Car wash and bigger tires question…

[EVnewb]

I went through the car wash earlier today and saw a Model X coming through behind me. It got me thinking: how do they do it? I mean, you’re supposed to put the vehicle in neutral as your front tires hit the conveyor belt. How do EV’s go into neutral? I thought RIVIAN had mentioned flat towing wouldn’t be possible due to the motors used. But then MT reported that the RIVIAN would actually be 4WD instead of just AWD so it would be possible to put atleast two of the motors into some sort of neutral mode.

Also brings up another question, for the physics nerds here. If you had that much control over which tires had power, wouldn’t it be more energy efficient to just send power to one tire? Kind of the way a normal 2WD vehicle works? Only one tire actually has power going to it unless you have pos-traction or some variation thereof (Mercedes and Toyota using the brakes comes to mind). Or would that one tire have to work so much harder that it would use more energy?

Last question, again for the physics buffs. How much more energy would be used to up the tire size by a few inches? For trucks and SUV’s I’ve owned in the past, the mpg loss was largely dependent on fuel type, gearing, and overall engine size. Probably sounds dumb to some, but I love lifted trucks. It would be awesome to be able to program the RIVIAN to raise the lowest suspension level to a custom height to clear some 36” or even 38” tires. ? Assuming fenders, bumpers, and other components didn’t cause rubbing. That way you could still lift it up to the max level for the bigger obstacles and lower it back down to an efficient level when you hit pavement.

Apologies if any of this has been talked about before!

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[crashmtb]

I went through the car wash earlier today and saw a Model X coming through behind me. It got me thinking: how do they do it? I mean, you’re supposed to put the vehicle in neutral as your front tires hit the conveyor belt. How do EV’s go into neutral? I thought RIVIAN had mentioned flat towing wouldn’t be possible due to the motors used. But then MT reported that the RIVIAN would actually be 4WD instead of just AWD so it would be possible to put atleast two of the motors into some sort of neutral mode.

Also brings up another question, for the physics nerds here. If you had that much control over which tires had power, wouldn’t it be more energy efficient to just send power to one tire? Kind of the way a normal 2WD vehicle works? Only one tire actually has power going to it unless you have pos-traction or some variation thereof (Mercedes and Toyota using the brakes comes to mind). Or would that one tire have to work so much harder that it would use more energy?

Last question, again for the physics buffs. How much more energy would be used to up the tire size by a few inches? For trucks and SUV’s I’ve owned in the past, the mpg loss was largely dependent on fuel type, gearing, and overall engine size. Probably sounds dumb to some, but I love lifted trucks. It would be awesome to be able to program the RIVIAN to raise the lowest suspension level to a custom height to clear some 36” or even 38” tires. ? Assuming fenders, bumpers, and other components didn’t cause rubbing. That way you could still lift it up to the max level for the bigger obstacles and lower it back down to an efficient level when you hit pavement.

Apologies if any of this has been talked about before!

You can roll It in neutral. how else would things like loading on tow trucks work?

there are 4 motors. One for each wheel. The ones with grip get the juice. should be way more responsive compared with brake or differential based torque vectoring.

i‘d expect larger tires to mess with everything, unless there’s a provision to tell the truck what size tires it’s got.

34” tires and 14” of ground clearance seems like plenty

 

[SANZC02]

I went through the car wash earlier today and saw a Model X coming through behind me. It got me thinking: how do they do it? I mean, you’re supposed to put the vehicle in neutral as your front tires hit the conveyor belt. How do EV’s go into neutral? I thought RIVIAN had mentioned flat towing wouldn’t be possible due to the motors used. But then MT reported that the RIVIAN would actually be 4WD instead of just AWD so it would be possible to put atleast two of the motors into some sort of neutral mode.

Also brings up another question, for the physics nerds here. If you had that much control over which tires had power, wouldn’t it be more energy efficient to just send power to one tire? Kind of the way a normal 2WD vehicle works? Only one tire actually has power going to it unless you have pos-traction or some variation thereof (Mercedes and Toyota using the brakes comes to mind). Or would that one tire have to work so much harder that it would use more energy?

Last question, again for the physics buffs. How much more energy would be used to up the tire size by a few inches? For trucks and SUV’s I’ve owned in the past, the mpg loss was largely dependent on fuel type, gearing, and overall engine size. Probably sounds dumb to some, but I love lifted trucks. It would be awesome to be able to program the RIVIAN to raise the lowest suspension level to a custom height to clear some 36” or even 38” tires. ? Assuming fenders, bumpers, and other components didn’t cause rubbing. That way you could still lift it up to the max level for the bigger obstacles and lower it back down to an efficient level when you hit pavement.

Apologies if any of this has been talked about before!

The Tesla has a special mode for loading on a ramp truck. It allows for the vehicle 2 freewheel when you get out of it.

 

[EVnewb]

Why would they have said no flat towing then? I don’t know enough about the motors used to understand how they work I guess. I’ve read a few conflicting posts in the forum about different types of motors used and whether or not they can be set to freewheel.

And I understand there are four motors in these vehicles. But there’s been speculation on whether you get the best range by the RIVIAN dropping power to the rear wheels. What you said makes sense to me though and I can see the computer constantly sending power where it is needed right to the wheel.

Adding a lift and bigger tires always changes how a vehicle handles and how the components interact. I’m just hoping it wouldn’t be a huge one since that suspension is extremely adjustable and ready for some serious clearance. Being able to adjust your tire size in the computer would be really handy. It could change everything even down to adjusting the speedometer for larger tires that way.

And yeah, stock, the RIVIAN will be more than capable off-road. I just like the look of a lifted truck/SUV with bigger tires as a matter of personal preference.

 

[EVnewb]

The Tesla has a special mode for loading on a ramp truck. It allows for the vehicle 2 freewheel when you get out of it.

Ahhh ok that makes sense. The guy didn’t get out of it though. I’m assuming it still works when you’re inside somehow?

 

[Greenwater]

Inside the car, on the gear shift (with typical park, drive, reverse) there's a neutral. You kind of do a half pull from drive or park to neutral. It's easy to miss it. At the auto car wash, you line up to the track and when your front wheel gets to the right spot you shift from park to that neutral. I bet lots of people have a little trouble with it.

 

[CommodoreAmiga]

Also brings up another question, for the physics nerds here. If you had that much control over which tires had power, wouldn’t it be more energy efficient to just send power to one tire? Kind of the way a normal 2WD vehicle works? Only one tire actually has power going to it unless you have pos-traction or some variation thereof (Mercedes and Toyota using the brakes comes to mind).

AFAIK, that isn't how physics work -- even with 2WD. In normal conditions, where both driven wheels have equal traction, power does go through BOTH wheels. If one wheel has significantly less traction (which makes it much easier to spin) then power causes that wheel to spin and not the wheel with all the traction. Torque is a bit like electricity, in that regard -- it takes any and all paths available to it.

Now before someone says "but wait, doesn't electricity take the path of LEAST resistance?" NO! That is a myth. Electricity takes ALL paths. However, the path with higher RESISTANCE will take less current. Now consider the 2WD vehicle. The traction between the tire and road is resistance. Drivetrain "power" is current. The wheel with higher traction (resistance) will transfer (flow) less power (current). The larger the difference in traction (resistance) between the driven wheels, the larger the difference in how much power transmits through each wheel. Make the difference large enough (like one wheel on a patch of ice) and there may not be enough power to spin the wheel with traction and effectively you get the wheel without traction just spinning.

Limited-Slip Differentials (Posi-track is one such trademark) are useful in those situations because they force at least some power (varies depending on implementation) to the wheel with traction. In normal driving, however, an "open" differential is fine and does "turn" both wheels.

 

[sub]

Free rolling in a Tesla is not a problem. The Tesla you saw behind you may have just left the car in drive. In the default drive mode in a Tesla, drive acts like neutral at low speeds. The car will roll forward or backward with minimal resistance if the driver is not pressing either pedal.

However you absolutely cannot flat tow a Tesla. You cannot get out of a Tesla and leave it in neutral. The moment you get out of the car it will automatically engage the parking brake. There is a "tow" mode that when engaged it will deactivate the parking brake long enough to be pulled up onto a flatbed truck, but it is on a timer and will reactivate the brake after a few minutes. Tesla will also not let you drive with the door open. All in the name of "safety". It is quite annoying - but their lawyers probably told them they needed to do it.

Even if Rivian does not force the parking brake on you, you still are not going to be able to flat tow a Rivian unless Rivian wants to let you do that. It is expected that Rivian will be using permanent magnet motors, which are going to generate electricity if you force them to spin. The faster you tow the vehicle, the more electricity the motors will generate which will make it feel like you are towing a vehicle with the brakes engaged. Plus, if the vehicle is not prepared to do something with all of that generated electricity (like charge the battery) it could build up and cause damage.

Rivian could write software to drive the motors at just the right speed to avoid generating electricity when being towed, but they probably are not going to.

 

[Jarico75]

Free rolling in a Tesla is not a problem. The Tesla you saw behind you may have just left the car in drive. In the default drive mode in a Tesla, drive acts like neutral at low speeds. The car will roll forward or backward with minimal resistance if the driver is not pressing either pedal.

However you absolutely cannot flat tow a Tesla. You cannot get out of a Tesla and leave it in neutral. The moment you get out of the car it will automatically engage the parking brake. There is a "tow" mode that when engaged it will deactivate the parking brake long enough to be pulled up onto a flatbed truck, but it is on a timer and will reactivate the brake after a few minutes. Tesla will also not let you drive with the door open. All in the name of "safety".

Even if Rivian does not force the parking brake on you, you still are not going to be able to flat tow a Rivian unless Rivian wants to let you do that. It is expected that Rivian will be using permanent magnet motors, which are going to generate electricity if you force them to spin. The faster you tow the vehicle, the more electricity the motors will generate which will make it feel like you are towing a vehicle with the brakes engaged. Plus, if the vehicle is not prepared to do something useful with all of that generated electricity (like charge the battery) it could build up and cause damage.

Rivian could write software to drive the motors at just the right speed to avoid generating electricity when being towed, but they probably are not going to.

I have a model Y and it will absolutely not go through a car wash in drive, unless I apply some perssure on the pedal which would scare the "P" out of me to do. Tesla announced recently that they enabled a car wash mode. I have not received the update that has this feature, but the Tesla you saw may have received the beta update.

 

[Zoidz]

Now before someone says "but wait, doesn't electricity take the path of LEAST resistance?" NO! That is a myth. Electricity takes ALL paths. However, the path with higher RESISTANCE will take less current. Now consider the 2WD vehicle. The traction between the tire and road is resistance. Drivetrain "power" is current. The wheel with higher traction (resistance) will transfer (flow) less power (current). The larger the difference in traction (resistance) between the driven wheels, the larger the difference in how much power transmits through each wheel. Make the difference large enough (like one wheel on a patch of ice) and there may not be enough power to spin the wheel with traction and effectively you get the wheel without traction just spinning.

It's a fair analogy, but you've got it backwards. An electric motor under load (in this analogy, the one with better traction) will draw more current and develop more power. A free spinning electric motor (on ice in this analogy) draws the least current. It's due to a physics phenomena known as Counter EMF or Back EMF. A magnetic electric motor, due to it's very nature, also acts as a generator, even when being powered as a motor. This generated power is the Counter or Back EMF, called this because it opposes the applied volatage. In multiphase motor power and control, the basic "rules" of electricy are substantially modified due to the electromechanical physics involved. It's much more complex than E=IR.

The inverter/motor controller in the Rivian (or any electronic motor controller) takes all this into consideration in hardware design, programming and tuning, and optimize the waveform applied to motor for the conditions at hand. Even though the Rivian has DC permanent magnet motors, the inverter is generating a "pulsed" DC waveform that conceptually is similar to an AC waveform, and allows fine speed and torque control of the DC motor, similar to a AC motor control.

Another interesting analogy is that positrac in electric vehicles is also implemented electronically, presumably by using the antilock brake rotation sensors to determine if a wheel is spinning faster than the others, and reduce power to that motor.

 

[Dark-Fx]

Why would they have said no flat towing then?

There is no physical disconnect for the motors. If the vehicle is off the cooling systems are also off, which will lead to a heat buildup in the motors and eventual failure. Jeeps can be flat towed because they have physical disconnects. If you forget to do those you're going to ruin something.

 

[sub]

I have a model Y and it will absolutely not go through a car wash in drive, unless I apply some perssure on the pedal which would scare the "P" out of me to do. Tesla announced recently that they enabled a car wash mode. I have not received the update that has this feature, but the Tesla you saw may have received the beta update.

Switch to roll mode. That is or at least was the default mode. Hold, which it sounds like you are using, was not added until a few years ago, and only newer cars have it.

 

[Jarico75]

Switch to roll mode. That is or at least was the default mode. Hold, which it sounds like you are using, was not added until a few years ago, and only newer cars have it.

Thanks for the information. I will check that out. We have been afraid to take the car through a conveyor belt car wash. My wife is afraid the rims will get scratched.

 

[Zoidz]

There is no physical disconnect for the motors. If the vehicle is off the cooling systems are also off, which will lead to a heat buildup in the motors and eventual failure. Jeeps can be flat towed because they have physical disconnects. If you forget to do those you're going to ruin something.

One of the components that will fail are the IGBT (transistors) in the inverter. Apparently Rivian has not designed the controller for extended flat towing, probably due to cooling challenges?

Here's an interesting video showing an IGBT failure - they arc and physically explode. I've seen this happen with industrial motor VFDs. The guy uses an IGBT to power a Tesla coil. Near the end, the IGBT overheats and blows up. You don't want this happening in your EV.

 

[twinprice]

Thanks for the information. I will check that out. We have been afraid to take the car through a conveyor belt car wash. My wife is afraid the rims will get scratched.

For better or worse, we take our Model Y through the conveyor belt car wash (one where you stay in the car) and you put the car in Neutral (as @Greenwater mentioned earlier in this post) by pulling 1/2 way down on the shifter stalk. Since you aren't getting out of the car it doesn't put itself back in park.
I'm not sure it has damaged the wheels at all (certainly not more than my wife's curb rash) but the car wash does mess up the black anodized aluminum door trim and make it discolored from the high pH in the car wash chemicals.
So, hopefully there will be a neutral setting for car washes.

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