Quad motor vs locking diff

[SoCal Rob]

There's more than a few "next level" maneuvers possible with quad motor and four wheel independent suspension and air struts with hydraulic anti-roll … as someone with a background in racing, not that I'm a chassis engineer, the suspension of the R1 is truly impressive and wildly too expensive for anyone to build independently … I can only hope that Rivian sponsors racers (they've done a couple) and encourages an enthusiast community of aftermarket upgrades for these vehicles to become legendary performers, not just goofy grocery getters for the lucky few.

This is what fuels my enthusiasm about the possibilities. As far as I know, this is the first production off-road vehicle with this mix of drivetrain and suspension capabilities. I just don't understand how anyone can legitimately say that this mix of hardware cannot do something based on past experience when nobody in the past has experienced this mix of hardware.

I think we all know that Rivian is continuing to make software improvements which use existing hardware differently or in more sophisticated ways. A year ago we could have had a similar debate about (admittedly trivial) ambient lighting. At that time someone could have said that all of the ambient lights in a Rivian can only be white because that's all they'd seen. Someone else could have speculated that hopefully Rivian was using RGB LEDs instead of simply white LEDs in some locations. Fast forward to Halloween of 2022 and we would have been able to prove that white was a software limitation rather than hardware. It certainly appears that Rivian's hardware has untapped potential in many areas, provided they develop the software to use it. Heck, it wasn't THAT long ago that people were debating whether or not Rivian included hardware for HomeLink-type functionality.

I may sound overly optimistic, but I think that the most reasonable course is to be less negative. At least until the update frequency drops dramatically because the system is more fully-developed.

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[ryanpei]

Great discussion..
i have to say I think R.I.P. is being naive to suggest that electronics cannot overcome this problem. Ever. Full stop.
And if I had to guess and put words in his mouth, I think White Shadow's beef is with that initial dogmatic statement that a full 3X locking diff will always be better. Because....that's just patently not true.
If the statement were "Rivian has a long way to go to match the capability of a triple diff lock setup for many tough off-road, rock crawling or off-camber situations." then I'm all in. I agree.
But to suggest it is not possible to have 4 motors spin at the same rate is ridiculous. There are countless applications of this in existence already. Rivian could mimic a locking diff in a week if they wanted to.
It is, rather, a matter of priority for Rivian. Where do they focus? Where do they put resources? I'm sure they will get there, but as an everyday Rivian driver, I'd prefer them to tweak a number of other issues first! Snow mode, for example; I would have been kinda pissed to have a "locking diff" mode come out before snow mode.

Last point: My R1T will never be TJ, RJ, CJ Etc.
If I wanted a TJ I would have bought one.
But my Rivian has an opportunity for constant improvement after purchase. (see snow mode above)
And I think we can all agree....a TJ will never be a Rivian.

 

[Zoidz]

Right, but if you're limiting the speed of the tire via software to a max of 1mph, does the slip really matter or lack of slip matter? You just slowly dial up how much rotation the wheels are allowed if you need. Elsewise, locking diffs don't prevent a wheel slipping either, they just prevent it overrotating in the event it does slip, which, I believe, my proposal does as well.

EDIT: The point I'm getting at here is that you can remove the need for any feedback and simulate full lockers in this manner. This obviously doesn't help with the simulation of an open diff.

Keep in mind that torque is what causes rotation, and higher torque is needed to move heavier loads. If all 4 wheels are free in the air, they will all spin at the same speed immediately. If all 4 have 100% traction, the vehicle will not move at all until enough torque is applied to move 7000 lbs. Hopefully we all agree on this.

In our slippery slope case, the speed control algorithm very slowly applies equal power and therefore torque, to all four motors with the goal of speed control by ramping up all four wheels from 0 mph to 1 mph max in 3 seconds. But .... the rear left is not touching the ground, the front left is very slippery mud, both on the right side have 100% traction in dirt.

Time=0 sec
No power applied, no motion on any wheels

T=0+ sec
Apply initial low torque=X on all wheels. If you apply way too much they will all spin, right? At low applied torque, the free wheel must start turning immediately because it has zero traction resistance to the ground. But the other 3 wheels have enough traction resistance on the ground that torque=X is insufficient to rotate those wheels. Right out of the gate, we've already failed at true locking diff emulation. How much power do we apply to the wheel that is now spinning? Do we stop it because the others are stopped, or do we let it spin? We can't speed control it to the others, they are not turning and there is no vehicle speed.....

T = 1 sec
Torque has ramped on the other 3 wheels to torque=5X. The front left with low traction now also starts to spin - it needs less torque because there is less resistance. The right side still has good traction but torque=5X is still not sufficient to cause rotation because it is not enough power to move the 7000 lb truck. Do we let the front left spin (again breaking our locking diff goal), or reduce power until it stops? OK, just let it spin and continue the full ramp up to 1 mph.

T= 2 sec
Torque is ramped up to torque=10X and only now the right side has enough torque to starting moving 7000 lbs of dead weight. Now that we know the speed of the right side wheels is starting to ramp from 0 to 1 mph, we can slave speed control the left wheels with low/no traction to match the right side.

T= 3 sec
From 2 to 3 seconds we attempt to speed match the left to the right side.

This is the challenge of software speed matching between differing dynamic loads. As the load changes, appled power must change to maintain the same speed in variable speed applictions.

 

[MXA121]

Anyone questioning this needs to watch some more videos of the R1T struggling in off-road situations that a locked out 4x4 would crawl over easily. (0:25 into this video for ex )

Will electronics ever prevail? Maybe, but not today.
The R1T still manages really well overall, but it's not a hardcore off-roader that some people think it is.

 

[racekarl]

Great discussion..
i have to say I think R.I.P. is being naive to suggest that electronics cannot overcome this problem. Ever. Full stop.
And if I had to guess and put words in his mouth, I think White Shadow's beef is with that initial dogmatic statement that a full 3X locking diff will always be better. Because....that's just patently not true.
[...]
But to suggest it is not possible to have 4 motors spin at the same rate is ridiculous.

You're making the same mistake that many people seem to be making in this thread. A "locking" differential is not the same thing as a "locked" differential. There is more to it than just "making 4 motors spin at the same rate" - that would be emulating a "locked" differential.

R.I.P's observations are more subtle than just "R1T can't spin all 4 motors at the same rate."

 

[R.I.P.]

I'm not qualified to speak on the questions raised in this thread, but a few month ago I came across this:



Engineering Explained on YouTube does a great job in breaking down (in simple terms to morons like me) the engineering and science related to a whole bunch of automotive technology.

The above video suggests that the Rivian's quad-motor setup is the "Holy Grail" of AWD systems.

Just another perspective with some science (instead of anecdotes) to back up his claims.

Great vid, and well explained. It is absolutely this logic that I bought lock-stock-&-barrel when I purchased the Rivian. All of what is explained is why my expectations were so high for the vehicle.

Now having one, finding out how challenged it is in the real world just makes my disappointment all the worse.

In the first vid posted Kyle does a perfect job of showing the system in full defeat mode. The tires with no traction spin helplessly, while the ones with traction fail to provide enough torque to move the vehicle. It is frustrating to see people pile on Kyle with criticism that he "can't drive", and has the "wrong tires". He did that for the video on purpose, to show the failure. You will note the street tires on the G-Wagon you posted, of course those are the wrong tires for off-roading, that is not the point. The point is the mannerly way the G-Wagon crawled over the obstacles with zero drama or spin, as opposed to the flailing the Rivian is prone to; even in the video you just posted.

I _love_ the tech. I bought the vehicle for the tech (it was supposed to replace my F250). The video you posted is _exaclty_ what I expected.

I am far less enthusiastic about the reality.

 

[R.I.P.]

Anyone questioning this needs to watch some more videos of the R1T struggling in off-road situations that a locked out 4x4 would crawl over easily. (0:25 into this video for ex )

Will electronics ever prevail? Maybe, but not today.
The R1T still manages really well overall, but it's not a hardcore off-roader that some people think it is.

Which, really, has been my point. I went up Buckhorn today to check the intake for my hydro. I would love to have not burned the gas I did and taken the Rivian. Unfortunately, even though I often take the old F250 up there, the Rivian flails horribly and scares the hell out of me on that road. There is a cliff on one side, and the R1T often just jumps sideways trying to find traction and that is bad juju on that road.

 

[Inkedsphynx]

Keep in mind that torque is what causes rotation, and higher torque is needed to move heavier loads. If all 4 wheels are free in the air, they will all spin at the same speed immediately. If all 4 have 100% traction, the vehicle will not move at all until enough torque is applied to move 7000 lbs. Hopefully we all agree on this.

In our slippery slope case, the speed control algorithm very slowly applies equal power and therefore torque, to all four motors with the goal of speed control by ramping up all four wheels from 0 mph to 1 mph max in 3 seconds. But .... the rear left is not touching the ground, the front left is very slippery mud, both on the right side have 100% traction in dirt.

Time=0 sec
No power applied, no motion on any wheels

T=0+ sec
Apply initial low torque=X on all wheels. If you apply way too much they will all spin, right? At low applied torque, the free wheel must start turning immediately because it has zero traction resistance to the ground. But the other 3 wheels have enough traction resistance on the ground that torque=X is insufficient to rotate those wheels. Right out of the gate, we've already failed at true locking diff emulation. How much power do we apply to the wheel that is now spinning? Do we stop it because the others are stopped, or do we let it spin? We can't speed control it to the others, they are not turning and there is no vehicle speed.....

T = 1 sec
Torque has ramped on the other 3 wheels to torque=5X. The front left with low traction now also starts to spin - it needs less torque because there is less resistance. The right side still has good traction but torque=5X is still not sufficient to cause rotation because it is not enough power to move the 7000 lb truck. Do we let the front left spin (again breaking our locking diff goal), or reduce power until it stops? OK, just let it spin and continue the full ramp up to 1 mph.

T= 2 sec
Torque is ramped up to torque=10X and only now the right side has enough torque to starting moving 7000 lbs of dead weight. Now that we know the speed of the right side wheels is starting to ramp from 0 to 1 mph, we can slave speed control the left wheels with low/no traction to match the right side.

T= 3 sec
From 2 to 3 seconds we attempt to speed match the left to the right side.

This is the challenge of software speed matching between differing dynamic loads. As the load changes, appled power must change to maintain the same speed in variable speed applictions.

I'm confused, can 100% of torque not be applied at ~0rpm in an electric motor? In that case, you apply maximum torque but control the maximum rotational velocity of each tire to an arbitrarily small number and in this case you've basically simulated a locked diff. If 100% of torque isn't enough to move the vehicle then nothing else really matters

 

[R.I.P.]

Inviting people on the internet to drive their truck to one of the most remote parts of the country to demonstrate how you are wrong is not earnest. Whether or not it is intended, it sounds incredibly snarky to suggest because it is completely detached from the reality in which we live.

I apologize if I offended you, but no, it is not detached from reality. There are a lot or Rivians around here, and a couple of them have come out to play with us. If you live too far away to do that, you may be able to find a local 4x4 club/group to hook into.

 

[R.I.P.]

Soooo, there seem to be a few references in this debate to a classic locking diff setup “knowing” which wheel has traction vs which does not.

How does it know?

I’d really like to know.

It doesn't. It is physics that causes the differential to send torque to the wheel with the least traction (resistance). Water does not need to "know" that it is on a hill to flow down it, it simply follows the rule of nature (gravity has a role in this case).

Using natural physics is a powerful tool. It requires no programing or guesswork, it just happens. If you are on an icy road, the physics of inertia+friction can be your enemy. On my trip to the top of the mountain today in an old F250, the natural, predictable physics of the 4-wheel-drive differential system worked as expected to help me overcome the physics of gravity, inertia, and the snow and ice I encountered. In prior attempts in the R1T (it was supposed to replace the F250), it's quad motors and computer flailed badly trying to figure out and react to the constantly changing traction needs of the four corners. _Terrifying_ when there is a cliff on one side, let me tell you.

 

[ryanpei]

You're making the same mistake that many people seem to be making in this thread. A "locking" differential is not the same thing as a "locked" differential. There is more to it than just "making 4 motors spin at the same rate" - that would be emulating a "locked" differential.

R.I.P's observations are more subtle than just "R1T can't spin all 4 motors at the same rate."

R.I.P: "A traditional four-wheel drive with fully locked diffs turns all four wheels at exactly the same speed. One wheel can never, and this is the point that is so very important NEVER slip. Any slip can mean falling off a rock and impaling your truck. Any slip starts to create holes that you have to dig yourself out of. When it is critical that you don't slip any wheel, four wheels locked into identical rotation speeds can never be approximated by a quad motor or software."


I would suggest that software and a motor at each wheel should be able to emulate any type of differential. Not suggesting it currently does, just that it should have that capability.

 

[SoCal Rob]

Great vid, and well explained. It is absolutely this logic that I bought lock-stock-&-barrel when I purchased the Rivian. All of what is explained is why my expectations were so high for the vehicle.

Now having one, finding out how challenged it is in the real world just makes my disappointment all the worse.

In the first vid posted Kyle does a perfect job of showing the system in full defeat mode. The tires with no traction spin helplessly, while the ones with traction fail to provide enough torque to move the vehicle. It is frustrating to see people pile on Kyle with criticism that he "can't drive", and has the "wrong tires". He did that for the video on purpose, to show the failure. You will note the street tires on the G-Wagon you posted, of course those are the wrong tires for off-roading, that is not the point. The point is the mannerly way the G-Wagon crawled over the obstacles with zero drama or spin, as opposed to the flailing the Rivian is prone to; even in the video you just posted.

I _love_ the tech. I bought the vehicle for the tech (it was supposed to replace my F250). The video you posted is _exaclty_ what I expected.

I am far less enthusiastic about the reality.

I think you may have missed this since I didn't see anything regarding the difference between Kyle's initial attempt and then off-handedly mentioning that he is is going to switch to what he determined to be the most off-roady settings.
: https://www.rivianforums.com/forum/threads/quad-motor-vs-locking-diff.12010/post-271334

I think that some issues are related to additional development needed and some issues are related to people not using the tech as intended. When Land Rover came out with Terrain Response (drive modes) a lot of hard-core people thought turning a little plastic knob connected to an electrical control couldn't possibly do anything like throwing a beefy metal lever connected to linkages... so they didn't use the right drive mode and thus "proved" the little plastic knob didn't do anything. Because they didn't use it.

 

[R.I.P.]

Getting back to Rivians, I think with off-camber loose and muddy surfaces the biggest issue is the ~7,000 lb. weight of Rivians. I say that coming from a ~6,000 Land Rover where the weight was enough to cause a rubble shelf road with a downslope outside curve to start collapsing as we drove on it.

You are not kidding! It can make for a scary combo.

In my case, I don't think the weight would be nearly as much of a handicap if the traction just behaved predictably. I think the same truck, with a dual motor setup and differentials would "dig in" on the downhill side and the weight might even help you. It is the truck deciding to put torque on the wheels that I am depending on to keep me from sliding off the hill that sets disaster in motion.

I have a reservation for a dual-motor R1S, as well as the Denali EV. Both of those vehicles use a traditional differential, and I expect them to be much better in those situations.

 

[R.I.P.]

Because you have access to a park and wheel all week - it would be great for you to make videos of the scenarios and post them here. I think that would help people understand more than the influencer videos that are currently being produced. Just simple A and B between a traditional locked rig and your Rivian.

Ah... the youtube thing.

I did that for a while, my old channel is still up. In the end I realised I just don't like hearing myself talk as much as some of these guys seem to. I think Kyle is doing a better job than I could. This whole thread came off of one of his vids that I just chimed in on and mentioned that I have found the same limitations to the system he has.

 

[ironpig]

Ah... the youtube thing.

I did that for a while, my old channel is still up. In the end I realised I just don't like hearing myself talk as much as some of these guys seem to. I think Kyle is doing a better job than I could. This whole thread came off of one of his vids that I just chimed in on and mentioned that I have found the same limitations to the system he has.

Yeah I get that - I find most YouTube videos too long and not to the point because the are geared towards monetization. I don't blame the creators by any means, but I just rarely find them compelling.

I would love a video of scenarios (like the ones you described) that was just back to back clips. First a traditional rig then the Rivian for each scenario. I think that would be more useful and a little less annoying than having to watch a video that was created to meet the inevitable YouTube ad revenue requirements.

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