Quad motor vs locking diff

[White Shadow]

I have lost all faith in Jeep's ability to innovate. IMO they have become purely reactionary and much more interested in the market segment of mall crawlers with 4x4 badges than the market segment of true off-road enthusiasts.

Yet Wrangler remains one of the most offroad capable vehicles you can buy.

The funny thing is that some of their smaller "Trailhawk" vehicles are actually pretty darn capable offroad. I've seen some pretty crazy videos of vehicles like the Compass and Renegade offroad doing things I never would have thought possible.

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[White Shadow]

I added the emphasis because that sentence right there is the problem. You continue to talk past everyone who tries to have a productive conversation by insisting this whole conversation is about fully locked diffs vs. quad motors, when that is simply not the case. You can continue to knock that straw man down all you want, but don't expect too many others to engage on it.

That's the premise of the thread. I didn't start this thread, R.I.P. started it. And that's exactly how and why he started it. So next time you claim that someone didn't say something, maybe you'd be better off reading the thread again for clarification.

 

[White Shadow]

Quote from the GMC website:

" On some electric vehicles with dual rear motors, such as the 2022 GMC HUMMER EV Pickup Edition 1, a mechanical locker is not required. The vehicle uses software to virtually lock the two motors that power the rear wheels together. The result is essentially the same as a mechanical locker. The front locker is an electronically controlled, mechanical locking differential. "

So again, if GM can do it on the Hummer, why can't Rivian do it on the R1T/S?

It's actually too bad that the Hummer isn't a quad motor vehicle. Because if it was, I'd bet that we'd see their virtually lockers on the rear and on the front. It's just a matter of time....

 

[Inkedsphynx]

I think we've beat to death the idea that it's far more technically feasible, though challenging, for Rivian to simulate engaged lockers.

The even more challenging thing that none of us have an answer for how to do is to simulate an open or partially open diff.

Can we all stop beating the full-locker horse now?

 

[White Shadow]

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 Mercedes I mentioned has three individual lockers, front, center, rear. That's extremely rare among vehicles being sold today.

One could argue that a traditional transfer case locks the front and rear axles together and that's true, but it's still not the same thing as a locking center differential. Why? Well, because locking the front and rear axles via the transfer case means you're in low gear. And low gear is basically useless on the street unless you stay under 20 mph. But a locking center differential locks front and rear axles together without having to go into low gear.

 

[White Shadow]

The even more challenging thing that none of us have an answer for how to do is to simulate an open or partially open diff.

What you want is to simulate a limited slip diff. Like I mentioned earler, even brake-based 4wd systems or AWD systems work pretty well on the street or moderate offroad in all kinds of weather. The entire idea is to make sure power is getting to the wheels with traction.

 

[R.I.P.]

That's the premise of the thread. I didn't start this thread, R.I.P. started it. And that's exactly how and why he started it. So next time you claim that someone didn't say something, maybe you'd be better off reading the thread again for clarification.

Please stop telling people why I started the thread. Your statements are not accurate. You seem to be watching a different movie than most of us, and you seem utterly hell-bound on drawing this conversation off-topic.

As the OP, I am asking politely; please stop.

 

[zefram47]

The challenge here is that as others have pointed out, with current hardware, the only real way to make sure all wheels actually turn at the same speed would be to use wheel speed sensors and aggressive application of brakes against the motors.

That's doable, but sounds really unpleasant and like it has a high potential for damage if misused. (To be fair - you could say the same about lockers).

The root problem here is that it's easy to send the exact same power to all four wheels. It's very difficult to ensure all four wheels turn at the same speed, and even more difficult to ensure that wheels with the least static friction receive the least power. Differentials (plus lockers, in the former case) get those powers basically for free.

Not sure why people keep saying this...it's not really accurate. In order for brushless motors to function without the risk of major cogging they'd have sensors built-in that will accurately be able to determine the position/speed of the rotor. I'd imagine the motors being used would be at least 8 pole or higher, so this should be possible with fairly high precision compared with something like an ABS sensor. Reading those data at thousands of times per second they'd easily be able to see when a wheel started turning and if it was turning too fast relative to the others either reduce power or even drive it in reverse to slow it down. You wouldn't need the brakes for this unless you were holding the vehicle on a steep grade. Even then, you could sense brake pressure at the point where it started rolling backwards and immediately apply sufficient torque to the motors to hold the vehicle.

 

[WSea]

I'm here for the 'car crash', and maybe learn something too.

 

[Ralph]

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.

You could make the same case for 15" of travel. About the same number of people will use it.

 

[graemebshaw]

I work in factory automation and control systems. My favorite saying?

"Just because you CAN automate something does not mean you SHOULD."

I tell all my companies that when possible, to let physics do all the hard work. When you can, choose to run downhill, rather than climbing uphill. Physics is a wonderful force multiplier. Just because I could design a robot to pull up my pants, it doesn't t mean it is a good idea.

 

[R.I.P.]

I tell all my companies that when possible, to let physics do all the hard work. When you can, choose to run downhill, rather than climbing uphill. Physics is a wonderful force multiplier. Just because I could design a robot to pull up my pants, it doesn't t mean it is a good idea.

Seems wise.
?

 

[graemebshaw]

What you want is to simulate a limited slip diff. Like I mentioned earler, even brake-based 4wd systems or AWD systems work pretty well on the street or moderate offroad in all kinds of weather. The entire idea is to make sure power is getting to the wheels with traction.

Actually no. In the case we have been discussing, we want power to the wheels with the least traction. We are trying to protect the reserve of static friction that the downhill wheels have (so they can protect us from sliding down the hill), so do NOT want to direct any power to them. As I'm sure you know, tires have a certain amount of friction they can use, which is shared between resisting driving torque and providing lateral support to prevent side slipping. So if you off-camber, the downhill wheels are the "rails" you are riding on. The uphill (slipping) wheels are providing the propulsion.

 

[zefram47]

Actually no. In the case we have been discussing, we want power to the wheels with the least traction. We are trying to protect the reserve of static friction that the downhill wheels have (so they can protect us from sliding down the hill), so do NOT want to direct any power to them. As I'm sure you know, tires have a certain amount of friction they can use, which is shared between resisting driving torque and providing lateral support to prevent side slipping. So if you off-camber, the downhill wheels are the "rails" you are riding on. The uphill (slipping) wheels are providing the propulsion.

Which still seems like they should be able to detect a very laterally sloped surface and treat the motors differently. Hell, maybe we need a "side slope mode" in the off-road section. WIth the right control logic it should certainly be possible to only drive the up-slope motors. The problem is that using that mode in the wrong situation could be damaging and Rivian isn't likely to make it as a result.

 

[Ralph]

.. snip

This can easily be fixed via software because it is software that is controlling the quad motors.

...snip

You have indeed made this statement many times. And many times people have attempted to explain. Just *maybe* it is not as simple as a few lines of code? Assuming that the Rivian engineers are just too stupid, lazy or incompetent to do something so "simple" seems like lazy thinking (to me).

Perhaps the sensors are not sensitive/fast enough or there are other problems to solve with the feedback loop required?

Several people have attempted to explain why there *might* be "torque available" limits. Those same people or others have suggested that *perhaps* there are component design limits with regard applying all available torque from no traction to (potentially) a full traction condition. Half shafts anyone?

Varying degrees of traction with torque on a heavy vehicle with lots of travel *may* produce levels of stress Rivian is finding they did not fully appreciate.

We simply don't yet know and therefore maybe speaking in absolutes is not warranted?

I'm assuming like most owners, that Rivian will indeed, over time, continue to improve the system. It *does* seem (to me) like the problem Kyle shows in his video could be improved upon; at least to some degree. But I don't *know* that it can "solved" with what is on the truck.

I hope so. In any case I've no doubt that if Rivian does improve it, even limited success will be seen as total vindication by some.

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