Quad motor vs locking diff

[Nix]

There is a lot going on in this video.

There are some choices he pointed out he made just to demonstrate the drivetrain, that wouldn't actually be good choices for hardcore wheeling. Stock tire PSI, stiff suspension, large diameter rims, and AT tires vs. MT tires in soft dry dirt is not at all optimal for wheeling on that specific trail. Both vehicles could have performed better just changing those factors.

None of that is helping, but setting that all aside and focusing on just the drivetrain and programming, I'm seeing a couple of problems.

1) Too much tire acceleration way too quickly. The tires are going from not turning to spinning quite hard at what appears to be the slightest jab of the pedal. A low geared off-roader would allow the tires to slowly scratch their way forward instead of spinning fast with no chances of regaining traction.

2) It wants to go too fast when it does hook up with traction. As soon as it hooks up, the driver must quickly let off the pedal. It isn't letting him moderate the power smoothly, and it isn't just his driving style. It isn't allowing him to keep applying steady moderate power.

3) Electric motors appear to be overpowering the spinning wheels, where brakes would normally lock up the spinning wheels in modern advanced traction control systems. Normally the idea would be to keep a moderate throttle and let the brakes overpower the spinning, and let the system find traction.

I have no knowledge of programming EV's. But I would be interested to see this test again with much less power going to the motors and much slower throttle response. It might not perform as well as a fully locked low range geared off-roader. But it seems like there is a lot of opportunity for significant improvement just through programming.



(my background -- lifted and locked Jeep TJ with re-geared axles and transfer case for extreme low range gearing, and two sets of wheels with both AT and MT tires)

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[CrazyOne]

I was going to suggest that, though less technically. I can see Rivian offering a virtual locker mode where all 4 wheels are kept at exactly the same speed regardless of reaction.

I don't think it can be implemented to perfection with software. If they had used a clutch between the two motors on an axle (like NSX), that would give true locking differentials at front and back. The center would still be software.

I bought mine for on-road torque vectoring. I knew what I was getting.

 

[Zybane]

I've already explained why a quad-motor Rivian will never be as good off-road compared to an off-road 4x4 with center, rear and front locking diffs in the off-road section.

Rivian quad motor acts more like a "good" AWD system. That is why it struggles even over some pretty small/basic obstacles.

 

[SoCal Rob]

I don't think it can be implemented to perfection with software. If they had used a clutch between the two motors on an axle (like NSX), that would give true locking differentials at front and back. The center would still be software.

I bought mine for on-road torque vectoring. I knew what I was getting.

I think it’ll come down to the resolution of the wheel speed sensors and how quickly the wheel speed can be controlled. I don’t know if perfection is possible, but my hope is that Rivian could do this closely enough that it makes no real-world difference.

@Zybane, I’m looking forward to getting our R1S since I’m used to difficult trails in a vehicle with independent suspension, different drive modes, and adjustable suspension heights. I know how important it is to set things up correctly and use just enough throttle to allow the vehicle to do what it needs to do. I’m used to the old-school, “you need solid axles, 3 lockers, and coil springs,” crowd telling me that I can’t possibly get to places I’ve already gotten to.

 

[Dark-Fx]

I think it’ll come down to the resolution of the wheel speed sensors and how quickly the wheel speed can be controlled. I don’t know if perfection is possible, but my hope is that Rivian could do this closely enough that it makes no real-world difference.

The inverter could be designed to do this entirely without consideration for what the wheel speed sensors are relaying since it's a static connection. It really just depends on how quickly they are sampling the output of the inverter and how quickly they can adapt the output. Second order changes in the data could indicate slip as it's just starting to occur. Traditional ICE traction control can't react nearly as instantaneously.

Whether or not the Rivian inverters have enough processing power or the equipment to do this? I have no idea. Just theorizing.

 

[EVnewb]

Maybe I’m completely missing something.. But isn’t the point of lockers to apply equal power to the wheels locked together for the sake of traction? How can the quad motors not be programmed (via OTA later) to “lock” or sync them up to turn at the same speed with the same torque applied to each wheel? I thought that’s why torque vectoring was so impressive. The software can control which wheel gets how much torque right? We’ve all seen the truck do a tank turn in the video, so why would it be impossible to digitally lock the truck up?

Sorry, I’m reading a lot of experience, knowledge, and physics from both sides so I’m confused.

 

[EVnewb]

And as others have said, I wish they would add a digital Low Range (4L) setting for more gradual throttle response in certain off road scenarios. OTA someday!

 

[R.I.P.]

I guess I should not be surprised at the number of people that weigh in with "well, I do not intend to watch the video, and I do not own one of these, but I read on a website something somewhere so I will tell you how it is".

To be expected for sure, but the information was presented for those looking for actual on-the-ground test data with real trucks. I hope it helped.

 

[R.I.P.]

(my background -- lifted and locked Jeep TJ with re-geared axles and transfer case for extreme low range gearing, and two sets of wheels with both AT and MT tires)

We are running very similar rigs; '99 TJ here, at least is was originally. Not much left from the factory lol.

As I said, my R1T is not my "off-roader", and the post was just some of my experiences doing some lite testing against such machines.

 

[dorsai]

I agree that if slipping is acceptable, your solution will work. But your speed control implementation would not ensure all 4 wheels start turning at the same speed in all possible conditions, which was the OP's concern and point, and is what a locking diff does.

Specifically from a standstill, with a mechanical locking diff, all 4 wheels will receive the exact same amount of power. On 4 wheel electric motors, from a standstill, there is no reference as to how much power to apply to each wheel. For example, If one wheel is in the air, and three others are firmly on ground, starting from standstill with equal and even minimal power applied will result in the tire in the air free spinning (no load on it) and the others will not spin (too much load for the minimal power applied). That's not emulating a locking diff. This same scenario extends to any situation where 3 wheeels have better traction than one. Even with speed control, different traction loads on each wheel will require a different amount of power applied to maintain the same speed and not spin, but you don't know how much until it starts to slip. This is OP's arguement.

With a high resolution encoder providing multiple pulses per motor revolution to the VFD - arbitrarily let's say every 5 or 10 degrees, you could detect the spinning wheel in a few milliseconds and adjust the power accordingly. Due to the way ABS sensors work, I don't think you could detect it quickly enough to satisfy the OP.

Do you know for a fact that Rivian has high resolution encoders on each motor? VFDs for DC motors can use other techniques to detect and control motor speed by using a single shaft sensor and back EMF detection.

You don't have to get wheel slip when you're measuring the motor position. If they're using a pure back-EMF design, they might have some trouble at exactly 0 speed, but advanced motor control ICs can measure the inductance at 0 speed as well to determine phase without encoders. With 12.6:1 reduction from the motor, you're entering a pretty decent measurable range as soon as you're moving at all, and again, you're not sensing wheel slip, you're sensing motor phase and controlling torque to keep the motor phases in 'sync'. (the phases are all independent, just would be locked to the relative positions when you turn on the 'locker mode').

With an unloaded wheel like in the first example, you would likely see about as much wheel movement from the speed sync control as you would see from the compliance of a 4WD axle system loading up.

 

[White Shadow]

I do understand that part. I’m just unsure if in an identical low traction scenario would a simulated locked diff with no mechanical connection behave differently compared to a physically linked axle due to the forces coming back from the surface into the wheel(s). In the former the return forces would stop at the motor and not reach the other wheel, whereas in the latter the forces would travel through the axle/diff over to the opposite wheel.

My guess is that it would depend upon the software. Electric motors make crazy torque basically from almost zero RPM, so in a sense it could be designed to keep the wheel moving no matter what. And if it was designed to replicate a true locked axle, then that's one way to accomplish that. But then again, you'd really have to be careful not to break stuff with motors turning the wheels at the same speed no matter the situation. I think in a practical sense, any manufacturer would be hesitant to design a vehicle like that due to the fact that people will probably cause damage to the vehicle by operating it in a way that it's not designed to operate, if that makes any sense.

 

[White Shadow]

I guess I should not be surprised at the number of people that weigh in with "well, I do not intend to watch the video, and I do not own one of these, but I read on a website something somewhere so I will tell you how it is".

To be expected for sure, but the information was presented for those looking for actual on-the-ground test data with real trucks. I hope it helped.

Kyle's video doesn't apply to some of the claims you're making. What I got out of the video was this----the way the Rivian vehicles is currently set up isn't even as good offroad as a basic brake-based 4wd or AWD system.

Proof? Easy---when Kyle took his R1T and put it at the bottom of the hill in that off-camber situation, he had one front tire and one rear tire spinning. The other two tires weren't moving at all. Even a basic AWD or 4wd system will brake the spinning wheel to send at least some torque to the wheel that isn't spinning. That's the easiest and most simple way to move torque across an axle. But since Rivian doesn't have wheels connected by axles, they can EASILY fix this with nothing more than software. I'm just surprised that they didn't bake it into their vehicles in the first place. There's no reason whatsoever that we should see any Rivian sitting there spinning one front tire or one rear tire, while the opposite tire does nothing....at least not in any type of offroad driving mode.

 

[dorsai]

My guess is that it would depend upon the software. Electric motors make crazy torque basically from almost zero RPM, so in a sense it could be designed to keep the wheel moving no matter what. And if it was designed to replicate a true locked axle, then that's one way to accomplish that. But then again, you'd really have to be careful not to break stuff with motors turning the wheels at the same speed no matter the situation. I think in a practical sense, any manufacturer would be hesitant to design a vehicle like that due to the fact that people will probably cause damage to the vehicle by operating it in a way that it's not designed to operate, if that makes any sense.

Fun detail of electronic locking like discussed is that there isn't anything more to break than in normal operation. The locked diff breaking things issue from 4WD vehicles is from the tires breaking and regaining traction as they slip a bit and shock loading the connections in the driveline. With independent motors, you don't have the diffs to break, and you have the motor acting as a spring and shock absorber for sudden driveline jumps.

 

[zefram47]

I want people to know these things, so they can choose wisely; without guys that don't even drive the truck yet loudly touting capabilities that are inaccurate.

Meanwhile, you're touting capabilities of a model that doesn't even exist yet. ? While what you claim could prove true, only those within Rivian know what its capabilities are.

 

[White Shadow]

Fun detail of electronic locking like discussed is that there isn't anything more to break than in normal operation. The locked diff breaking things issue from 4WD vehicles is from the tires breaking and regaining traction as they slip a bit and shock loading the connections in the driveline. With independent motors, you don't have the diffs to break, and you have the motor acting as a spring and shock absorber for sudden driveline jumps.

Yup...hard on the motors for sure.....no doubt about that.

As for breaking a more traditional 4wd system---I've broken my transfer case before and that's no fun. My transfer case essentially locks the front and rear axles together, 50/50.

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