Flat towing a Rivian

PaulMLAS

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For those of us that are actually RVer's there are really just 2 important factors in this thread in my opinion:

1. Wow, the Rivian can be flat towed!

2. Per most (all?) state towing laws the Rivian MUST brake itself regardless of whether it's regenerating or not.

It's fun to theorize the interesting ways Rivian could assist in accomplishing #2 outside of the currently available aftermarket solutions. So for those that question the relevance of braking, it's not just a relevant discussion, it's something that Rivian must consider when accomplishing #1.
 
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CappyJax

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Because that isn’t right. In order for the Rivian regen breaks to be engaged, the RV brakes would have to be engaged also.
Why? Every large electrically activated trailer I have ever pulled allowed for the trailer brakes to be applied manually through the brake controller. Also, the regency braking could be applied in the brake travel of the towing vehicle before that vehicles brakes are applied.

It is also unlikely that the Rivian would break with only the Regen but I’ll just give you that one. At the absolute very best, the Rivian would account for a quarter to a third of the braking power on the downhill.
How so? It has 700 horsepower. That means it can provide up to 700 hp of braking power with the regenerative braking.
 

Hmp10

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The batteries are a limiting factor in how much regenerative braking energy can be returned to the batteries for storage. I don’t see that accounted for in any calculations here, and in most EV’s the batteries can only absorb electrical energy at about half the rate they can release it.
 
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CappyJax

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The batteries are a limiting factor in how much regenerative braking energy can be returned to the batteries for storage. I don’t see that accounted for in any calculations here, and in most EV’s the batteries can only absorb electrical energy at about half the rate they can release it.
Well, 700 HP of regenerative braking would be a very aggressive deceleration. But Rivian has stated their batteries will be able to charge at 360kW at 800V, which means the batteries can absorb 480 HP worth of energy. And because of the regenerative breaking only being about 70% efficient, that is 490HP back into the batteries if you could get 700HP of regeneration from the wheels.
 

Hmp10

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It also depends on how fully charged the batteries already are. Unless Rivian is going to use graphene ball or solid state battery technology, there will be protective circuitry to slow the charge fed to the batteries as they near full state in order to prevent their accelerated deterioration. Batteries are generally pretty poor receptors for regenerative braking energy, which is why Tesla is experimenting with adding capacitors to their battery packs, as capacitors are more optimal for capturing regenerative braking energy, especially if it’s soon going to be called upon for acceleration.

I have read that Rivian will cut back battery charge rates once the battery pack reaches 80% of capacity. I don't see that accounted for in any of the above calculations.
 
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stank65

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Why? Every large electrically activated trailer I have ever pulled allowed for the trailer brakes to be applied manually through the brake controller. Also, the regency braking could be applied in the brake travel of the towing vehicle before that vehicles brakes are applied.



How so? It has 700 horsepower. That means it can provide up to 700 hp of braking power with the regenerative braking.
You’re assumptions are just over the top. Assume the cat is a sphere. Real world applied engineering does not work like this.

I agree with PaulMLAS. It’s great the Rivian can be flat towed. If it can regen while being towed in a safe and legal manner that is a bonus.

I don’t want an over engineered (more expensive) solution, and I want a solution where I can hook and go as easily as possible.

All of this functionality can be accomplished with minimal deviation from the Rivian baseline (only software and connections) and with existing proven tow arms.

The revolutionary parts for the car industry could be a vehicle that comes from the factory with these connection points integrated and hidden, and software driven communication between RV and toad.
 

ajdelange

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RIvian has to be careful to not fall into the trap Tesla did with the Model X by putting in way too many features in the initial version. An admission by Elon himself. He even had a name for it that escapes me at this moment.
I wound up here as this thread was referenced in a more recent one on flat towing and this comment caught my eye. While this thread is ancient, Elon's name for the X's excesses might still be of interest. He refers to it as his "Faberge Egg".

But reading on in this thread I see dozens of posts about how regen braking works that are way off in left field so as this is of current interest I thought I'd try to sketch out how it actually does work.

The motors have a fixed outer hollow cylinder with teeth pointing towards the axis. On these teeth are wound 3 (I'm very much simplifying) coils of wire at 12 o'clock, 4 o'clock and 8 o'clock. This assembly is called the stator. It is fixed to the body of the car. By adjusting the currents in the 3 coils properly a magnetic field can be induced in the cylinder in any orientation. Thus there could be a North pole at 2:30 with a South pole at 8:30 (they are always opposite one another). If the current waveforms to the coils are periodic the magnetic field can be made to rotate. It is the function of the inverter to provide those currents.

Now we install another solid cylinder inside the stator. This is the rotor and is attached to the wheels. The rotor also contains a magnetic pole pair. Picture the car as observed from its right side (passenger in North America) moving, at uniform speed, to the right. The wheels are spinning clockwise as is the magnetic pole pair in the rotor. And so also is the magnetic pole pair in the stator. Driving at constant speed requires that power be supplied to the wheels so the magnetic pole pair in the stator is, at all times, slightly ahead of the pole pair in the rotor e.g. if the rotor N pole is at 2 o'clock at a particular instant of time the stator south pole might be at 3 o'clock. As a south pole attracts a north pole the rotor will be pulled in the clockwise direction, i.e. torque will be applied to the wheels in a direction that moves the car forward. If you have a pair of bar magnets experiment with them.

If you want the car to accelerate you move the stator poles a bit away from the rotor poles in the clockwise direction. The torque depends on sin of the angle between the pole pairs. Thus if poles are at 3 o'clock and 4 the torque is proportional to sin(30°) = 0.5. But if they are 2 hours apart the torque is proportional to sin(60°) = 0.87. To get the poles farther apart we speed up the rotation of the stator pair by increasing the frequency at which the inverter operates. The torque goes up and the vehicle accelerates. Conversely, if we want to slow the car we move the stator poles counterclockwise by reducing the frequency of the inverter.

Regen.png


Perhaps this crude sketch will help. Note that it resembles a PM synchronous motor more than an induction motor in which the rotor poles are induced (there are no magnets in the rotor of an induction motor).

On the left of the sketch we have a motor. When the voltage applied to a stator coil is positive current flows into the coil and energy is absorbed. In actuality it is converted to magnetic energy which flows across the gap between stator and rotor and hence to the rotor, wheels and road. On the right we have a generator. When the voltage applied to a stator coil is positive current flows out of the coils. Here energy is taken from the road by the wheels which transfer it to the rotor, across the gap and into the coils. The inverter, which, in motor mode, took DC current from the battery and changed it to AC to produce the rotating poles now takes the AC current from the machine and rectifies it to DC which charges the battery. Note that it is the same circuit but the currents, AC and DC flow through it in the opposite direction. What controls the situation is where it puts those stator poles relative to the rotor poles.

Thus using regen to charge the batteries in a towed BEV equipped with it is a matter of developing some software that allows one to tell the vehicle how many HP you want it to borrow from the towing vehicle. It is a relatively simple matter to compute from the speed the corresponding torque and to set the motor controllers to produce that much torque. It should also be a relatively simple matter to take the signal from a brake controller output pin and turn it into a torque command if it is only desired to use the motors as brakes rather than as a means for charging. But note that once the batteries are full one can no longer brake it regeneratively. So the interface would have to work with the friction braking system too.
 

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For those of us that are actually RVer's there are really just 2 important factors in this thread in my opinion:

1. Wow, the Rivian can be flat towed!

2. Per most (all?) state towing laws the Rivian MUST brake itself regardless of whether it's regenerating or not.

It's fun to theorize the interesting ways Rivian could assist in accomplishing #2 outside of the currently available aftermarket solutions. So for those that question the relevance of braking, it's not just a relevant discussion, it's something that Rivian must consider when accomplishing #1.
1599858098008.jpeg

Totally agree with your first point. If I can’t tow it, I can’t buy it. Not only must it be able to be flat towed, it must be able to inter operate with my current RV stowing systems. Besides considering normal braking, support for emergency braking in the case of break-away is also needed.
As far as regeneration is concerned, it would be important to be able to switched off by the owner. IMHO the trade off between the charging batteries effective load, diesel fuel costs, and RV performance. Should be considered.
Also, I would really like to see Rivian consider how to hide all the connection points when the R1T is no longer being towed.
 
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CappyJax

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1599858098008.jpeg

Totally agree with your first point. If I can’t tow it, I can’t buy it. Not only must it be able to be flat towed, it must be able to inter operate with my current RV stowing systems. Besides considering normal braking, support for emergency braking in the case of break-away is also needed.
As far as regeneration is concerned, it would be important to be able to switched off by the owner. IMHO the trade off between the charging batteries effective load, diesel fuel costs, and RV performance. Should be considered.
Also, I would really like to see Rivian consider how to hide all the connection points when the R1T is no longer being towed.
It will have a lot of autonomous features. So, a break-away would probably result in it pulling over and stopping.

I don't understand why you would want to turn off regeneration. It would reduce the wear and tear on the RV and Rivian, and it would be collecting free energy. It would only be applied when breaking.
 

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It will have a lot of autonomous features. So, a break-away would probably result in it pulling over and stopping.

I don't understand why you would want to turn off regeneration. It would reduce the wear and tear on the RV and Rivian, and it would be collecting free energy. It would only be applied when breaking.
Hi Crappy Jax,

Unfortunately nothing’s free. Low friction rolling is best. It’s a bit like when I’m walking my dog versus dragging her.

To be clear, I’m not against it for those that see the benefits as long as I can disable it if I want.

As far as the wear and tear, my current solution provides proportional braking as well as emergency break-away.

As I activate the RV brakes, a small piston connected to the TOAD proportionally applies its brakes. It’s a small piston as the TOAD equipment I use supplies vacuum to the brake booster. It also transmits a signal to a receiver in my RV to inform me When the R1T brakes are applied.

If I lose both the air connection and the break-away line disconnects, the brakes will lock up.
 
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CappyJax

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Hi Crappy Jax,

Unfortunately nothing’s free. Low friction rolling is best. It’s a bit like when I’m walking my dog versus dragging her.

To be clear, I’m not against it for those that see the benefits as long as I can disable it if I want.

As far as the wear and tear, my current solution provides proportional braking as well as emergency break-away.

As I activate the RV brakes, a small piston connected to the TOAD proportionally applies its brakes. It’s a small piston as the TOAD equipment I use supplies vacuum to the brake booster. It also transmits a signal to a receiver in my RV to inform me When the R1T brakes are applied.

If I lose both the air connection and the break-away line disconnects, the brakes will lock up.
It is CappyJax. And the resistance will not change with regen on or off until you step on the brake pedal and engage the regen. Then the Rivian will apply regenerative braking and charge the batteries. It only works when the brakes are engaged. What that means is that it can slow down both your rig and the Rivian without the need to apply the mechanical brakes on either vehicle.
 

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It is CappyJax. And the resistance will not change with regen on or off until you step on the brake pedal and engage the regen. Then the Rivian will apply regenerative braking and charge the batteries. It only works when the brakes are engaged. What that means is that it can slow down both your rig and the Rivian without the need to apply the mechanical brakes on either vehicle.
Apologize for my fat fingers and bad eyesight! Really destroyed your name!
I think several members of this group have different views on regen beyond just braking.
Like I said previously, I just want the ability to configure it to off when it’s a Toad.
But frankly, I don’t buy that regen braking would safely stop BOTH my 50k RV and the R1T. Trust me, I’ve seen RV drivers accidentally pull their TOADS without knowing that theTOAD’s wheels were locked.
I would think if it was all that easy, why put mechanical brakes on the R1T in the first place? What are the impacts to ABS features?
I’ll love using the R1T’s regen braking when I’m in it driving.
 

thrill

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Mechanical brakes are required for safety, and the total amount of braking from it is unknown to us right now. You couldn't use regen anyway once the battery is full.
 

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Regen will work up to a point, and then friction brakes would be engaged in addition.
It would be transparent to you as to whether the Rivian was using regen, friction, or a combination. As stated above, unless you are braking it would be freewheeling. If the battery was full, braking would be entirely friction. If you were braking gently, it would be entirely regen. At some point, the friction brakes would also be engaged to supply additional stopping power. It's not an either or scenario, regen and friction would be used in combination.
 
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