Quad motor vs locking diff

[Yellow Buddy]

Can you explain this? I’m not following.

I oversimplified it in my example. But to start we have different sized motors. The type of differential will also matter in terms of power distribution, where it's situation the power distribution, driveline factor losses, etc. That's my understanding of it.

I was simply trying to understand the available power when it comes to standard motor design.

My understanding based on the replies..and I still may be wrong.

Using just the rears the maximum power available on one wheel on a quad motor will be limited to that single motor for that wheel. Vs the maximum power available to one wheel on a dual motor w/ locking diff is still limited to that single motor, less any losses/tuning due to the diff. But that motor may have higher power naturally because it's designed to typically handle two wheels.

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[R.I.P.]

These folks have outstanding reviews, and it looks like it's only a 35 minute drive from you ...

https://www.leafly.com/dispensary-info/canna-king

Might help to chamfer those sharp corners

(Side note: also answering your 4xe question)

Hehe, got me thinking, trying to remember if I have ever actually paid somebody else for weed. I don't think so.

Those new boutique places don't really cater to my type anyway, the little girl behind the counter would likely call the cops. I appreciate the tip 'though.

 

[R.I.P.]

It's solid. If I may derail the thread a bit ...

It was our first EV, after deciding we wanted to give EV tech a shot. Funny enough, my original motivation came from all the friends/associates I knew with EVs (mostly Tesla) but from a Dad's Day gift a cordless, electric blower (to replace a broken corded blower).

Got other Greenworks yard tools without the battery/charger, making them super inexpensive (and I figured with my light use, I could easily share), so I scored a string trimmer and a pole saw, eventually got a mower for, a killer deal came up on Woot (that included TWO batteries and charger). Then something clicked, batteries, no smell, no gas ... I could drive a vehicle like this! YES!

But we were a little apprehensive to go full on BEV. About this time, our previous 2nd car/family/wife's ride, a DD/RT was coming off lease, we decided we'd just try something without 3-rows, we both kind of wanted a Wrangler (around here, every 4th car is a Wrangler ...) We bought the DD/RT off lease, and turned around and sold it to Carvana for a $12K profit

So we got the 4xe, we were a pretty early adopter, got ours in April '21, got the $7500 tax credit immediately as a cap reduction, even a discount (this was pre-multiple-price-hikes).

Once I got the 50a circuit installed, and a 40a EVSE it was a total epiphany. So much so, that I sold my '19 GT convertible and picked up an M3P (they share the Grizzl-E EVSE, for the Tesla, I just pop on the adapter that was supplied with the car). I went with an outlet vs. hardwired, and got an N6-50 adapter for the Tesla mobile connector, so I have an L2 charging backup.

Anyway, back to the 4xe, we LOVE it, I installed an electric hoist in the garage, so removing the top is easy (and it stays up and out of the way), we use it for beach cruising, heading down to the point for a little SUP-ing, to lug shit around, and it's almost exclusively our around town car. It's extra great around here too, where there's constant flooding issues, it just chugs through water no problem. It's so nice quietly cruising around the beach areas, the only sound the ocean and the occasional tires squeaking in the sand.

It runs about 95% of the time on battery only, and yes, it doesn't have a ton of range, but we rarely use it beyond that range (we work from home, daughter's school is ~6 miles away).

So that means in the last 1200 miles or so, it's used less that 1/2 tank of gas.

I probably don't have to tell people on this board, how fun Wranglers can be, all the aftermarket, modular body parts, super durable interior, and heck, ours is a big sissy model with heated seats and a fancy stereo system But it's still very much a Wrangler, a real 4WD system, low range, and when the gas AND electric systems kick in, it's had a f*** ton of power (it's shockingly quick on the road too, for a big brick).

Thanks for the run-down!

Jeep has a "cool" factor that is popular, but they are notoriously inefficient to drive around as a grocery getter. The 4xe is a fantastic solution, and seems to be selling well; so many more EV drivers on the streets, many of which would never have considered a BEV when they purchased it.

But; they are a gateway drug. I still own a Volt that I let family members have (brothers, sisters) when they need to use a car. That experience has prompted three of them to end up buying Teslas.

Electric is addictive!

Beautiful rig BTW.

 

[R.I.P.]

I oversimplified it in my example. But to start we have different sized motors. The type of differential will also matter in terms of power distribution, where it's situation the power distribution, driveline factor losses, etc. That's my understanding of it.

I was simply trying to understand the available power when it comes to standard motor design.

My understanding based on the replies..and I still may be wrong.

Using just the rears the maximum power available on one wheel on a quad motor will be limited to that single motor for that wheel. Vs the maximum power available to one wheel on a dual motor w/ locking diff is still limited to that single motor, less any losses/tuning due to the diff. But that motor may have higher power naturally because it's designed to typically handle two wheels.

Your understanding is correct.

 

[downranger12]

Being a flatlander, I have more experience with snow and ice.
Yesterday I tried to move my now rarely driven 2012 Frontier Pro 4x hooked up to a ten foot trailer in hard frozen snow over intermittent ice on a slight upgrade. Surprisingly 4wd would not take me out. Locking the rear differential about the same, though the rear end began to kick sideways with that terrain. Rocking a number of times got me out.
In contrast, this morning I took the R1T on a 30 yard half circle in the same type of snow, snow mode in snow up to the floor pan.
I have read a very detailed engineering account of the torque vectoring currently on board. I distinctly felt three different wheels either slipping or engaging which I couldn't tell. My guess would be increasing speed of those quadrants at the proper time. It was risky a bit, but for snow, even deeper snow at low speed, the Rivian worked better for me. I walked right out.
Nissan was wearing Cooper ATS 4 plus, half worn out. The Rivian was wearing 21 inch street tires half worn out.

Good review, illustrating very well why quad motors can't really compete with full locking diffs. It is interesting to see in the comments that people believe this can be "fixed" with software updates. I guess it is just a factor of people not understanding the technology.

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.

While this creates a handicap the quad motor Rivian can never overcome off-road, it has a similar handicap on road. When conditions become super slippery, (snow and ice is an example), a traditional four wheel drive can lock both front and rear axles together while not locking the differentials. In this mode there are at least two wheels at any given time providing lateral stability while the other two attempt to either increase or decrease speed. Again, four disconnected wheels on motors and software can never really approximate this very important ability.

I doubt that Rivian will ever admit that there is more than just a cost savings motivation behind the new dual motor version. As cool as the quad motors are, and as useful as they are in some circumstances, they really do carry some very basic handicaps.

Thank you Kyle for a decent demonstration of at least the first concept.

 

[SamDoe1]

I'm not reading 20 pages of this thread to get to this point but as a former Jeep owner (will own one again in the future once the 4xe isn't stupid expensive) who has taken it off roading in a wide variety of terrain, the issue isn't differentials or wheel speed. A lot of the issue is that even though the R1T has 830hp...each wheel only has about 210hp. If one wheel has grip, your 210hp is all you got to move your 7k lb truck up and over an obstacle. With locking diffs, you get all the motor's torque and power to one wheel. Also, low range helps a lot.

I've taken my Jeep through some serious stuff and can count on one hand the number of times I've used the lockers. Top of the World, no lockers. Hell's Revenge, no lockers. Poison Spider, no lockers. Imogene Pass, no lockers. Black bear pass, no lockers...etc. The big things that are needed for effective off roading is articulation and tires. The Pirelli's that come stock aren't great and the articulation of a R1T is nowhere near the same as a Jeep Rubicon with the sway bar disconnected. Having a wheel on the ground is far more important than a locked diff for a variety of reasons.

Lastly, I don't EXPECT the Rivian to be anywhere close to as good as a Jeep off road. It's not at all meant to be that way. The Rivian is 1000000000000000000000000000x better on road than a Wrangler which is where the vast majority of them will live. It certainly can go off road in moderate situations but I doubt you'll see any Rivians traversing the Rubicon trail anytime soon. The Wrangler gives up a lot for being an off road beast, the majority of what it gives up is in on road behavior. The Rivian trades that off road performance for better on road performance.

 

[zefram47]

I've taken my Jeep through some serious stuff and can count on one hand the number of times I've used the lockers. Top of the World, no lockers. Hell's Revenge, no lockers. Poison Spider, no lockers. Imogene Pass, no lockers. Black bear pass, no lockers...etc.

Anyone who believes you need lockers for Imogene or BBP needs to go take an off-roading course or something.

 

[SamDoe1]

Anyone who believes you need lockers for Imogene or BBP needs to go take an off-roading course or something.

Agreed though my point was that lockers really aren't a necessity by any means as long as you have the the ability to keep your tires on the ground.

 

[Kaprice]

I don't understand. I thought the point of 4 motors is that software can completely control each motor. So why can't software completely mimic a locking differential? Why does it matter if a wheel knows how much slippage there is. Just tell the software to spin each tire at the exact same speed.

What am I missing?

 

[KootenayEV]

This has certainly been tossed around, but most agree there are drawbacks.

In a 100 foot stretch of road you may bank in opposite ways a couple of times and change sides that have the most traction 50 times. A differential will easily and automatically swap sides it is delivering power to all 50 times. Trying to mimic that manually on the fly would not only require you to actually know where the traction is at the moment (difficult, even impossible), but would be a lot of work at a potentially busy time in the drive.

I do wonder if in software they can sense the vehicle angle and/or ‘road off camber angle’ and decide to switch torque to the uphill wheels in that situation.

 

[MountainBikeDude]

I do wonder if in software they can sense the vehicle angle and/or ‘road off camber angle’ and decide to switch torque to the uphill wheels in that situation.

Interesting thought occurred to me.

Rivians obviously have some sort of MEMS gyroscope built in to determine levelness of the vehicle for uses of say camp mode for example. Why couldn't observations from such a device as well as observed Mu as the vehicle progresses along a trail, say the last 15 seconds, be used to determine appropriate force to the wheel likely to have the least traction, but most potential for forward motion (higher wheel) over the lower wedge wheel let's call it, and just provide those lower wheels with enough power to allow them to freely move with the vehicles forward motion?

Great minds? Don't worry, you'll be shot down for even suggesting such a work around any moment...

 

[R.I.P.]

I don't understand. I thought the point of 4 motors is that software can completely control each motor. So why can't software completely mimic a locking differential? Why does it matter if a wheel knows how much slippage there is. Just tell the software to spin each tire at the exact same speed.

What am I missing?

The principal of rolling friction.

 

[R.I.P.]

I do wonder if in software they can sense the vehicle angle and/or ‘road off camber angle’ and decide to switch torque to the uphill wheels in that situation.

Sure. The vehicle and computer system could then make an educated guess as to which wheels to put into "freewheel" mode and when.

I wonder, do you think there would ever be a situation where the dynamic changes, and the wheels with the most grip all of a sudden change with it? Oh, let's say the side that has the least grip all of a sudden encounters some gravel on the road. Now, in addition to knowing which way the vehicle is tipped, it also needs to identify the sudden presence of gravel under one of it's wheels to be able to mimic what the differential does through basic physics alone.

Now let's add the fact that in slippery situations things can go very bad very fast in a split second. Is the computer going to act fast enough, every time?

As I keep saying, maybe a differential can be mimicked, but the problems are not monolithic. We have a heavier system (2 extra motors) that is far more complex trying to do what the lighter, more efficient system does natively. There is such a thing as trying to push a tech in the wrong direction, and these quad-motor issues feel like that to me.

 

[KootenayEV]

Sure. The vehicle and computer system could then make an educated guess as to which wheels to put into "freewheel" mode and when.

I wonder, do you think there would ever be a situation where the dynamic changes, and the wheels with the most grip all of a sudden change with it? Oh, let's say the side that has the least grip all of a sudden encounters some gravel on the road. Now, in addition to knowing which way the vehicle is tipped, it also needs to identify the sudden presence of gravel under one of it's wheels to be able to mimic what the differential does through basic physics alone.

Now let's add the fact that in slippery situations things can go very bad very fast in a split second. Is the computer going to act fast enough, every time?

As I keep saying, maybe a differential can be mimicked, but the problems are not monolithic. We have a heavier system (2 extra motors) that is far more complex trying to do what the lighter, more efficient system does natively. There is such a thing as trying to push a tech in the wrong direction, and these quad-motor issues feel like that to me.

very fair points

Reminds me of Tesla trying to mimic a simple rain sensor through camera vision and software. In that case, no amount of software or machine learning will overcome the basic limitation that the camera is sampling a tiny fraction of the windshield and that distribution of water etc on the windshield is not homogenous.

Can’t program your way out of design choices…

 

[R.I.P.]

very fair points

Reminds me of Tesla trying to mimic a simple rain sensor through camera vision and software. In that case, no amount of software or machine learning will overcome the basic limitation that the camera is sampling a tiny fraction of the windshield and that distribution of water etc on the windshield is not homogenous.

Can’t program your way out of design choices…

BINGO! Exactly. Tesla has given a numerous examples, and are about to offer up another one for us; the Tri-motor CT. That is the model I have reserved, but I intend to change that (when they let me) after my experience with the Rivian. Tesla has some pretty good programmers, but I don't think the tri-motor CT will be immune from the problems the tri-motor Hummer is having, so gimme a good-ol differential please. Want more power? Put a bigger motor on that differential, but for those of us that drive in the slick stuff anyway, I do not believe we have found a working replacement for it yet.

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