Actual Off-Road Testing: climbing & descending (@ Hollister Hill RSVA - 10/23/23)

[Setok]

Yep that pretty much sums it up.

A mechanical lock on both front and rear (and middle if you have a middle diff like most full time 4x4) means each wheel is turning synchronized with each other wheel, regardless of if that wheel has traction or not.

Most importantly this means that every bit of forward thrust produced by the wheels is produced at the same time and at the same speed. Wheel torque is distributed in "real-time" and only offset by the amount of twist and slack in the driveshafts.

Consider this scenario: If you would have a 4 wheel vehicle, and two of the wheels spin at say 10RPM and the other two at 15RPM, then what is the rate of forward motion? Sometimes 10 or 15RPM, but not 12.5RPM, meaning that the faster wheels are working against the slower ones.

Now with a motor per wheel the "syncronisation" of the wheels should technically be possible, but to do so means you need very high sensor feedback to modulate the motor inverter output to match the torque load on each wheel. This would have to be in the ms control loop range. A locking differential on each the front and rear motor on a DM CT, would be much simpler and likely perform better than a QM in low speed rock crawling situations with high wheel articulation, however, in other off-road conditions, a QM with torque vectoring would operate better when you need more wheel slip and need to steer.

In the end from a control perspective, so long you can basically "look" (aka sense) at the ground in relation to the vehicle, and see what direction and velocity you have, you should be able to calculate exactly the wheel position from a rotary encoder etc, and steering angle sensor, and then use the motor controller to observe current (torque) and then limit switching frequency to lock in a rpm that matches what movement it is doing across the ground. This will however only work if it can maintain the right amount of torque per wheel, so it's likely that either a high ratio gearing needs to be used per wheel (like a portal axle) or that it is a high power motor that can achieve full torque at low RPM.

This differs considerably in complexity from a normal ICE, with just one motor and a low range gear, where with lockers you can have every bit of torque from the engine going to any one wheel or any combination of them in an instant mechanically. That's why if you have a how power ICE you can snap shafts.

Technically, without lockers, a dual motor Cybertruck is only a 2WD, with one front and one rear driven wheel only. A Tri-motor is only 3WD without locker, and only a QM is really 4WD. Traction control really doesn't cut it, but for many less serious users they probably won't know how it is meant to be, so unless they experience lockers wouldn't understand just how much better they are.

My suspension comments also come into consideration here as well, because if you can't get your wheels on the ground, you really don't have a 4x4 anyway. More like a AWD. But even there, my son has a modded sub 4sec WRX, and that comes standard with a rear LSD to get that power down. So definitely useful.

It’s not necessarily fair to say it is 2WD. A Model 3 mostly is, but I’m not sure what Cybertruck’s implementation is. It might be that in offroad mode it will default to sending the same torque to both axles. That means that less per wheel torque is required to get the vehicle moving, which means less likely for each vehicle to slip (which is why full time 4WD is useful).

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[Dmcgee]

I was really speaking to your point about lockers being common in 1/2 ton pickups. I can see LSDs being common, but I don't really think lockers are, but maybe I'm wrong.

My spouse peered over at this and asked, 'who would know best about locking axles being useful' then she said 'military!' then she ran off to figure out what vehicles the military uses that have locking axles...

'Marines on beaches and multi-axle trucks,' she came back with. 'That is all I can find.'

The only reason you use locking axles is because you don't care about cornering, you only want to scrabble forward or backwards. There's no reason a truck with motor per wheel cannot simulate it.

Besides, experience with the vehicle and track overcomes alot of slip. Knowing where to put your tires is just, the most important thing.

Having a locking differential won't stop you from spinning your tires if you put them in the wrong spots with the wrong torque.

-Crissa

I agree with Bill837 ... 'Lockers' are NOT common on most full size trucks and you generally have to find one optioned for a run of the mill LSD which need regular maintenance they never get (so most are roasted after 20-30Kmiles). Also, part time (air or electric) lockers are only useful when doing something that is clearly sketchy to begin with ... how many people are really going to be putting there 9000# $90K rig into such a predicament .... uh.... never mind.

The Humvee has Torsen 'lockers' which are actually a 'part time' traction control differential which is also known as a 'Detroit Locker' ... like the kind I've had on my Jeeps for the last 20+ years. Look on YouT for torsen differential and you can find some really interesting engineering vids about how it works.

Personally, I don't think the CT needs factory lockers. Based on what I've seen my wife's M-Y do on black ice, it works better than my JK or JT dealing with really poor traction (and no I'm not going to take the MY off road to prove anything).

 

[Trekboy]

The inherent difference between mechanical locker and traction control is that a locker is instantaneous torque on the wheel with the most traction, and traction control is a delayed torque, after brake intervention with the wheel with least traction.

The suspension setup also seems way to stiff and doesn't comply and articulate with the undulations of the road well. Some of this can be adjusted by using the airbag suspension, but if they do not have a way to disengage of reduce the impact of the torsion sway bar it will still want to hang on the high parts and dangle the opposite wheel. Wheel articulation is critical in keeping as many wheels as possible on the ground with traction.

Now with a locker this would simply mean the compressed wheel has more traction and it would use that to drive further. In a traction control setup, this will mean the dangling wheel has no traction and would spin, and the traction control would see this, reduce throttle and activate the brake to stop it from spinning, which in turn puts torque onto the compressed wheel with traction, and moves the vehicle forwards.

This delay produces intermittent loss of traction on all the drive wheels which impedes forward propulsion, as power is constantly being reshuffled between the drive wheels that have traction, and energy is lost through the use of brakes, which also causes heat and wear.

The difference is quite large, in that a locker needs far less power to achieve the same amount of forward momentum, and as such would consume far less battery to achieve the same task.

The combination of having a non-articulating suspension, limited by the sway arms and airspring settings, and not having lockers makes this simple hill a chore for the CT.

This is also why the Range Rover just idles up there with apparently no traction loss. Because it's wheel articulation is excellent, and at a minimum it has a rear locker/LSD.

For an off-road EV, a mechanical locker is a efficiency device, because no dive inertia is lost through traction control interference which has to be compensated for by more individual wheel acceleration, and no brake interventions cause energy to be converted into heat instead, along with brake wear.

So although traction control is an easy add on to any vehicle with ABS by just adding code, it is in no way a replacement for a locker type system.

You can see this in the following video which shows the traction only version in low range at 0:50 and the locker version going up the same hill with the same car at 5:20.



The only way to achieve the same as a locker with a EV, and in fact it would likely be even better, is by using a individual motor per wheel setup like the QM Rivian. That way you can not only "lock' the drive wheels in synchronization with each other, but also torque vector, in that it would also be "locked" to the steering angle and would make sure each wheel is spinning at the exact speed required to turn, being faster on the outside and slower in the inside of the corner. This is like tank turn, but limited to the steering angle of the wheels.

Further to this, a disconnect-able sway bar also is beneficial off-road, but is required to stop the vehicle from leaning out of a corner on-road. There are a few options for this as well. My old 2001 Discovery already had this, in that when they were owned by BMW, they added an active hydraulic actuator to the sway bar that would lean the vehicle into the corner. This also leaned the body against a side slope whilst crossing sideways along a hill. But being cross linked, also meant that the wheel articulation (that is the front wheels leaning one way and the rear leaning the opposite way) was excellent and not impeded by the active sway bar in any way.

The alternative version is to have a passive system, that like the locally developed Kinetic system was used in Toyota production vehicles with KDSS in Landcruiser, can do so by simply cross linking a hydraulic actuator on either sway bar. This takes advantage of the fact that on-road a vehicle leans out of the corner, compressing the two outside wheels equally in the same direction, and opposite to the two inside wheels, which in turn is met with the full resistance of the actuator and heavy sway bar.

Conversely, off-road, wheel articulation is such that the front suspension is opposite to the rear, meaning that the cross link of the actuator line wo opposite corners allows full wheel articulation without the sway bar trying to resist it.



So given the above, I'd rate the CT as an "average" off-roader in it's current configuration from what we have seen so far.

I am obviously hopeful they will sort some of these issues out with further testing, optimizations and iterations, but I doubt that active hydraulic brake actuation and modulation is able to achieve the levels of torque distribution offered by a solid mechanical connection or single motor per wheel torque vectoring.

GREAT videos. I can totally attest to mechanical lockers. I once had a full-size Bronco, which stock, was a total nothing-burger. I then installed a Detroit locker in the rear and Dana Posi in front, and it became a beast of a machine. Keeping my fingers crossed with the Cybertruck. Maybe the Tesla team will come up with something we haven't thought about.

 

[Crissa]

Unfortunately I believe you are wrong. A motor per wheel cannot simulate it. This takes a bit of thought to realise (I believed wrongly as well, at first). The key to realise is that the same torque, when applied on a wheel with low or no traction, will spin it fast, while the wheel on solid concrete will move forwards at a steady pace. WIth a locked differential torque is not sent equally and separately to both wheels, but interestingly is sent to the wheel with most traction. Instantly (due to physics). This cannot be really replicated with computers and motors, as for the system to realise that torque should not be sent to a wheel with less traction, the wheel has to slip. Otherwise you'd need some kind of probe to figure out the wheel is going to slip. It still means the other wheel can drive the vehicle forward, but the slipping wheel can dig down into material of weak traction (mud, snow etc), which can result in the vehicle being stuck (you'll be surprised how little snow is needed for that to happen), or slipping sideways.

A locked differential will absolutely stop your wheels from spinning, as long as even one wheel has enough traction to move the vehicle. The others will not slip, but merely turn at the same speed.

Most military work is driving from A to B. Sometimes over rough terrain, yes, but not necessarily always on the trickiest offroad. Plus those vehicles are driven by all kinds of random people who will often not have any clue how to use locks. So possibly the decision is the risk is not worth it, as keeping a lock on when driving on asphalt can damage the transmission. Then again militaries that use old Defenders have central diff locks, and those using G Wagons may additionally have rear locks. So perhaps only things like Humvees don't have them (I'm honestly not sure)?

Big paragraph to say that you don't think that electric motors can't be controlled in sub-full rotation. Or move a connected wheel a sub-full rotation.

Being that is something robots do all the time, well...

Those who believe that a motor on each wheel can "simulate" a locker are being proven way wrong with the Hummer and the Rivian quad motor.

The QM Rivian behaves so badly in low mu situations and low speed obstacles that I traded it for a loss for a new dual motor Enduro version, because dual motors with traction control is much better than that quad motor disaster. Based on this, because I am an off-roader, I will _not_ be selecting the TriMotor version of the Tesla!

While I don't discount your experience...

...It doesn't match their demonstrations, nor create a theoretical limitation.

"Hummer didn't do it" isn't "is can't be done"

Lightning supposedly has it standard. Maybe you should have gotten one of those.

-Crissa

Gosh darnit, why does this forum sometimes delete random characters or sentences while I'm typing?

 

[JBee]

Big paragraph to say that you don't think that electric motors can't be controlled in sub-full rotation. Or move a connected wheel a sub-full rotation.

Being that is something robots do all the time, well...


While I don't discount your experience...

...It doesn't match their demonstrations, nocreated a theoretical limitation.

"Hummer didn't do it" isn't "is can't be done"

Lightning supposedly has it standard. Maybe you should have gotten one of those.

-Crissa

Gosh darnit, why does this forum sometimes delete random characters or sentences while I'm typing?

You're still hilarious Crissa! ?

 

[Bill837]

Ford Chevy and Dodge all offer rear locker options for virtually every 4x4 pickup they sell.

Sure do, any idea on the take rate for that option? Just like eight foot beds. Here's the thing. You don't get all the magic options in every truck, or the price goes through the roof. Lockers, on the fly manually adjustable air. At some point, design wise, you have to decide what your market is. And the folks who want to run the Rubicon flat stock are a small enough percentage that trying to outfit your vehicle to that spec means you lose a lot of sales. Even trying to set up production with those things as options means added complexity to your supply and production line that pretty soon you eat into margin.

How far do you go in making the vehicle more costly and complex and less comfortable/capable on road do you go for that 1% of customers? Its not that they dont know the questions to ask, its that they came to different conclusions to you. They didn't ask "How do we build the worlds most capable off road EV bar none". They asked "How can we build the worlds most overall capable EV pickup in a slight smaller footprint and still make money on it".

























how we can deliver the worlds most overall capable

 

[GuyV]

Unfortunately I believe you are wrong. A motor per wheel cannot simulate it. This takes a bit of thought to realise (I believed wrongly as well, at first). The key to realise is that the same torque, when applied on a wheel with low or no traction, will spin it fast, while the wheel on solid concrete will move forwards at a steady pace. WIth a locked differential torque is not sent equally and separately to both wheels, but interestingly is sent to the wheel with most traction. Instantly (due to physics). This cannot be really replicated with computers and motors, as for the system to realise that torque should not be sent to a wheel with less traction, the wheel has to slip. Otherwise you'd need some kind of probe to figure out the wheel is going to slip. It still means the other wheel can drive the vehicle forward, but the slipping wheel can dig down into material of weak traction (mud, snow etc), which can result in the vehicle being stuck (you'll be surprised how little snow is needed for that to happen), or slipping sideways.

A locked differential will absolutely stop your wheels from spinning, as long as even one wheel has enough traction to move the vehicle. The others will not slip, but merely turn at the same speed.

Most military work is driving from A to B. Sometimes over rough terrain, yes, but not necessarily always on the trickiest offroad. Plus those vehicles are driven by all kinds of random people who will often not have any clue how to use locks. So possibly the decision is the risk is not worth it, as keeping a lock on when driving on asphalt can damage the transmission. Then again militaries that use old Defenders have central diff locks, and those using G Wagons may additionally have rear locks. So perhaps only things like Humvees don't have them (I'm honestly not sure)?

I do not think you are crediting how fast computers are in relationship to the mechanical world, how fast motion can be sensed and how fast electric motors can be controlled.

 

[GuyV]

There are dozens of pages of why the engineers were wrong, and why in practice it is a failed concept on the Rivian forums. TFL's recent shootout with a lowly Chevy trail Boss shows just how anemic the poor quad motor is. The dual motor with differentials pulls itself over that type of obstacle with no fuss whatsoever.

However, separate motors on each wheel causes a problem far greater than being unable to act like a "true locker". When decelerating down a slippery bank or an off camber icy situation in my differential equipped vehicle, I can open them up, and allow the wheel with the most traction to maintain my lateral stability. I can use the wheels with the least traction to either increase or decrease my speed, without compromising my lateral stability. By design, differential equipped vehicles are inherently laterally stable because of this.

You absolutely, utterly give this up if the wheels are not connected with a differential to use physics in this way. The result is a vehicle that will suddenly and wildly yaw sideways at the worst possible moment. When on the trail, we have to treat quad motor rivians the same as the spooled Yoda's, and actually winch them down into and out of certain obstacles because of their lack of lateral stability.

I never understood people who used Lincoln lockers for this reason, and it is even more egregious in a production vehicle.

Yeah, yeah, yeah, and they told Elon you can't reuse rocket boosters. He will Dojo this. ?

 

[HaulingAss]

Well yes, if you connect them, you solve the problem lol.

I said, "Let's remain logical here", not "Let's try to create the appearance you knew what you were talking about all along"!

 

[Setok]

Big paragraph to say that you don't think that electric motors can't be controlled in sub-full rotation. Or move a connected wheel a sub-full rotation.

Being that is something robots do all the time, well...


While I don't discount your experience...

...It doesn't match their demonstrations, nor create a theoretical limitation.

"Hummer didn't do it" isn't "is can't be done"

Lightning supposedly has it standard. Maybe you should have gotten one of those.

-Crissa

Gosh darnit, why does this forum sometimes delete random characters or sentences while I'm typing?

I didn’t say motors can’t be controlled sub-full rotation. I said that in order to detect one wheel has less traction, it will have to slip. Whereas with a locked axle it won’t. That’s why you can’t simulate a locked axle with independent motors on each wheel.

That’s what JBee and I have been trying to explain.

 

[Setok]

I do not think you are crediting how fast computers are in relationship to the mechanical world, how fast motion can be sensed and how fast electric motors can be controlled.

I’m well aware of how fast they are. Though reliable sensing isn’t always trivial. Still, the fact remains you need to have a slip to sense it.

This is easily noticed by a human on a Model 3 between front/rear axle, where you can sense the slip before the vectoring tries to compensate. Very different from the Defender I drive when central diff is locked.

Now obviously the M3 is not a Cybertruck so there may be tweaks, but the core fact remains. The Rivian is proof of the limitations in that approach (still a nice vehicle from what I’ve seen).

 

[seanrarey]

I said, "Let's remain logical here", not "Let's try to create the appearance you knew what you were talking about all along"!

Wow. You are remarkably hostile.

We have spent hundreds of thousands of dollars, and thousands of hours in the last year trying to be one of the first to do true BEV over-landing things. We have purchased two Rivians, a Hummer EV and a Lightning, and even worked with a couple of the manufacturers to try to get these rigs to do what we need them to do. I have a first day reservation for a CT, and we will add it to the stable.

I stopped in here to see what other people are doing, and any knowledge they have gained, only to find, well, what seems like a bunch of talkers.

One more place identified where there does not seem to be much actual usable data, moving on.
?

 

[CyberGus]

The superiority of a locked axle is irrelevant, since the Cybertruck will not have this capability. Software-based traction control must suffice.

 

[HaulingAss]

Wow. You are remarkably hostile.

Hey, you are the one who claimed that obviously a motor for each wheel couldn't replicate a locking differential since the Hummer and the Rivian didn't do it!

If it's remarkably hostile for pointing that out, then maybe you have a problem with reality itself.

We have spent hundreds of thousands of dollars, and thousands of hours in the last year trying to be one of the first to do true BEV over-landing things. We have purchased two Rivians, a Hummer EV and a Lightning, and even worked with a couple of the manufacturers to try to get these rigs to do what we need them to do. I have a first day reservation for a CT, and we will add it to the stable.

It sounds like you have more money than brains! I'm sorry if that sounds "hostile" but it should be obvious by now a gas truck is a better fit for your remarkably niche application. Tesla is trying to solve the problems that the majority of truck owners face, not the less than 1%'ers.

 

[JBee]

The superiority of a locked axle is irrelevant, since the Cybertruck will not have this capability. Software-based traction control must suffice.

We don't know if it might have a limited slip diff in the rear or not. If it has nothing there will be a sizeable aftermarket for them.

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