Quad motor vs locking diff

[Biturbowned]

Big difference is Mercedes is still planning to give it low range as well. So all the talk of Rivian possibly not having enough starting torque goes out the window. The amount of wheel torque a locked 4x4 can generate in low-range and 1st gear is incredible...but you really need a torque-converter automatic to let the revs come up in order to use it. For example, look at the gearing on something like a Unimog with multiple stages of gear reduction. Those things can climb up steep rocks/hills/tree stumps at idle because they're geared so low.

Absolutely agree.

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[R.I.P.]

@R.I.P.

Btw, on thinking more about the dual-motor vs quad-motor capability, in the event that the lack of wheel spin seen on the quad-motor videos is truly a limitation of the individual motor torque (as it might be under some circumstance based on my math earlier in the thread, but this is not a certain), then a dual-motor system might fare even worse.

A dual-motor system, with one motor per axle, coupled with an open differential at each end, complemented with a brake based traction control system, can at most send 50% of the motor torque to each tire. So unless the motor is more than twice as powerful as the individual motors are, or the axle-ratios are changed, then the total torque available to the tire not-spinning in the videos would be less than what we have in the quad motor setup.

Perhaps I'm misunderstanding the torque delivery characteristics of an open differential (my understanding is that it delivers equal torque [not power] to each tire, regardless of the load on either tire... this applies whether regardless of the load on each tire, or it's source [whether or not the tire is completely locked, spinning in the air, spinning but in full contact with ground, or being artificially slowed by the braking system.])

EDIT: Also, to be clear, I do NOT know what the torque specs of the new dual motor system will be. They have been announced to have less total horse power than the quad motor systems, but this does not exclude the possibility of them still having more torque [particularly at the very low/zero RPM where this situation occurs.] Therefore, a higher torque capable dual-motor system might be able to out perform the quad motor in this scenario.

That is the Rogue River below me, taken from my driveway. It is over 1,500 feet below me, and my steep driveway winds down to it to hit the pavement. Slipping off my driveway could easily be fatal.

I am far less concerned with the Rivian's rock climbing ability than I am with it's ability to keep me on this road when conditions deteriorate. The quad-motor's penchant for freaking out & going sideways is the thing I could not live with.

That is where I hope the dual motor will help me most. I am hoping it will handle more like my Y, which handles these conditions brilliantly BTW.

Does the visual help?

 

[White Shadow]

And, update.

My infantile whining about the vehicle not serving the purpose I purchased it for has had an interesting upshot. I received an offer for it I would be insane to turn down, so, for all my grumbling, it looks like I shall be sans Rivian until my next one gets built.

With more familiar tech in the R1S I ordered (differentials), I am expecting a happier ownership experience.

Back to driving an ICE or a car. <sigh>

I'd love to see a picture of it before it goes. Or better yet, a picture of it on a trail.

 

[R.I.P.]

I'd love to see a picture of it before it goes. Or better yet, a picture of it on a trail.

She did not do well on trails lol, but that has been covered. She is back on her street 22s & all cleaned up. The trails remain the domain of my far more expensive rig:

 

[White Shadow]

100% with you. I’ve been humbled in the past on things which seemed similar at first, but got much more difficult than I could have possibly thought. I’m sure that the Mercedes engineers will be benchmarking their current G when they’re working on the EQG. I have to assume those guys know what they’re doing and that there is a good argument for quad motor over dual with lockers.

edit: I still haven’t convinced myself that quad motor is
Inherently inferior to lockers for all of the cases mentioned in this thread.

It doesn't have to be inferior if it's set up right. Think about it---there's absolutely no more capability possible then you can get with an individual motor powering each wheel. But again, it's all about how it's set up to operator under various different conditions.

That said, a vehicle like the Mercedes G-Wagon is a rare breed, factory equipped with front, center, and rear locking capability. There are very few vehicles produced today with that type of capability. A quad motor Rivian will obviously never have true mechanical lockers of any kind because of individual motors controlling each wheel, but at the same time those motors can be made to perform in a way that constantly monitors traction and outputs torque where it is needed. This is new technology and it will get better.

My understanding of the Hummer is that it uses a differential in the back since it has one motor to power two wheels. And in the front, it's more like a Rivian with a motor at each wheel. It will be interesting to see which approach other EV manufacturers take for their "offroad" capable vehicles.

 

[White Shadow]

She did not do well on trails lol, but that has been covered. She is back on her street 22s & all cleaned up. The trails remain the domain of my far more expensive rig:

Nice, but surely you must have a picture of it on a real trail with real offroad tires, no? Or at least something other than the OEM 22" wheels & street tires. I had this impression in my head that you really tested your Rivian offroad.

 

[R.I.P.]

It doesn't have to be inferior if it's set up right. Think about it---there's absolutely no more capability possible then you can get with an individual motor powering each wheel. But again, it's all about how it's set up to operator under various different conditions.

That said, a vehicle like the Mercedes G-Wagon is a rare breed, factory equipped with front, center, and rear locking capability. There are very few vehicles produced today with that type of capability. A quad motor Rivian will obviously never have true mechanical lockers of any kind because of individual motors controlling each wheel, but at the same time those motors can be made to perform in a way that constantly monitors traction and outputs torque where it is needed. This is new technology and it will get better.

My understanding of the Hummer is that it uses a differential in the back since it has one motor to power two wheels. And in the front, it's more like a Rivian with a motor at each wheel. It will be interesting to see which approach other EV manufacturers take for their "offroad" capable vehicles.

Ford, GM, Jeep, Toyota, are all making "Fully locked" versions of off-road vehicles. I guess if you consider that these versions cost more than the "street" versions, they do sell in less numbers, but I am not sure I would agree that there are "very few" out there.

The Hummer has the two motors in the rear, not the front, and suffers the same issues the Rivian does. TFL did a pretty good video pitting it against the Bronco with lockers.

 

[R.I.P.]

Nice, but surely you must have a picture of it on a real trail with real offroad tires, no? Or at least something other than the OEM 22" wheels & street tires. I had this impression in my head that you really tested your Rivian offroad.

Your impression was wrong. We tested the Rivian on Toyo Open Country M/T's on 20"s. I would have preferred smaller wheels (more sidewall flex), but it is what we could get our hands on at the time. The Toyo MT's are what most of us prefer for our area, so it was an apples-to-apples comparison.

I will go over it again, but the R1T's penchant for moving sideways on slippery camber's meant it could barely get off the trailhead. Mine was one of three that has made attempts, but I am here to tell you it is not fun slipping sideways toward the banks in this mountainous terrain.

I am aware that you are just continuing to "poke", for whatever your reasons are, but I am trying to just give you a polite re-hash of my experience. If you were closer, maybe you could come out and join us on a run. That would enable us to stand on real mud and snow, and talk about real physical things. I even have a cabin you could stay in for your visit, for free.

Beyond that, I am not sure what I can do for you. I get the feeling that my experiences have somehow offended you, but at the same time, it seems unreasonable to have done so. If it is any consolation, it would seem that my "quad motor experiment" is over, so you should not be submitted to any more of my complaints.

 

[racekarl]

It doesn't have to be inferior if it's set up right. Think about it---there's absolutely no more capability possible then you can get with an individual motor powering each wheel.

I really want to believe this, too, but I am not totally convinced that it is a true statement, and that even if it were that the Rivian has the hardware required.

For a quad motor system to achieve what mechanical differentials can, it needs to have the ability to "read" the static friction of each tire against the ground *without moving the tire at all.* (or at least, without causing it to exceed the static friction and spin or slip).

(Perhaps, as some have imagined, there could be some sort of "sensor fusion" using camera or accelerometers to accomplish the same thing, but even that would be almost impossible to program to account for all the situations that a mechanical differential just handles by design.)

I am not an EE, so I have no idea if that is possible. And if it is theoretically possible, does the Rivian have the hardware to enable it (sensors, etc.)? And even if they could do all that, would they when there is a much cheaper and obvious mechanical solution available?

 

[brancky3]

Ford, GM, Jeep, Toyota, are all making "Fully locked" versions of off-road vehicles.

It may make sense in the future for Rivian to make a more 'offroad' trim, but this is so extremely niche for the market they're targeting so it would be absolutely detrimental to their profitability to spend any amount developing lockers / etc. We're talking about a very very minuscule number of people that would want such a thing. Yes, there are *some* people that want to offroad it at extreme levels, but we're talking about <0.05% of owners most likely.

 

[R.I.P.]

I really want to believe this, too, but I am not totally convinced that it is a true statement, and that even if it were that the Rivian has the hardware required.

For a quad motor system to achieve what mechanical differentials can, it needs to have the ability to "read" the static friction of each tire against the ground *without moving the tire at all.* (or at least, without causing it to exceed the static friction and spin or slip).

(Perhaps, as some have imagined, there could be some sort of "sensor fusion" using camera or accelerometers to accomplish the same thing, but even that would be almost impossible to program to account for all the situations that a mechanical differential just handles by design.)

I am not an EE, so I have no idea if that is possible. And if it is theoretically possible, does the Rivian have the hardware to enable it (sensors, etc.)? And even if they could do all that, would they when there is a much cheaper and obvious mechanical solution available?

You have an absolute perfect grasp on the problem. I think it is a difficult concept to wrap your head around, so congratulations LOL.

 

[Zoidz]

I really want to believe this, too, but I am not totally convinced that it is a true statement, and that even if it were that the Rivian has the hardware required.

For a quad motor system to achieve what mechanical differentials can, it needs to have the ability to "read" the static friction of each tire against the ground *without moving the tire at all.* (or at least, without causing it to exceed the static friction and spin or slip).

(Perhaps, as some have imagined, there could be some sort of "sensor fusion" using camera or accelerometers to accomplish the same thing, but even that would be almost impossible to program to account for all the situations that a mechanical differential just handles by design.)

I am not an EE, so I have no idea if that is possible. And if it is theoretically possible, does the Rivian have the hardware to enable it (sensors, etc.)? And even if they could do all that, would they when there is a much cheaper and obvious mechanical solution available?

Given time and money, in theory accelerometers and cameras (perhaps even the existing ones) could be used to augment a traction control/locking diff algorithm. Lots of time, and lots of money - but it's a use case that few will use, not worth Rivian's investment.

I'd also be concerned about software flaws. We've seen major issues including fatalities with Tesla FSD. Rivian Driver+ has some "near miss" reports of erratic steering adjustments, etc. A little miscalculation in "Off Road Locking Diff Assist" mode could be disasterous...

 

[White Shadow]

I really want to believe this, too, but I am not totally convinced that it is a true statement, and that even if it were that the Rivian has the hardware required.

For a quad motor system to achieve what mechanical differentials can, it needs to have the ability to "read" the static friction of each tire against the ground *without moving the tire at all.* (or at least, without causing it to exceed the static friction and spin or slip).

(Perhaps, as some have imagined, there could be some sort of "sensor fusion" using camera or accelerometers to accomplish the same thing, but even that would be almost impossible to program to account for all the situations that a mechanical differential just handles by design.)

I am not an EE, so I have no idea if that is possible. And if it is theoretically possible, does the Rivian have the hardware to enable it (sensors, etc.)? And even if they could do all that, would they when there is a much cheaper and obvious mechanical solution available?

No, not with a locked differential. A locked differential reads nothing. It's mechanically locked and that's that. The same exact thing can easily be accomplished with a motor at each wheel. Make them turn at the same speed no matter what.....and that's essentially the EV version of a locked differential.

What you're talking about is more of a reactionary type system, which is in fact much more common in today's 4wd (and even more so in AWD) vehicles, in which loss of traction is sensed and then the system reacts by apportioning torque to where it can benefit most. That also can be done with a quad motor system.

 

[White Shadow]

It may make sense in the future for Rivian to make a more 'offroad' trim, but this is so extremely niche for the market they're targeting so it would be absolutely detrimental to their profitability to spend any amount developing lockers / etc. We're talking about a very very minuscule number of people that would want such a thing. Yes, there are *some* people that want to offroad it at extreme levels, but we're talking about <0.05% of owners most likely.

Wait for Jeep to do it then. I can 100% guarantee you that Jeep will be building very offroad capable EVs once they really get into the EV game. I mean, that's what the Jeep brand is built on in the first place.

Today, you can get a Jeep Wrangler or a Jeep Grand Cherokee that's a hybrid electric vehicle. And on the Grand Cherokee 4xe (their hybrid designation), it comes with the top 4wd system available in a Grand Cherokee, including their eLSD, which essentially can act like a truly locked rear axle.

 

[Inkedsphynx]

This is probably totally bonkers but this thread still has me trying to think up novel ways you could use the technology to improve the situation.

This idea is very specific to a very specific set of circumstances so obviously doesn't address everything in this thread, but...

What if there was an ability to individually test each wheel's traction in a stopped position? You're stopped on a trail that's off-camber and before you proceed, you enable some mode that runs a traction test individually on all 4 tires. Slowly ramps up torque until the wheel starts to slip, then stops. Now it knows how much traction that specific tire has. Do that all the way around the vehicle, and now you know the traction situation and could modulate the motors in some fashion to account for that.

I guess it'd be mostly pointless since that all goes out the window once you start moving, but for those off-camber situations, you're likely riding over the same/similar conditions the entire way, so it's really about the vehicle being able to know which tires are more likely to slip and accounting for that. Maybe this would need to be coupled with a special mode for this specific situation.

I don't know, this could be entirely pointless, since the conditions could change. This obviously doesn't help if you're already moving. My brain doesn't like unsolved problems

I do enjoy though that the tech allows me to think up these crazypants ideas. The one big benefit the quads have over traditional lockers is this flexibility, so it's just a matter of whether or not a system can be reasonably designed that can perform the same functions as lockers.

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