Quad motor vs locking diff

[White Shadow]

.....Independent electric motors will NEVER be as good as locked axles off-road.

You're one of those guys who shits on technology and praises old school stuff right up until the technology becomes superior, and then you disappear with your tail between your legs, never to be seen again.

One thing I've learned in life is to avoid speaking in absolutes whenever possible. To say that independent motors will never be as good as lockers is ignorant at best and just completely stupid in actuality. Having a motor at each wheel gives the most capability possible. We're in the infancy stages of quad motor vehicles. Their off-road ability will definitely improve. How's that for an absolute statement? I believe there are reasons why Rivian set up the software to work the way it does, but I'm also sure it's just a matter of time before we see quad motor vehicles with e-locker setups that will function like a conventional locked drivetrain. It's definitely 100%possible to do so.

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[Zybane]

My biggest fear around our current 4 motors system is that the lack of wheel spin we see in the videos may in fact be a real limitation of the total torque potential of the respective motors.

I ran some numbers to get a sense. You can check my math on the image to keep me honest.

Full disclaimer: I'm not a scientist, or a mathematician, or a magician, etc. although I did graduate from high-school.

The bottom line from my analysis (assuming I didn't screw up that math, and I'm sure someone here will correct me if I did) the problem may really be that the motors just arent' powerful enough (especially once you start loading up vehicle with weight and introducing other factors like mud/rolling resistence, etc.) to spin the tires in some situations.

This is not the answer I wanted to see, as it would diminish the hope for clever software fixes (unless they can increase the total torque.)

(Incidentally, the smaller radius, non-AT tires would have a small advantage in having more available force due to the smaller torque arm.)

Yes, that is another facet. Here are a few posts I made last year about the topic:

https://www.rivianforums.com/forum/threads/rivian-off-road-struggling.5026/post-136087

https://www.rivianforums.com/forum/threads/rivian-off-road-struggling.5026/post-136410

Basically once you start losing traction on one or two wheels, Rivian torque drops exactly 25 and 50%. The Rivian torque numbers at the wheels aren't really that impressive for off-roading and IMO lead to motor stall due to current limits, thermal limits, or both.

 

[Zybane]

You're one of those guys who shits on technology and praises old school stuff right up until the technology becomes superior, and then you disappear with your tail between your legs, never to be seen again.

One thing I've learned in life is to avoid speaking in absolutes whenever possible. To say that independent motors will never be as good as lockers is ignorant at best and just completely stupid in actuality. Having a motor at each wheel gives the most capability possible. We're in the infancy stages of quad motor vehicles. Their off-road ability will definitely improve. How's that for an absolute statement? I believe there are reasons why Rivian set up the software to work the way it does, but I'm also sure it's just a matter of time before we see quad motor vehicles with e-locker setups that will function like a conventional locked drivetrain. It's definitely 100%possible to do so.

Speculative with no physical theory to back anything up. You're out of your depth.

Not only are the 3 locking members turning in unison, the entire mass of the drivetrain has a flywheel effect. Axles, driveshafts, gears, the actual flywheel, the crankshaft, it all has weight and momentum acting in unison. That weight is multiplied by the gear ratios in the transmission, transfer case, and axles.

This flywheel effect moderates the rpm's so the ICE motor doesn't have to react instantaneously to changing torque demand. The flywheel effect keeps the ICE motor from either speeding up or slowing down instantly. As long as the driver can keep the tires turning, the wheels and tires add to the flywheel effect too.

The guys who make every obstacle look easy are using small throttle inputs and gearing to keep the wheels slowly churning as a steady pace, clawing for traction. No big changes in RPM, no big spins. As long as the driver doesn't get too ham-fisted on the throttle, 2, 3, even all 4 tires can all take turns chirping and slipping and still maintain forward progress. Gear backlash, drivetrain wind-up, and tire/tread block flex will keep giving the tires chances to hook up again. It also maintains that anchor effect even with all 4 tires locked.





One of the efficiencies of an electric motor and quad motors is how light the rotating mass is throughout the drivetrain and motor. This is good for efficiency, but the moment a tire loses traction there is much less physical with that flywheel and gearing effect to keep the tire from quickly spinning very fast. Much faster than would give the tires any chance to hook up again without cutting the power. Cutting the power ends the momentum, putting forward progress back to zero.

I don't know how to program EV motors. But something would need to be programmed in to simulate those gearing and flywheel effects, and to keep the RPM's from either speeding up or slowing down too quickly without actually ever completely cutting power and losing that all-important constant forward momentum.

Indeed!

 

[Nix]

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.

My goal is to use a Rivian to replace our pickup and a high performance sedan with one vehicle. Trying to use it to also replace our dedicated off-roader would probably be one bridge too far. Like you, I've modified my TJ so much that it is a purpose-built tool. I wouldn't expect any stock vehicle to be as much of a purpose built off-road tool as a fully built rig. Even a stock Rubicon.

I'm not a Rivian owner yet, so I'm very limited on how much I can contribute on this topic. All I can really do it is compare what I see on videos with my ICE experience.

 

[White Shadow]

Speculative with no physical theory to back anything up. You're out of your depth.

Sure. Keep telling yourself that, kiddo.

 

[R.I.P.]

My biggest fear around our current 4 motors system is that the lack of wheel spin we see in the videos may in fact be a real limitation of the total torque potential of the respective motors.

I ran some numbers to get a sense. You can check my math on the image to keep me honest.

Full disclaimer: I'm not a scientist, or a mathematician, or a magician, etc. although I did graduate from high-school.

The bottom line from my analysis (assuming I didn't screw up that math, and I'm sure someone here will correct me if I did) the problem may really be that the motors just arent' powerful enough (especially once you start loading up vehicle with weight and introducing other factors like mud/rolling resistence, etc.) to spin the tires in some situations.

This is not the answer I wanted to see, as it would diminish the hope for clever software fixes (unless they can increase the total torque.)

(Incidentally, the smaller radius, non-AT tires would have a small advantage in having more available force due to the smaller torque arm.)

This same thing is hinted at in one of the videos from Kyle. One of the trucks doesn't have the power to pull itself up against the rock, you hear him saying in the video that it ran out of power.

 

[srwaxalot]

You ain’t cool, unless you go full spool.
My Friend daily drove a spooled F-150 in college.

 

[Dark-Fx]

One of the efficiencies of an electric motor and quad motors is how light the rotating mass is throughout the drivetrain and motor. This is good for efficiency, but the moment a tire loses traction there is much less physical with that flywheel and gearing effect to keep the tire from quickly spinning very fast. Much faster than would give the tires any chance to hook up again without cutting the power. Cutting the power ends the momentum, putting forward progress back to zero.

I don't know how to program EV motors. But something would need to be programmed in to simulate those gearing and flywheel effects, and to keep the RPM's from either speeding up or slowing down too quickly without actually ever completely cutting power and losing that all-important constant forward momentum.

It really depends on how the control is implemented. ICE motors are generally driven based on torque control and requested torque. You can take this same idea and implement it with EV motors but it will have extremely delayed reaction to tire slip in the way that you are suggesting.

Oversimplifying the issue, in that situation, the inverter is told to produce X amount of torque and will rapidly increase motor speed to maintain that torque when a tire slips. Typically in the span of a second, an inverter is capable of sending somewhere on the order of ten thousand different "commands" to the motor. If you're waiting to react to readings that say your tires are spinning faster than you want them to, you've already missed hundreds of opportunities to reduce the output. You can have limitations on ramp rate versus what torque you're commanding, but generally those are set much higher than you would set if the system was expected to always act as a locked differential.

Electric motors can be controlled to react to unexpected changes in speed nearly instantly if you are instead have an inverter design that can be commanded specific speeds instead of torque. Every individual pulse sent to the motor can be varied so that you are maintaining the specific speed instead of a specific torque. Typically these types of controls are used in things like industrial machining servo motors, but there's no reason they couldn't do the same thing in an automotive space.

 

[AlexF]

My biggest fear around our current 4 motors system is that the lack of wheel spin we see in the videos may in fact be a real limitation of the total torque potential of the respective motors.

I ran some numbers to get a sense. You can check my math on the image to keep me honest.

Full disclaimer: I'm not a scientist, or a mathematician, or a magician, etc. although I did graduate from high-school.

The bottom line from my analysis (assuming I didn't screw up that math, and I'm sure someone here will correct me if I did) the problem may really be that the motors just arent' powerful enough (especially once you start loading up vehicle with weight and introducing other factors like mud/rolling resistence, etc.) to spin the tires in some situations.

This is not the answer I wanted to see, as it would diminish the hope for clever software fixes (unless they can increase the total torque.)

(Incidentally, the smaller radius, non-AT tires would have a small advantage in having more available force due to the smaller torque arm.)

Finally, some math! It seems a lot of people forget that any given wheel is only driven by one motor. If only one wheel has traction, max torque is a quarter of total vehicle torque (ignoring any programmed safety limitations), which may simply not be enough given the gearing and vehicle weight.
A locked axle (even in the dual motor setup) would increase the available torque to either wheel on the axle.

 

[R.I.P.]

Finally, some math! It seems a lot of people forget that any given wheel is only driven by one motor. If only one wheel has traction, max torque is a quarter of total vehicle torque (ignoring any programmed safety limitations), which may simply not be enough given the gearing and vehicle weight.
A locked axle (even in the dual motor setup) would increase the available torque to either wheel on the axle.

Really good point, and strengthens my conviction that moving to a dual motor architecture will help. With power split between axles rather than wheels, you would have at least 50% power to any given wheel rather than 25%.

 

[Dark-Fx]

Really good point, and strengthens my conviction that moving to a dual motor architecture will help. With power split between axles rather than wheels, you would have at least 50% power to any given wheel rather than 25%.

With brake locking instead of diff locking, your torque still halves when one tire slips.

 

[R.I.P.]

With brake locking instead of diff locking, your torque still halves when one tire slips.

Well, no LOL. The way spider gears work, they can split torque between the axles, or provide 100% torque to either side.

The motor or engine will output X torque. That torque will go toward the wheel with the least resistance. If the left wheel is locked, either by a brake or bolting it to a building (the mechanism does not matter) then 100% of the available torque provided to the system travels to the right wheel.

A way to visualize how this works is to put a crowbar between 2 rocks. Now apply 100lbs of pressure on the crowbar. If no rocks move, is each rock only submitted to 50lbs of pressure? No. Because they are both braced against the crowbar, if 99lbs of pressure will move one of the rocks it will. The torque is only split if both rocks move.

Spider gears act in a similar fashion, by bracing one side against the other. If one side is kept from moving by any mechanism, all available torque will be braced against both until one moves.

Yet another way to explain this is that if you immobilize one side of a differential there's actually no mechanism for absorbing 50% of the torque. It just braces off the other side through the spider gears and sends the torque in that direction.

 

[Dark-Fx]

Well, no LOL. The way spider gears work, they can split torque between the axles, or provide 100% torque to either side.

The motor or engine will output X torque. That torque will go toward the wheel with the least resistance. If the left wheel is locked, either by a brake or bolting it to a building (the mechanism does not matter) then 100% of the available torque provided to the system travels to the right wheel.

A way to visualize how this works is to put a crowbar between 2 rocks. Now apply 100lbs of pressure on the crowbar. If no rocks move, is each rock only submitted to 50lbs of pressure? No. Because they are both braced against the crowbar, if 99lbs of pressure will move one of the rocks it will. The torque is only split if both rocks move.

Spider gears act in a similar fashion, by bracing one side against the other. If one side is kept from moving by any mechanism, all available torque will be braced against both until one moves.

Yet another way to explain this is that if you immobilize one side of a differential there's actually no mechanism for absorbing 50% of the torque. It just braces off the other side through the spider gears and sends the torque in that direction.

Well, yes LOL. Motor RPM halves for the same speed. You are doubling the gear ratio. Rivian has even stated that the torque is going to halve.

 

[R.I.P.]

Well, yes LOL. Motor RPM halves. Rivian has even stated that the torque is going to halve.

Um, I don't think we are talking about the same thing. Rivians don't have a differential.

 

[MountainBikeDude]

Um, I don't think we are talking about the same thing. Rivians don't have a differential.

To @Dark-Fx point, the dual motor will have a differential, and even with brakes holding the in air wheel, you won't get more than roughly 50% of the power from the axel to the planted wheel.

The clip starts where Mason, Rivians drivetrain VP attests to that fact.

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