Let's talk tow ratings? Why would the Dual-Motor and Tri-Motor have different CVWR?

[Rutrow]

At the CyberTruck reveal Tesla listed the tow capabilities for the Dual-Motor CT at 10,000 lbs. and the Tri-Motor CT at 14,000 lbs. but I can't get my brain to accept that there should be such a wide difference between the two.

To understand tow ratings it's important to know the components of testing used to determine their limits. The intuitive thought is that tow rating is how much weight can the vehicle get up to highway speed, but that is least important, and often the easiest test for vehicles to pass. More important metrics and more difficult to achieve relate to safely stopping and maneuvering the maximum load. Let's first touch on the components of SAE's J2807 standard.

SAE J2807 is a voluntary standard agreed to by vehicle manufacturers to allow buyers to objectively compare different models. The testing, pertinent to CyberTruck* includes:

• 0-30 mph within 12 seconds
• 0-60 mph within 30 seconds
• Passing acceleration from 40-60 mph within 18 seconds
• Five 16' launches up a 12% grade within 5 minutes. In both forward and reverse
• The parking brake alone must hold on that 12% grade, forward and backwards
• 0-20 mph stopping distance less than 80' without leaving a 11.5' traffic lane
• Safely** steer around an arc at 0.4g of lateral force
• Davis Dam Grade*** test without overheating
  • 12 mile uphill grade, averaging 5% slope
  • Done on a day over 100˚ F
  • A/C set to maximum cooling
  • Maintain a speed >40 mph
  • No warning lights, no diagnostic codes, without loosing any coolant

I have searched unsuccessfully to find which of these components cause the most common failures, but given the trend towards larger and larger grills on Heavy Duty ICE trucks, I tend to think the Davis Dam Test is what determines the tow rating for most trucks.

If I may, for purposes of this discussion, (given that three Model 3 motors [Tesla Semi] can propel >82,000 lbs.) we'll assume that even the Dual-Motor CyberTruck will have no problem achieving ALL of the minimum acceleration components at weights far greater than the Tri-Motor CyberTruck's maximum tow rating, and I also doubt that the Davis Dam Grade will result in any overheating for CyberTruck. In my opinion, the limiting components will be the stopping and maneuvering tests. These tests rely on the vehicle tire's ability to maintain enough friction with the road surface. And that is the nut of my feeling that both DM and TM CT will have nearly the same ability.

Friction at the tire patch, if both versions use the same sized tires, is entirely dependant on the downforce on the tires. Greater vehicle weight will increase the downforce, but will also contribute a higher inertial force resisting the tire friction. I am totally ignorant about whether increased weight would result in increased stopping/maneuvering ability or decreased stopping/maneuvering ability, but my gut tells me it's the latter (I'm open to discussion about this). Reviewing ICE truck CVWRs, every optional component (4WD, larger fuel tank, heavier axle/differentials, etc.) on a truck seems to reduce the payload and trailer capability by about the weight of that optional item. Is that because braking friction doesn't appreciably increase? or is it that the weight hurts them in Davis Dam test?

Regarding the two versions of CyberTruck, a large difference in tow rating could be explained by the two versions having markedly different grade of suspension/frame components. But this would be very un-Tesla-like. Tesla tends to make as many components identical across all trims, even across many models (S & X, 3 & Y). I think that suspension, cooling, steering parts will be the same for all CyberTrucks. Unless the Dual-Motor has vastly underpowered motors, I can't see why the tow rating would have any difference at all. If anything, the larger battery on the 500 mile range version should REDUCE the payload/trailer rating!

Unless...

Tesla decided to placard the two trims differently to create a market for the more expensive (higher margin) version. My thinking is that if think you need the Tri-Motor, not because of range, but because of its tow rating, you might want to put a Dual-Motor, 10,000 lb. tow rated truck to the J2807 test at the trailer weight you need. For instance, you're a plumbing contractor who tows a backhoe to jobs around your area (<100 miles/day). If it CAN safely stop, maneuver, and hold, even a 14,000 lb trailer, why would you spend the extra $20-30,000 for the top trim? I'd love to see somebody like Munro, or TFLTrucks put a Dual-Motor CyberTruck to the test.

For me, it will be the range I'm after. I want to tow a 9,000 lb Airstream around the country so getting from SuperCharger to SuperCharger in the rural west is my determining feature. But if the 500 mile version doesn't come out soon (by the time my RN11275... reservation comes up) I just might give the Dual-Motor a try. I recently put down another reservation (RN119 ) just for this reason. If a 350 mile truck won't work for me, at least I'm in line for a longer range version in the future.

I'd love to hear folk's thoughts on this, but instead of simply telling me "You're WRONG!" I'd like to hear the logic behind your opinion.

Thanks!

*non-dually, tow rated for >3000 lbs.
**maintain understeer along a 300' circle
***State road 68 and 163 from Bullhead City AZ into Nevada

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[ldjessee]

More motors, means you can do stronger regen braking, thus a heavier load can be safely towed. That is what I assumed.

 

[Rutrow]

More motors, means you can do stronger regen braking, thus a heavier load can be safely towed. That is what I assumed.

I had considered what effect regenerative braking and even torque vectoring traction control due to two rear motors might have on J2807 tests, but I couldn't think of any benefit they would provide. For the parking brake hill holding test, I could see that if locking the movement of all motors can be included as "parking brake" taking advantage of all wheel parking brake would help on that test, but I've had to have my single motor Model S with a dead 12v loaded onto a flatbed tow truck and can attest to the fact that one motor will COMPLETELY prevent the rear wheels from turning. Jumping the battery did allow us to release the parking brake, but the car refused to go into "tow mode". One motor held and neither rear wheel turned more than ~3˚-5˚.

Whether Tri-Motor torque vectoring could have any effect on the stability test on the skid pad, I'm also skeptical. I'm in the camp that, for purposes outside of F-1 racing, torque vectoring with brake application can be just as precise as the fine control you could get from motors applying the correction instead. Many on this forum have gone to great lengths to assert that Quad-Motors are essential to get decent torque vectoring out of the CyberTruck but I don't buy it. The skid pad test would be the only test in my mind where torque vectoring would have anything to contribute, but since it requires that the tow vehicle UNDERSTEER, traction control would be of limited help.

Thanks for commenting. I appreciate the input.

 

[JBee]

Nice post and write up. I've asked this a few times already as well, but I don't think there are many here that understand the trailer mass certification process, so we might have to investigate that ourselves.

One quick way to at least get an idea of what is possible is to do a comparison table with ICE vehicles with similar tow capacity. There you will find that a DM 700hp CT is in no way power constrained, so that it is a reason to limit tow capacity.

Primarily, it will be tongue load, wheelbase/overhang geometry, sustained friction braking capacity before fade or overheat, (regen might be excluded from this?), possibly some suspension limits.

Braking also needs to include weight transfer, uneven loads in the trailer bed etc, trailer sway etc.

Typically braking distance is just available tyre traction vs brake force, so depending on where you add mass, and how you deal with weight transfer whilst braking, it's not just a single argument either or, "if" extra mass increases braking distance. Extra mass might also get your tyres warmer, or in a load range that give more traction, on top of the mass induced traction. Tyre pressures should also be adapted for load. Surface traction is also subject to change, with aquaplaning, snow etc. So it's a fairly complex scenario with lots of variables.

As for using brakes for traction control, this is not comparable with torque vectoring (TV) QM at all. For example: you cannot maintain a controlled power drift with a hydraulic brake system, because it is to slow to modulate the brakes to match available traction in real time, it's always adjusting to far behind. QM TV actually stops traction losat from occurring in the first place, at most speeds. Although it also has limitations at low rock crawling speeds offroad, as discussed on the offroad thread.

This is because at speed a QM can rpm lock all four wheels to match ground speed, meaning intermittent patches of low traction (ice etc), will not affect it as much as a open diff. By the time the brakes react, you have already lost traction, which leads to more force on other wheels etc, whereas the motor controller can change torque in the kHz range, so can compensate not only faster than brakes, but for both acceleration and deceleration. Essentially a QM TV setup would do a "tank turn" together with the 4WS steering angle on all 4 wheels at all times, so no wheel is doing what it shouldn't. However there are some lateral stability issues in edge cases and alternating surface traction conditions that need to be addressed, and programmed out as well.

BTW ESP can actually steer in the opposite direction of the steering angle of the wheels, because the orientation of rotation is less capable of changing direction than the contact patch angle of attack. (Otherwise a stationary Rivian tank turn wouldn't work either) Especially when you already have traction loss.

There's also the benefits of the rear wheel steering to consider here for trailer stability (crab steer on hwy use, tongue weight tolerance etc) that should all equate to a much higher CT tow rating.

I'm hoping we'll find out some more next month, but overall I strongly suspect a DM won't "technically" have a different tow rating to a TM/QM unless Tesla wants it so for marketing purposes, or if by some extra safety factor they offer they can increase it for TM/QM.

 

[cvalue13]

I’d first add that we’re trying to explain spec differences at 2019 unveil, which we’ve learned aren’t necessarily a good guide to the specs that will be unveiled on Nov. 30.

At this point, it’s completely possible that both variants have identical payload / tow ratings. Not merely in theory, but also in available evidence. For example, we’ve seen only a dual CT towing only a ~10,000lb trailer at the dam grade test location. That’s not definitive (the test can be done on paper in special test facilities). But it’s interesting.

Second, the explanation can come down entirely to slight modifications in suspension setup. The Lightning has far more power than any ICE F150 with higher lb tow rating.

The F150 "MAX TOW" option is basically a larger fuel tank/tranny cooler/trailer hook ups/elec locking diff/tranny and engine oil cooler/etc that has nothing to do with the trucks GVWR or rawr. For the F250, these same ‘ungraded’ apply but more to the effect is the heavier weight of the truck, suspension, frame, plus - importsntky - the longer wheelbase.

Looking at those as relates to the CT:

Weight: with identical packs, we might expect the ‘performance’ to weigh LESS than the base - if they do speed-related component weight reduction. But here seems better to assume they’ll prob weigh materially the same, for any degree related to towing stability/traction.

Frame: again, presumably roughly the same, but?.

Wheelbase: here, certainly the same

Comes back to suspension mostly

With electronically controlled air suspension, this could in theory be just a trim differentiating software difference? That would be weird, though.

But I’ll come back to power, too: another spot where software + power related differences could provide better control against eg trailer away, etc.?

But all-in-all, I’m left leaning towards doubting whether the base and performance will have spec differences in payload/towing

 

[Aces-Truck]

Antidotally, I've heard it's bad to try to tow a trailer that's heavier then the tow vehicle. I understand that with trailer brakes maybe this is not that critical. But if those brakes fail, stopping could be compromised by the tire friction on the tow vehicle.

If you follow this idea. perhaps the trimotor version, by having a 500-mile range (bigger, heavier batteries), would be seen as having more towing capacity by virtue of the heavier vehicle weight.

 

[Rutrow]

Antidotally, I've heard it's bad to try to tow a trailer that's heavier then the tow vehicle. I understand that with trailer brakes maybe this is not that critical. But if those brakes fail, stopping could be compromised by the tire friction on the tow vehicle.

If you follow this idea. perhaps the trimotor version, by having a 500-mile range (bigger, heavier batteries), would be seen as having more towing capacity by virtue of the heavier vehicle weight.

That might be nice rule of thumb, but, anecdotally, every Heavy Duty and Super Duty Pickup I've towed with says that's not a fact. <9000 lb trucks that tow >20,000 lb trailers.

 

[Rutrow]

Primarily, it will be tongue load, wheelbase/overhang geometry, sustained friction braking capacity before fade or overheat, (regen might be excluded from this?), possibly some suspension limits.

Braking also needs to include weight transfer, uneven loads in the trailer bed etc, trailer sway etc.

Typically braking distance is just available tyre traction vs brake force, so depending on where you add mass, and how you deal with weight transfer whilst braking, it's not just a single argument either or, "if" extra mass increases braking distance. Extra mass might also get your tyres warmer, or in a load range that give more traction, on top of the mass induced traction. Tyre pressures should also be adapted for load. Surface traction is also subject to change, with aquaplaning, snow etc. So it's a fairly complex scenario with lots of variables.

These are all good points to keep in mind when towing, but don't come into play when manufacturers determine their tow ratings. They use a standardized trailer and standardized payload distribution. If they're trying to maximize their tow rating they're likely to use an idealized set up that their average customer can't match, but these days, with liability the way it is, I'll bet they take a lot of user error into account when stamping the CVWR on their vehicles.

As for using brakes for traction control, this is not comparable with torque vectoring (TV) QM at all. For example: you cannot maintain a controlled power drift with a hydraulic brake system, because it is to slow to modulate the brakes to match available traction in real time, it's always adjusting to far behind. QM TV actually stops traction losat from occurring in the first place, at most speeds. Although it also has limitations at low rock crawling speeds offroad, as discussed on the offroad thread.

This is because at speed a QM can rpm lock all four wheels to match ground speed, meaning intermittent patches of low traction (ice etc), will not affect it as much as a open diff. By the time the brakes react, you have already lost traction, which leads to more force on other wheels etc, whereas the motor controller can change torque in the kHz range, so can compensate not only faster than brakes, but for both acceleration and deceleration. Essentially a QM TV setup would do a "tank turn" together with the 4WS steering angle on all 4 wheels at all times, so no wheel is doing what it shouldn't. However there are some lateral stability issues in edge cases and alternating surface traction conditions that need to be addressed, and programmed out as well.

BTW ESP can actually steer in the opposite direction of the steering angle of the wheels, because the orientation of rotation is less capable of changing direction than the contact patch angle of attack. (Otherwise a stationary Rivian tank turn wouldn't work either) Especially when you already have traction loss.

There's also the benefits of the rear wheel steering to consider here for trailer stability (crab steer on hwy use, tongue weight tolerance etc) that should all equate to a much higher CT tow rating.

I'm hoping we'll find out some more next month, but overall I strongly suspect a DM won't "technically" have a different tow rating to a TM/QM unless Tesla wants it so for marketing purposes, or if by some extra safety factor they offer they can increase it for TM/QM.

I'm an open minded skeptic when it comes to the wonders of motor controlled torque vectoring (MCTV) versus brake controlled torque vectoring (BCTV). I have read every word that MCTV proponents have written and it all sounds like salesperson hype to me. Take your point about how slow hydraulic fluid is. Sure, any piston actuated mechanism will be slower than computer controllers, but even if the computer is sending the electric motor commands at kHz speeds, it's the inertia of the motors rotor that is constrained by physics. Even if the computer tells the motor to reduce it's rotation in 3 ms, an iron rotor turning at 16,000 rpm is going to take FAR longer to reach that speed, and in my mind, slower than a hydraulic piston would.


Forewarning. Anyone who want me to believe anything they say about Torque Vectoring will need to SHOW me that it's superior before I'll be willing to pay you a nickle for it. I'm not from Missouri, but I'm close enough to the "Show me state" that some of it has rubbed off on me.

 

[JBee]

These are all good points to keep in mind when towing, but don't come into play when manufacturers determine their tow ratings.

Not sure what you are trying to say here seeing that J2807 includes both braking performance and tongue weight, which is what I highlighted in my comments. There's also a few other standards covering towing like J2664 and even still for the tow vehicle itself as well.

From what I can tell from the posts I found online, the standards that are used are fairly open, and far from really definitive in locking down the safety of towing. The braking speed tests are a joke to say the least, being only from 20mph.

I'm an open minded skeptic when it comes to the wonders of motor controlled torque vectoring (MCTV) versus brake controlled torque vectoring (BCTV).

So did you want me to persuade you of the benefits of QMTV or not?

Just briefly, TV does not exclude the use of the brakes, rather it uses both. So it's not a brake "versus" motor traction control situation. The brakes are already onboard and are used "when" they are of extra benefit.

The second factor is that brakes only have one main cylinder and pulse the individual wheel caliper to modulate torque/rpm. This means that it needs to vary main cylinder pressure to the required average of all the wheel calipers, or as is most common, simply pulse the brakes. This leads to the wheel stepping and braking traction in the process, and is also why ABS increases the braking distance, but allows you to still steer. Further brakes cannot modulate torque separately from RPM, whereas a electric motor can, to the point it can also produce reverse torque as well. So a motor can both accelerate and decelerate the rpm of the wheel, OR change the torque of the wheel whilst maintaining the same RPM. Note that a torque need not move, and so like using a torque wrench, a nut and bolt can hold a tension without moving.

As for motor inertia, this is not reduced when using a brake system, when the motor is a direct drive without a clutch. The brake still has to deal with both motor rotor and wheel rotation inertia. In this case though the wheel moment of inertia is higher than the motor one, seeing that a electric motor rotor is nearly always lighter than a wheel, and gearing is relatively low ratio, because of the motors low range torque.

The motors on the Plaid for example accelerates the MS faster than the brakes (with regen) can decelerate it, because it is already on the limit of tyre traction.

Accordingly, inertia only plays a secondary role, in that the real limit, as I postulated previously, is about maintaining enough braking or acceleration force to match tyre traction. The difference here is that QMTV gives finer graduated control over each individual wheel than braking can by itself.

Pay attention to the terms they use in this video describing the Rimac TV system.



Now if you combine this with a position detection system of the trailer (could just be CV camera or US) you'd have the ability to also use it to reduce sway, or to restabilise a vehicle. There are also specific toing ESP modes in some vehicles, that would benefit from the added performance of a QMVT. Together with much better offroad capability, that a brake system just simply can't keep up with in the real world at all.

 

[Crissa]

More motors, means you can do stronger regen braking, thus a heavier load can be safely towed. That is what I assumed.

While this is true, in practice regen is limited by the battery's ability to absorb it more than the motors' ability to create it.

-Crissa

 

[newwave1331]

While this is true, in practice regen is limited by the battery's ability to absorb it more than the motors' ability to create it.

-Crissa

If 4680 packs have the ability to discharge at higher rates than 2170s, should we assume that regen could be stronger with higher charge rates? Under normal/non-towing driving this won’t be a big impact due to driving comfort. If you are towing at full capacity, can we expect to recapture a higher percentage of regen? I look at tests like TFLs Ike Gauntlet and think a higher regen capability could help towing range. In places like the mountains, you could use recapture a higher percentage of energy on downhill slopes. It also could nearly eliminate the need for huge brakes. Will regen er

Also a topic came up on a X space about regen. Can a tri motor capture a higher percentage of regen than a dual motor now that 4680 packs charge at higher rates? Could thermal management be the limiting factor? The host had a theory that tri-motor will be more efficient because it can capture more regen. I could imagine if motor temperature was a limiting factor, this could be true.

 

[ldjessee]

While this is true, in practice regen is limited by the battery's ability to absorb it more than the motors' ability to create it.

-Crissa

I believe one of the upgrades that was reviewed was that the regen can dump extra to generate heat and that be disbursed by the heating/cooling system… but I could have that wrong.

 

[Crissa]

If 4680 packs have the ability to discharge at higher rates than 2170s, ...

I don't think we know yet.



But braking power will be higher on the front axle than the rear one. So the potential energy the rear axle is probably not going to exceed a single motor's capability. (This is why you have smaller/fewer braking surface in the rear than the front).

I believe one of the upgrades that was reviewed was that the regen can dump extra to generate heat and that be disbursed by the heating/cooling system… but I could have that wrong.

I don't think that applies to regen, specifically. They use the motors in a 'less efficient' manner via the motor controller's alternating power pulses to heat up the motors within tolerance of the coolant system.

I have never heard of that being used in regen mode.

-Crissa

 

[PilotPete]

I don't think we know yet.



But braking power will be higher on the front axle than the rear one. So the potential energy the rear axle is probably not going to exceed a single motor's capability. (This is why you have smaller/fewer braking surface in the rear than the front).


I don't think that applies to regen, specifically. They use the motors in a 'less efficient' manner via the motor controller's alternating power pulses to heat up the motors within tolerance of the coolant system.

I have never heard of that being used in regen mode.

-Crissa

I believe I red this is a new feature that is intended to give the same regen/single pedal feel and feature when the battery is at 100% as it is when the battery is at less than 100%.

 

[JBee]

I believe I red this is a new feature that is intended to give the same regen/single pedal feel and feature when the battery is at 100% as it is when the battery is at less than 100%.

Sounds counter productive to run your whole cooling and drivetrain system to get rid of heat generated by regen, while you have completely normal brakes onboard that will do the same with minimal wear and tear?

I'd be the first one to turn it off. Rather replace brake pads than motors, inverter, and cooling system etc.

Sounds like one of those hypotheticals again, and all just to improve pedal feel, which would probably be the worst reason.

What they should do is have charge stations on the way down the hill, so you can pull over and charge up the vehicles going up the hill if your battery gets to full. V2V would be good for that too. Then at the top fill up the bed with water, and you'd have perpetual hill climbing capability...

But honestly...just don't fill up your EV to 100% if you know your going down a big hill. That solves the whole problem and avoids losing energy to any heat generation. Better for battery cycle life as well not to charge it full.

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