vectoring control
What I did not understand before, is that you lose #2 in the quad motor design, and you lose a critical element of #3.

TEST #1: The bolder-filled creek

Place classic 4x4 on 4 large boulders, one under each wheel. Once the vehicle is atop the boulders, remove support from one of the boulders, meaning it will roll freely if the wheel attempts to put torque on it.
-Instructions to driver: 4-wheel locked mode, apply enough power to the wheels to get off the rocks.
-Result: Torque is routed to all wheels equally through the transfer case and drivelines. With the diffs locked, the wheel on the loose bolder will rotate at the exact speed of the other three that are propelling the vehicle forward. The net result is zero torque to the wheel on the loose boulder, and it does not slip or "spin" that bolder out of position.

How does the vehicle know to put zero torque on that wheel? It does not, it is the simple physics of having all wheels driven at the same rotational speed. In the creek crossing scenario that we just demonstrated, as each wheel in ln turn encounters these loose/slippery rocks, no spinning occurs, the wheels with traction continue to drive the vehicle forward, even if the wheels with traction swap places 10 times during the crossing.

Place quad-motor vehicle on the same obstacle.
-Instructions to driver: Apply enough power to crawl off the obstacle.
-Result: Since no physical connection exists between the wheels, the vehicle can only control torque; trying to adjust as quickly as possible once they start to spin. As was demonstrated perfectly in Kyle's video, the wheel on the rolling bolder will spin, kicking that bolder up under the well and causing a potential hang affect while immediately falling into the hole the bolder was just pulled out of. This will be repeated with every loose/slick bolder the truck encounters in the crossing. The result is a MESS, and with no way to lock rotational speeds, is not curable with any amount of software.

TEST #2: The side hill

This trail is an off-camber approach to the creek. It is very steep, and although kind of a gut-check, I have seen even cars like Subarus and Ford Explores make it just fine. It is kryptonite for my R1T Here is why:

-Instruction to driver: Select 4-wheel-drive, first gear, traction control off, DIFFERENTIALS UNLOCKED! it is very steep, but whatever you do, do NOT touch the brakes! Aim for the opening in the trees at the bottom, and trust the vehicle.
-Result. Providing the driver does not touch the brakes, the left side tires tend to dig into the downhill side of the camber and just roll, as all torque from the driveline is transferred automatically to the wheels with less traction (in this case the uphill side). These free-wheeling wheels provide the lateral and directional control needed to make it to the bottom without sliding off the trail. If the driver freaks out and touches the brakes, those left side tires stop rotating and the hapless vehicle immediately slides off the trail and into the brush.

Climbing back out is pretty much just the reverse. People attempting the obstacle with a locked diff just slid off the trail. Your torque-vectoring open differential is your friend on these off-camber trails.

At least, this is how the scenario plays out when driving a classic. A quad-motor machine has no way to "loose" its downhill traction wheels. There is no way of applying the slowing forces to just the uphill wheels, and it IMMEDIATELY slides off the trail. Same problem trying to climb back out. There is not way to keep torque from going to the wheels needed for lateral control, and away she goes sideways down the hill.

No, software can't fix this. There is no way for the vehicle to react to the surface the way a differential does. It is impossible for the truck to know which wheels happen to be on the more slippery surface the way a differential-equipped vehicle can.
 
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I think this is a mostly silly argument honestly. Locking diffs may be better for a very specific set of circumstances that a Rivian driver may encounter. For many of the other use cases that the Rivian is capable of, the quad motors are a massive improvement over a solid axle.

Quad motors allow for optimum flexibilty depending on use case. Sure that makes the Rivian more of a Jack of All Trades, but that's also what makes it so unique.
Well... see, this just turns out not to be the case when you get out of the forum and onto to trail. I have the keys to the off-roading park the tests above were performed at, and I am retired; meaning I can meet you there pretty much any day of the week to allow you to put your own vehicle to the test.
 
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Unless I’m misunderstanding your explanation, the crux of the issue is that the quad motors work independently until slip is detected on one or more then power is pulled from the slipping wheel(s). This means the rotation of the 4 tires doesn’t stay in perfect sync as they would on a vehicle with 3 locking diffs. Is that right?
Edit:typo
That is one of the symptoms of having no physical link/control between the wheels, yes. Refer to the "creek test' above.
 

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As an owner of a Jeep Rubicon with lockers, you do have a point here. However, I don't plan to take a new Rivian into the kind of rocks where this makes a difference.

But, on slippery side slopes, like a banked road covered in snow, the lockers can be a detriment. When both tires turn and slip, there's nothing to hold the axle from sliding sideways. Sometimes open diffs do a little better in situations like that because the non rotating tire offers some resistance to the axle sliding sideways.

How does the quad motor handle that traction situation?
 

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So, I have to admit, I thought so too. But, like you, I did not think it through enough. Now what I have on-the-ground experience with it, I will walk you through some of the scenarios that are happening.

Located at 7777 Galice Road in Merlin, Oregon is a 4x4 park where all the "forum wisdom" is washed away and real 4x4s gather to negotiate real obstacles on the ground. These obstacles are a mix of real-world overlanding experiences, and some man-made obstacles that test a vehicle's ability in a variety of situations. We have had a number of R1Ts out there now, including mine, and they do not fare well. Some of that is weight; soft surfaces are not their friend. But it came as a genuine surprise to me at how often it is the quad motor design that stops them.

For purposes of this discussion, lets call your standard Jeep/Bronco/Yoda old-school 4x4s the "classics". The classics typically have 3 differentials that are controllable (lockable) and a power source. It can be gas, diesel or electric, this is not a EV vs. the rest thing. What turns out to be the overreaching advantage of the classics can be summed up with three areas of control;
  1. Torque to the wheels
  2. Rotational speed control of the wheels
  3. Torque vectoring control
What I did not understand before, is that you lose #2 in the quad motor design, and you lose a critical element of #3.

TEST #1: The bolder-filled creek

Place classic 4x4 on 4 large boulders, one under each wheel. Once the vehicle is atop the boulders, remove support from one of the boulders, meaning it will roll freely if the wheel attempts to put torque on it.
-Instructions to driver: 4-wheel locked mode, apply enough power to the wheels to get off the rocks.
-Result: Torque is routed to all wheels equally through the transfer case and drivelines. With the diffs locked, the wheel on the loose bolder will rotate at the exact speed of the other three that are propelling the vehicle forward. The net result is zero torque to the wheel on the loose boulder, and it does not slip or "spin" that bolder out of position.

How does the vehicle know to put zero torque on that wheel? It does not, it is the simple physics of having all wheels driven at the same rotational speed. In the creek crossing scenario that we just demonstrated, as each wheel in ln turn encounters these loose/slippery rocks, no spinning occurs, the wheels with traction continue to drive the vehicle forward, even if the wheels with traction swap places 10 times during the crossing.

Place quad-motor vehicle on the same obstacle.
-Instructions to driver: Apply enough power to crawl off the obstacle.
-Result: Since no physical connection exists between the wheels, the vehicle can only control torque; trying to adjust as quickly as possible once they start to spin. As was demonstrated perfectly in Kyle's video, the wheel on the rolling bolder will spin, kicking that bolder up under the well and causing a potential hang affect while immediately falling into the hole the bolder was just pulled out of. This will be repeated with every loose/slick bolder the truck encounters in the crossing. The result is a MESS, and with no way to lock rotational speeds, is not curable with any amount of software.

TEST #2: The side hill

This trail is an off-camber approach to the creek. It is very steep, and although kind of a gut-check, I have seen even cars like Subarus and Ford Explores make it just fine. It is kryptonite for my R1T Here is why:

-Instruction to driver: Select 4-wheel-drive, first gear, traction control off, DIFFERENTIALS UNLOCKED! it is very steep, but whatever you do, do NOT touch the brakes! Aim for the opening in the trees at the bottom, and trust the vehicle.
-Result. Providing the driver does not touch the brakes, the left side tires tend to dig into the downhill side of the camber and just roll, as all torque from the driveline is transferred automatically to the wheels with less traction (in this case the uphill side). These free-wheeling wheels provide the lateral and directional control needed to make it to the bottom without sliding off the trail. If the driver freaks out and touches the brakes, those left side tires stop rotating and the hapless vehicle immediately slides off the trail and into the brush.

Climbing back out is pretty much just the reverse. People attempting the obstacle with a locked diff just slid off the trail. Your torque-vectoring open differential is your friend on these off-camber trails.

At least, this is how the scenario plays out when driving a classic. A quad-motor machine has no way to "loose" its downhill traction wheels. There is no way of applying the slowing forces to just the uphill wheels, and it IMMEDIATELY slides off the trail. Same problem trying to climb back out. There is not way to keep torque from going to the wheels needed for lateral control, and away she goes sideways down the hill.

No, software can't fix this. There is no way for the vehicle to react to the surface the way a differential does. It is impossible for the truck to know which wheels happen to be on the more slippery surface the way a differential-equipped vehicle can.
So in your example of the vehicle on the slippery hill. What would be the difference if the driver had an “advanced” setting where he/she could turn off individual motors? If the driver turned off the wheel. The other wheel on the “axle” would do whatever you want it to do by controlling with your throttle. You could theoretically turn off all but the one wheel if you wanted to.
 
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We get it, you much prefer your locking diffs over the quad motor setup.

I've watched literally every video Kyle has done on the T including some of the hours long live streams. The conclusions you can draw from not only those, but this singular video as well, is that the drivetrain characteristics have advanced dramatically from his first drive a year prior to the attempts in the video, and on top of that, to what they are as of today. Rivian is literally the only company building a 4 motor drive vehicle for the consumer, and as such, deserves a little leeway in getting that new tech dialed in.

I know. Locking diff is easy, quad is hard. Just furthering the discussion (because that's what we are doing here, not gettin all heated) watch/listen to the MotorTrend interview with Max Koff. To his credit I wrote him off as a legacy leftover of the Rivian old guard, but he has some decent credentials/background in vehicle development etc. Anyway, while quad motor is at a disadvantage currently in the odd use case, it's superior in its ability to deliver a far quicker precise reaction to changing terrains. https://www.rivianforums.com/forum/...n-the-inevitable-podcast-by-motortrend.11979/

I'll help you with your rebuttal "but full lockers will have perfect reaction time because all wheels turn at the same speed regardless of Mu etc" 😜
Well, see, now you are being silly. What I "prefer", is to make it through the overland trail. I am a big fan of technology, so I am usually the first to jump on the next new thing and try it out; hence the R1T I am driving.

To be fair, the quad-motor does great at places like Moab. Wet, sloppy Oregon is for sure not its fortey. However, refer to the tests above that we have done for details on where and why the quad-motors are failing to negotiate the obstacles.

So, at the end of the day, as much as I like new tech; only if it ends up being better. The R1T is failing to negotiate pretty basic obstacles, so no. The real world is throwing cold water on what i had thought was going to be great tech.
 

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Well... see, this just turns out not to be the case when you get out of the forum and onto to trail. I have the keys to the off-roading park the tests above were performed at, and I am retired; meaning I can meet you there pretty much any day of the week to allow you to put your own vehicle to the test.
It could be possible, just very remotely possible, that I was referring to things not involving a trail. I'd bet greater than 50% of Rivian owners will never take their vehicle off-road, let alone on a trail that requires a locking diff, and are much happier with 4 corner torque vectoring.
 
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As an owner of a Jeep Rubicon with lockers, you do have a point here. However, I don't plan to take a new Rivian into the kind of rocks where this makes a difference.

But, on slippery side slopes, like a banked road covered in snow, the lockers can be a detriment. When both tires turn and slip, there's nothing to hold the axle from sliding sideways. Sometimes open diffs do a little better in situations like that because the non rotating tire offers some resistance to the axle sliding sideways.

How does the quad motor handle that traction situation?
Awful. Simply awful. See "test #2" above.

This is why not only is the vehicle _not_ a great off-roader, but it is downright evil even on the road under slippery conditions. Snow Mode helps, but as you pointed out; without diffs to instantly adjust your traction from side to side, this thing is like Bambi on ice.
 
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It could be possible, just very remotely possible, that I was referring to things not involving a trail. I'd bet greater than 50% of Rivian owners will never take their vehicle off-road, let alone on a trail that requires a locking diff, and are much happier with 4 corner torque vectoring.
Sure, I did not really buy the vehicle for that. I just could not help putting it through the tests... just so I know. I have a '99 Jeep TJ for actual off-roading.

To your point, however, test #2 has great relevance for on-road performance in slippery conditions... which is why I will be trading "up" to a dual-motor version when available.
 

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If indeed you are in that camp, then you don't quite understand the mechanics at work.
...
Again, if physically locked together with large gears and drive shafts, there is no possible way for one wheel to break free in that manner...
As soon as you turn the steering wheel, it is no longer possible for a full locking dif to maintain traction on all 4 wheels. Turn navigate a corner, each wheel must rotate a different amount. Your understanding of the mechanics is limited to a straight line! Ridiculous!

I too am in the camp that the 4 motors will have more advantages than limitations in the over-all driving experience. Hands down...
 
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So in your example of the vehicle on the slippery hill. What would be the difference if the driver had an “advanced” setting where he/she could turn off individual motors? If the driver turned off the wheel. The other wheel on the “axle” would do whatever you want it to do by controlling with your throttle. You could theoretically turn off all but the one wheel if you wanted to.
That has been talked about... _a lot_. Here is where we are on that thought:

_If_ we could manually "freewheel" the downhill motors we could simulate what the diff is doing and make it work. Figuring out how to tell how fast the uphill motors are going would be real tricky (engine RPM is how the classics are doing it), but it would probably work.

The thing is, i think a dual-motor R1T would roll right down it, and probably climb out with ease because it has two differentials. Turn traction control off and go for it. Since you are also dropping weight and complexity with a dual-motor version, if seems like a no-brainer to me.
 
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As soon as you turn the steering wheel, it is no longer possible for a full locking dif to maintain traction on all 4 wheels. Turn navigate a corner, each wheel must rotate a different amount. Your understanding of the mechanics is limited to a straight line! Ridiculous!

I too am in the camp that the 4 motors will have more advantages than limitations in the over-all driving experience. Hands down...
I spend an average of 50 to 60 hours a week off-roading, and have been doing it since the 1970s. I have built dozens of rigs, although my back has become a limiting factor in the heavy work. I also have the keys to the gate of the local off-roading "show off" park. Being retired I can meet you there pretty much any day of the week for you to come and demonstrate for me the folly of my ways.

But to your point, my Jeep has ECTED diffs. Each diff has a dial on the dash. I can dial each diff from fully open to fully locked, and anything in between. It is common to square up to a steep obstacle while fully locked for maximum traction at all four corners, only to reach up and flip the diffs to "limited slip" when getting to the top and wanting to negotiate a tight turn.

Yessir, I am quite familiar with how diffs work on the ground; locked and otherwise.
 
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That has been talked about... _a lot_. Here is where we are on that thought:

_If_ we could manually "freewheel" the downhill motors we could simulate what the diff is doing and make it work. Figuring out how to tell how fast the uphill motors are going would be real tricky (engine RPM is how the classics are doing it), but it would probably work.

The thing is, i think a dual-motor R1T would roll right down it, and probably climb out with ease because it has two differentials. Turn traction control off and go for it. Since you are also dropping weight and complexity with a dual-motor version, if seems like a no-brainer to me.
Apologies to make you re-tread old ground there (pun intended), I guess this forum moves faster than I do. Genuinely appreciate the real world experience. Also no argument at all about the lower weight of the dual motor being a benefit.
 
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Apologies to make you re-tread old ground there (pun intended), I guess this forum moves faster than I do. Genuinely appreciate the real world experience. Also no argument at all about the lower weight of the dual motor being a benefit.
No worries at all. Love me some forward thinking tech, and was _really_ looking forward to playing with this quad-motor stuff. It is not without merit, for sure. Max power and traction on dry pavement, this thing is nuts! Want to beat Corvettes in the quarter mile? The quad-motor is for you!

I was just seeing way too much miss-information on the forums about the "off-roading" capabilities of this machine. Sadly, a lot of it is from people that are not even driving the truck yet.

I am extremely excited about the dual-motor version. My use case is not for drag racing, I am more interested in the vehicle being manerly on slippery roads and in light off-roading. Based my experience, the dual-motor will be better on both... is less money, is lighter, and should be more efficient. I want people to know these things, so they can choose wisely; without guys that don't even drive the truck yet loudly touting capabilities that are inaccurate.
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