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Discussion in 'Rivian General Discussions' started by CappyJax, Mar 4, 2019.
Pretty much everything in that how stuff works article is wrong -- comically wrong.
The article does not apply to a Rivian or the new Tesla motor because they use a permanent magnet motor.The car uses AC to operate the motor, when the motor is not being powered by the inverter from the battery and wheels are turning in a forward direction the motor is producing an AC current which is converted to DC to feed the battery. The braking function results from a counter electro motive force that naturally occurs when electrons are flowing through the motor that tries to slow down or stop the motion that is making the electron flow.You essentially have to dump the electricity some where when the battery can't take it or open the circuit so no electrons will flow and shut off the magnetic field that is working against the motor so it will coast when you need it to coast. This is not hard to do electrically.
What opens the circuit to allow the car to coast?
I accept that now.
The problem for me is what to believe when researching things on the internet. One would think, for instance, that an article sponsored by the engineering department of a university would be reasonably authoritative. But then you find a lack of understanding even there of what a single-speed motor is and why most electric cars use transmissions.
Since one cannot query the authors of these disparate articles, the only way to try to flush out the most credible answer is to probe and repeatedly test the positions of people who can answer -- in other words, to keep debate going on a forum such as this.
I don't have all the details or the electrical schematics, its could be a simple switched relay or a more complex variable transistor which could feed a portion of the current to battery, bleed off to resistors or capacitors, or open up and save nothing. My dads RV bus talks to his Jeep that he tows wirelessly for signaling brake lights and such, There would have to be some kind of communication between the two vehicles.
This is what I would love for them to have. A wireless connection with a simple plug into the RV would be a slam dunk. Not really any additional hardware required on the Rivian depending on how the communication is done and hopefully hidden tow mounts. Put the Rivian in town mode and physically connect the RV to the truck and go.
If you could monitor the car from your phone and turn off/on exhaust style breaking that would be brilliant.
Notification on the phone when the car was nearing full or setting the level to get to full.
Not sure why you would want wireless when you can just use the brake controller signal.
Many people writing articles don’t use the proper terminology. I have read that the Rivian will be direct drive because it only has a single speed gearbox. Well, that is not direct drive. Most people think a transmission has to have more than one gear. Anything with a gear is a transmission.
Because wireless you can get acceleration, coast and brake inputs instead of just brake vs no brake. It is one less physical connection you have to make between vehicles, and less parts to fail. If the connection does fail, it is significantly easier to replace a wireless component than take the RV in for service to diagnose the electrical connection issue (I had to do this last year).
It’s also one less specialized hardware component to be integrated into the Rivian.
Also, if Rivian is really smart (which I think they are) the flat towing connections will be somehow integrated with recovery hooks on the front of the truck. This would make them effectively hidden, and also have the towing hooks connected through a reinforced area of the frame.
So the actions of the tow driver in just taking his foot on and off the brake and accelerator pedals in his vehicle is not sufficient to induce a response in the towed EV beyond mere braking, even with a brake control signal? Some more complex control signal has to be sent to the EV's systems? Isn't this what I've been asking all along?
You could do this with sensors in the Rivian. Wouldn't need anything transmitted from the tow vehicle. The only thing from the tow vehicle you would need is whether to brake or coast going downhill and that could be handled through the brake lights or brake controller wire. And I would ALWAYS prefer a mechanical connection over wireless. I can go to any hardware store and replace a wire in the unlikely event it were damaged. With wireless, you are waiting for new equipment.
You probably choose wireless connections in 1000 different aspects of you life over wired. Wired is inherently limited and not upgradable. I get what you are saying, but you are assuming you are actually able to diagnose and fix the problem. Most RV drivers would be either unwilling or unable to make these hypothetical fixes themselves. Getting appointments to get RVs serviced can take weeks. I’d take an express mailed new wireless connection or an over the air update to my Rivian over that any day.
You also want to avoid sensors in the Rivian controlling the behavior. Translating driver input back to the Rivian is the most effective way to do this. Sensors in the Rivian would lag the driver input.
Example: Driving downhill, driver lets foot off the gas — RV continues to accelerate, but driver input is coast. Input to the Rivian is acceleration via sensors, but driver input is coast.
Again exhaust breaking is the perfect comparison. It kicks in whenever the input from the driver is coast or break regardless of whether the RV is accelerating or decelerating. It mirrors driver intent.
No — we are talking about something more advanced beyond just simple flat tow. For simple flat tow all you need is Brake or No Brake input. That is it.
I thought the discussion always went beyond simple flat towing, as points were brought up by various posters about using towing to regenerate the EV battery, using the EV motors to create additional braking for the two vehicles going downhill, and one poster even brought up using the EV motors to generate propulsion to assist the two vehicles going uphill. This is why I never could understand why there was an argument about having to send more complex signals than just brake on and off signals to the EV systems to accomplish all these things.
The key here is “having to” or not. You don’t have to send any additional info to the Rivian other than brake or no brake for everything to work including regen. The one scenario outside of that which we are now talking about is what to do during “coast”. If you want special behaviors during coast you need additional input that is not currently provided through standard wired connections. This is honestly a luxury — it is not needed to be considered a fully functional solution.
Also, propulsion is a no go. Tow arm for flat tow are not built to handle propulsion from the vehicle being towed, and in general it is just a bad idea. Too complex a dance and dangerous for any benefit it could give you. Would likely be made illegal if it isn’t already due to safety concerns.
There are a few things about this that haven't quite been mentioned. Depending upon the state, supplemental braking in the toad (towed vehicle) is legally required. So if Rivian is going to allow for flat towing, whether they have an integrated or non-integrated braking system, they have to be aware that the Rivian will be required by law to brake, no matter whether it charges or doesn't charge the battery.
Those not familiar with RV'ing lifestyle might be surprised at how unsophisticated the supplemental braking systems can be. I've used a supplemental "Brake Buddy" that physically jams between the drivers seat and the brake pedal on the floor and has a plunger that jams on the brakes whenever an inertia sensor determines that sufficient deceleration is triggered by the brakes of the RV. The force of the "plunge" onto the brake pedal and the amount of inertia to trigger braking has to be estimated by the user and adjusted by dials on the device. The force on the brake is also determined by how far forward you have your seat. When it goes wrong you'll get the toad to lock up all 4 wheels with even mild braking of the RV. On the other end of the spectrum you might set it to brake too lightly or not at all. This is a very simple and very unsophisticated system but helps stop in emergency braking situations and complies with towing laws. I mention this as some of the discussion here implies that complex and dynamic braking of a toad is a requirement.
The more sophisticated (and expensive) systems integrate brake pressure to roughly match the amount of braking in the toad to the RV.
There's a lot of talk in this thread as to how the Rivian would "know" that it's time to couple and engage the regenerative braking. That could be as simple as whether or not the brake lights are activated (which is also required by law to occur on the toad vehicle unless you bolt some auxiliary lights on the toad.) So the Rivian would easily know whether the RV is braking simply by whether the brake lights are activated. On our diesel pushers with exhaust brakes, the brake lights are activated whenever the exhaust brakes are engaged even though the driver hasn't hit the brake pedal.
So Rivian could go simple and apply a minimal amount of braking whenever the brake light is activated or as complicated as using the array of sensors in the Rivian to provide dynamic amounts of brake pressure and even stability control and steering systems to keep the Rivian in line during evasive maneuvers. Keep in mind that there are also issues as to just how hard the Rivian can brake as the towing equipment is only rated at so many pounds. The heavy Rivian could potentially brake hard enough to exceed the capacity of the tow bars. These engineers have to know all this and have to take all this into consideration. I think my heavy duty tow bars are rated at 10,000 pounds. With these Rivians coming in at more than 7000 pounds, I doubt they're going to do much more than brake themselves. I doubt there'll be any braking of the RV by the Rivian tugging on it through those tow bars.
Bottom line is that it's not very complicated for the Rivian to figure out when it's time to brake. The real discussion will be just how dynamic the braking will be. It will have to be able to use the friction brakes when the battery is full, otherwise it'd not meet legal requirements for towing. It'd be hard to imagine the Rivian regenerating while coasting or accelerating as it'd be tricky business to not exceed the capacity of the tow bars. As far as the Rivian "pushing" the RV up a steep hill, I highly doubt this would be any consideration. The tow bars have a capacity of precisely zero for pushing forward--the tiniest amount of pressure forward will bend the bars. For example, absolutely no backing is allowed when the toad is hooked up--not even a foot or you will likely bend the bars. Unless of course Rivian creates its own tow bars (but then there'd be the issue of how well the trailer hitch on the RV handles forward pressure.)
Enjoying your discussion and looking forward to having a nice toad to replace my Tacoma.
Great post. Thanks for capturing this in one place and communicated better than I have been able to.
Also, I forgot about the exhaust brakes turning on the brake lights. That makes the complex solution for coast unnecessary. Just brake or no brake.
I assumed as much. I've towed a trailer a few times, and the last thing you want is the trailer pushing the tow vehicle in any circumstance.
I will disagree with one premise here. Towed vehicles and/or trailers ALWAYS transfer energy from towed to the towing vehicle during braking, and there is no problem with this. This is the reason why it's important for tow bars to be close to level; if not, the force/momentum from the toad can either lift the back of the coach or potentially be lifted/levered up into the back of it.
The reason you can't back up with a toad has to do with lateral force, not the front/back.
However, propulsion from a toad would be complicated, and not worth the effort IMO.
I'm with ya on that. As I mentioned early on when this thread was started ...