Quad motor vs locking diff

[SeaGeo]

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.

Professionally speaking, I agree this is a concern for me as well.

because the Rivian just flat goes sideways under certain conditions and as a death trap.

So, I'm not sure this is entirely due to the quad configuration, but I haven't put much thought into figuring out the "why."

Small sample size, and somewhat anecdotally here but the trucks seem to want to slide sideways on slippery surfaces more than I'm used to even when not moving. I've seen at least a few people describe their truck sliding sideways while basically parked on a road, and I've been in mine when it's shifted on an ice surface when I wasn't moving. I obviously couldn't do an A/B comparison with another vehicle at the moment, but it surprised me. I very quickly ditched the ATs in winter because of concerns about the inertia of the truck on ice in general, but the desire to move when stopped on a relatively flat surface surprised me a bit. I'm sure it's weight related, but whether it's also cog and/or camber isn't something I've fully thought though yet.

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[R.I.P.]

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>

 

[MountainBikeDude]

Professionally speaking, I agree this is a concern for me as well.

So, I'm not sure this is entirely due to the quad configuration, but I haven't put much thought into figuring out the "why."

Small sample size, and somewhat anecdotally here but the trucks seem to want to slide sideways on slippery surfaces more than I'm used to even when not moving. I've seen at least a few people describe their truck sliding sideways while basically parked on a road, and I've been in mine when it's shifted on an ice surface when I wasn't moving. I obviously couldn't do an A/B comparison with another vehicle at the moment, but it surprised me. I very quickly ditched the ATs in winter because of concerns about the inertia of the truck on ice in general, but the desire to move when stopped on a relatively flat surface surprised me a bit. I'm sure it's weight related, but whether it's also cog and/or camber isn't something I've fully thought though yet.

Where rubber meets the road is usually the weak point. Even in my Super Duper Xterra! on a slight icy camber, while stationary, my BFG AT's had me slipping toward the gutter on the Stanley Park causeway.

Talking out my ass here... but, on an off camber situation the downward forces holding a vehicle in place should be relative to the tendency for it to slide down a hill. so a 2000lb vehicle on the same tires same/parameters should slide as much as a 7000lb vehicle? no? It would come down to it's ability to hold lateral stability via rubber, but then also drivetrain setup once in motion.

I've had a few beers, so if this makes any sort of sense to anyone but me, then heck yeah!

 

[escapee]

It makes sense to me. Thanks for all the explanations. This has been quite a mental exercise for me.

My first engineering job was writing firmware for stepper motors. I later moved on to designing hardware for AC motor controllers, among other things. This was an engineering lifetime ago, so I decided to call a buddy of mine that has much more experience in this sort of thing, and as an added bonus, he is a huge EV enthusiast.

Initially, he was pretty keen on the idea of being able to exactly emulate a locking differential in an EV application like Rivian. However, as we continued the discussion, there were a few caveats, mainly questions about precision of the hardware control and feedback, which we can only speculate about. In the end, we thought it may be doable, but it clearly isn't now.

Could the issue be related to the starting torque of the motor(s) with traction given the weight and incline or do you think it's purely a control issue?

 

[R.I.P.]

Talking out my ass here... but, on an off camber situation the downward forces holding a vehicle in place should be relative to the tendency for it to slide down a hill. so a 2000lb vehicle on the same tires same/parameters should slide as much as a 7000lb vehicle? no? It would come down to it's ability to hold lateral stability via rubber, but then also drivetrain setup once in motion.

I've had a few beers, so if this makes any sort of sense to anyone but me, then heck yeah!

Continuing down that path, I think the heavier vehicle would need a larger "traction patch" than the lighter one to hold more weight against gravity...? Like you, not 100% sure on that one, but one thing I know for sure. Once the traction is broken, the vehicle is on it's way and it now takes force to stop it. Since the quad-motor always puts power to both rear wheels, they both break free and if you are on camber, the vehicle goes sideways. Exactly as if the diffs in my Jeep were locked. Problem is, you cannot "unlock" the motors on one side in the Rivian, so it behaves like a locked up rig would.

 

[graemebshaw]

To a certain extent, it depends on the allocation of the trucks weight among the different tires. The truck starts slipping on one tire and then breaks away. Also, the heavier vehicle could actually cause the ice surface to melt under the tire and lower the mu even more. There is a lot at play here.[/QUOTE]

 

[zefram47]

And btw, some other posters have seen similar behavior on-road in snowy conditions with lots of camber. Snow mode may have helped some of that, but I don’t know.

I have been quite public about my fear of a 7000lb vehicle on icy highways at close-to highway speeds, especially if regen can’t be turned off entirely. High loads at high speeds with high rates of change in the forces involved (torque, traction, braking, regen, etc.) and super low-mu is terrifying.

I can attest to the terrifying part, and I am quite accustomed to some pretty hairy off-road stuff LOL. I have called the handling "evil," because the vehicle will just go completely sideways on you in ways I have never felt a vehicle do before. It just gets a mind of its own.

Edit:
Actually, I have been in vehicles before that acted this way. Vehicles that have a welded rear spool.

@SeaGeo @MountainBikeDude...similar situation.

Probably worth a mention given these two posts. While on-road that was basically a sheet of ice on a relatively steep downhill with slight camber towards the road edge my 4Runner has slid towards the edge of the road before. I was going maybe 5-10 mph in 4WD and open diffs. First time I've experienced that one, but it's definitely not just a Rivian problem. I wound up pirouetting the truck pointing back up the hill and went home...wasn't worth trying to get into the office. That was on Falken AT3W and a rig weighing around 5400 lbs kitted out.

 

[SeaGeo]

Continuing down that path, I think the heavier vehicle would need a larger "traction patch" than the lighter one to hold more weight against gravity...? Like you, not 100% sure on that one, but one thing I know for sure. Once the traction is broken, the vehicle is on it's way and it now takes force to stop it. Since the quad-motor always puts power to both rear wheels, they both break free and if you are on camber, the vehicle goes sideways. Exactly as if the diffs in my Jeep were locked. Problem is, you cannot "unlock" the motors on one side in the Rivian, so it behaves like a locked up rig would.

The patch changes as a function of pressure assuming a constant psi. Rivian recommends higher than typical for a normal consumer vehicle, though so you're engaging probably engaging the same patch area but with a higher pressure.

 

[MountainBikeDude]

@SeaGeo @MountainBikeDude...similar situation.

Probably worth a mention given these two posts. While on-road that was basically a sheet of ice on a relatively steep downhill with slight camber towards the road edge my 4Runner has slid towards the edge of the road before. I was going maybe 5-10 mph in 4WD and open diffs. First time I've experienced that one, but it's definitely not just a Rivian problem. I wound up pirouetting the truck pointing back up the hill and went home...wasn't worth trying to get into the office. That was on Falken AT3W and a rig weighing around 5400 lbs kitted out.

Changed from BFGs to Falken wildpeaks a couple years ago. Far better wet/slippery surface traction than the BFGs.

 

[kylealden]

[Deleting this - I thought I was closer to the current page than I thought. Ignore me. Not trying to dig up old discussions.]

 

[MountainBikeDude]

FWIW I have seen this pattern a half dozen times in this thread alone:



Y'all are talking past each other.

Hehehe

 

[madgrey]

Could the issue be related to the starting torque of the motor(s) with traction given the weight and incline or do you think it's purely a control issue?

Good question, but I really would rather not speculate, and I know when I'm out of my depth. Too many people speaking in absolutes in this thread and all kinds of well meaning misinformation.

I've been an engineer long enough to know that few things are even remotely as simple as they initially seem. I think this is an epic problem, one that Rivian has not solved. The differential was invented almost 200 years ago, let's hope it takes less time for Rivian to build a workable E-diff. I really need it to get out of my driveway the next time in the snows.

 

[Biturbowned]

Good question, but I really would rather not speculate, and I know when I'm out of my depth. Too many people speaking in absolutes in this thread and all kinds of well meaning misinformation.

I've been an engineer long enough to know that few things are even remotely as simple as they initially seem. I think this is an epic problem, one that Rivian has not solved. The differential was invented almost 200 years ago, let's hope it takes less time for Rivian to build a workable E-diff. I really need it to get out of my driveway the next time in the snows.

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.

 

[OrthoBlock]

@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.

 

[popoga]

I won't opine too much, and maybe it's been said, but here's a counterpoint to the locking-diff superiority convo.





I do think a locking diff provides some baseline benefits, but in 5 years we won't miss it in the EV world.

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