Quad motor vs locking diff

[HarrySiii]

Forgot to say.....you do realize that a limited slip diff in no way behaves like a locked diff, right? If you're truly worried about sliding off the cliff of death, you most definitely would want limited slip over locked for sure.

Edit: hum, actually that is rolling resistance, not rolling friction. Actually even though they are cross referenced in that link, they are completely different concepts.

I am quite sure some of you can explain this better than I can, so feel free to chime in but here goes:

Rolling Friction:
Although I am sure there is a more appropriate scientific term for it, we enthusiast types use this term to refer to the lateral resistance a rolling wheel retains as long as it is moving.

A static wheel has static friction, which is greater than sliding friction.

A freely rolling wheel maintains it's "lateral static coefficient" even when it is rolling; up until the friction is broken.

Once broken, the wheel will move in whatever direction forces act upon it, regardless of which direction it happens to be turning.

This is why a braking car on ice is out of control until the driver lets up on the brakes and lets at least 2 of the wheels spin with no resistance what-so-ever. If you can get at least a front & rear wheel "connected" and spinning again in this way, you will regain as much lateral control as the surface will allow. The other two can continue trying to slow (or accelerate) the vehicle while the "anchor" wheels maintain lateral control (i.e. keep you from sliding over the edge).

How did I do, is that close?

This is slick surface driving 101. NO! You do NOT want to use any more brake than you absolutely have to. NO!! You do NOT want your diff in any kind of locked or LS condition! You MUST have your diffs open, to allow complete free-wheeling of the wheels with the most traction to keep your vehicle from spinning out, moving sideways or falling off the trail. If you cannot gain enough traction with your other 2 wheels, you will be looking for other options (winch), but unless you are that nuub that just bought a Jeep and wanted to tag along with the big boys, you do NOT lock your diffs; or over the side you go.

I know I did not do as well at explaining this as some of you may be able to do, but I have come to see that there are those not aware of how this works.

Ok, so I finally made it to the end...for now...page 21, after 2 or 3 days of reading. Mostly great info and conversation.

I have a couple of questions on the two quotes above which sound contradictory to other quotes in this thread about locked versus open, etc.

Are you saying (RIP) that in your case you'd want an OPEN diff? I though through the rest of this you were saying that you'd only trust a LOCKED diff in your example. The example being off-camber snowy/slippery surface. I would think a LOCKED diff would also cause you to slide off the cliff in the case that all 4 wheels were slipping underneath (obviously not if any one wheel had traction). Otherwise, a limited-slip would be best, correct?


In that case, what others are saying about a speed-applied 4 wheel setup could also work, because it can add speed only keeping speed with the wheel with the most traction (PROACTIVE). Of course, if at any time this acts in a REACTIVE fashion, it would be best to have only the crawling mode implement this at extremely low speeds in low traction situations. (again, as others have said, product managers and engineering need to decide this is a high enough priority to code and implement)

Alternatively, if there were enough room in the packaging, they could add E-locking diffs between the two motors (and maybe even a driveshaft front-rear) to satisfy this use-case scenario. That would severely change design and cargo capacities of the vehicle, which they wouldn't do for the masses, but maybe another off-model once they truly make it "big" (enough).

Edit: Additionally, you'd want the latter type of setup at least electronically connected the same-axle motors to achieve more power output to a single wheel.

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[HaveBlue]

The patents showed that locking diffs were possible in the housings. They elected not to implement them. An after market carrier will be the only other solution for dual motor.

 

[R.I.P.]

"Are you saying (RIP) that in your case you'd want an OPEN diff?"

Yes. The power of an open diff is the biggest thing the quad motor design is missing, and the reason it is incapable of safeguarding rolling friction.

 

[MountainBikeDude]

It's official... @R.I.P. is actually ChatGPT in disguise!

 

[R.I.P.]

It's official... @R.I.P. is actually ChatGPT in disguise!

Sorry, remember, I am an old Oregon Longhair... So not as up on this online stuff as most... What is "Chat GPT?"

 

[MountainBikeDude]

Sorry, remember, I am an old Oregon Longhair... So not as up on this online stuff as most... What is "Chat GPT?"

No worries man, check out the link. Also this YouTube channel does great work

 

[Acon]

Can someone just post a summary so I don't have to go through 21 pages of posts...

 

[MountainBikeDude]

Can someone just post a summary so I don't have to go through 21 pages of posts...

No! Haha

One must read the previous 20 pages to understand the struggles in getting to 21.

 

[R.I.P.]

Can someone just post a summary so I don't have to go through 21 pages of posts...

The chat GDP quote above does a decent job of summary. It is not as specific as the testing I did, but definitely gives you the idea.

 

[Zybane]

I think this video shows the issues with Rivian's quad motor setup well at the 7 minute mark:




Rivian struggling with a little bit of loose rocks on an almost level surface. Notice the right rear wheel/tire accelerating and decelerating at rapid speeds, tossing rocks out, tons of shock loads on that driveline (bad).

An ICE off-road vehicle in low range with a locked rear differential would have literally slowly idled over that without touching any pedals.

 

[R.I.P.]

I think this video shows the issues with Rivian's quad motor setup well at the 7 minute mark:




Rivian struggling with a little bit of loose rocks on an almost level surface. Notice the right rear wheel/tire accelerating and decelerating at rapid speeds, tossing rocks out, tons of shock loads on that driveline (bad).

An ICE off-road vehicle in low range with a locked rear differential would have literally slowly idled over that without touching any pedals.

Yup. They do fine on some stuff, but fail shockingly (at least to me) in seemingly very benign areas.

 

[MountainBikeDude]

I think this video shows the issues with Rivian's quad motor setup well at the 7 minute mark:




Rivian struggling with a little bit of loose rocks on an almost level surface. Notice the right rear wheel/tire accelerating and decelerating at rapid speeds, tossing rocks out, tons of shock loads on that driveline (bad).

An ICE off-road vehicle in low range with a locked rear differential would have literally slowly idled over that without touching any pedals.

Sure, but you also would have been going near dead straight or dragging wheels on any sort of cornering manouver, putting similar "bad" loads on the driveline despite the loose surface.

Driving in that same outing as the video in a conventional setup, you would have likely been in 4 high, open diffs, until you got stuck, then locked the axel and tried again.

My current vehicle would have done something similar to the Rivian, it just wouldn't have tossed the rocks as far becuase my vehicle is gutless. As a result, I don't think this is much of gotcha or benign failure of the system over the terrain.

 

[rivmok]

R1T did better than the Jeep Gladiator Mojave w/rear locker in this video:

 

[R.I.P.]

R1T did better than the Jeep Gladiator Mojave w/rear locker in this video:

When the Rivian can get its power to the ground, it does well. This is one of those scenarios where it shines.

Where the quad motor struggles is low traction conditions where it can't get that power to the ground.

Another really good example of what we're talking about here.
?

 

[White Shadow]

Of course, an open diff would not help Kyle in his video and lockers would help. That was never in doubt. I think everyone’s frustration with your posts stem from the fact that we moved on from that issue 10 pages ago.

But FWIW, later on (maybe further up the hill) I recall Kyle mentions that the stationary wheels were actually being driven with max available torque but were still unable to pull the vehicle up and over the obstacle. So his Rivian was not exactly acting like it had an Open diff, but rather like a LSD with insufficient torque. I have no means of checking if that is the case, but that was the calculation that an earlier poster was trying to confirm.

But in any case, most of us are more concerned with the challenges of replicating an open diff in certain low-mu scenarios.

You can't move past an issue that's the root cause of the problem. Until Rivian changes the behavior of it's quad motor design, then it will never be comparable to today's other "offroad capable" vehicles. That's my whole point.

The other issue you mentioned did indeed seem like there was no torque available....but we know that isn't the case with these high-torque electric motors. What I suspect is happening is that the vehicle is intentionally limiting torque to the motors for whatever reason. It might be to keep the motors from overheating, who knows? I'll tell you this---I did read an article recently that Rivian is building a gearbox to enable torque multiplication through gearing instead of relying on the motors alone. Did anyone catch that article? Very interesting....

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