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Comment by dinom | Hacker News Reader

> able to get a tow truck capable of transporting my car (turns out you need one that has a full bed because of the regenerative breaking

Sounds incredibly short-sighted that the regenerative braking can't be temporarily disabled by the driver... two people can push a "regular" car to a nearby gas station in a pinch. Even one person can push a smaller car around a parking lot if necessary.

That said, they often send out full/flat bed tow trucks these days for almost any reason.

Edit: fixed "breaking" to "braking"

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It's common for cars with AWD systems where the rear drivetrain can't be disconnected by a clutch to require flatbed towing, less because the car can't physically be moved with the power off and more because spinning the wheels at a sustained high speed will cause damage to the drivetrain and potentially the engine. E.g. my moldy old Subaru requires flatbed towing, because spinning the rear wheels while the fronts are immobile will destroy the limited-slip center diff, but it's totally possible to push it along with all four wheels on the ground.

I think AWD Teslas have an independent motor and drivetrain for the rear wheels. Naively, one would expect that if you unload the rear axle motor/generator you could just tow the car with little resistance and no damage, but in practice the picture is probably more complicated. I would love to hear from an expert.

Tesla cars are way heavier. Nobody is pushing one with their friend. Unless it's downhill.

I'd need to do some quick math to double-check but very rough ballpark estimates still make me think Teslas can be pushed by a single person. After all, a single strongman can pull a passenger airliner, and any car is nearly impossible to push uphill -- you have to really get some momentum built up first to let them coast up driveways/etc.

It's not about weight or incline, it's the mechanical gearing in the "transmission" that can't turn the wheels without 100 percent power from the engine. You'll be able to push it and think it's really heavy, but it's just the transmission fighting you.

standards and automatic trans have neutral that disconnects driveshaft from engine, removing any resistance from the engaged gear. Most EVs either don't have neutral or a poor implementation of it, because of single gear transmission/direct drive.

> Most EVs either don't have neutral or a poor implementation of it,

I'm sorry, do you mean that neutral and "car wash/freeroll mode" are not 100% equivalent?

This really confounds me... if my car has a problem while driving down the road, I'd really like to have a chance of pushing it onto the shoulder if necessary.

A car that you can't do this with sounds very poorly designed to me.

They aren’t equivalent. Most EVs have a free roll mode, where you can push the car or take it through a car wash. You are still spinning the electric motors though.

This is fine for low speeds and short distances but is not advised for towing, like with tow trucks that leave two of the car’s wheels on the ground.

> Unless it's downhill.

Wouldn't that be ok since the parent's root problem was no cell-phone signal?

Edit: of course walking is an option too :-)

>To make matters worse I was in a mountain valley and had no phone signal

I read the above as an indication that they were in a valley where mountains were blocking any chance of a signal. Going further "downhill" wouldn't help in that case.

Fair enough, I was thinking back to the time something similar happened to me "mid-mountain".

This Tesla must've been in one heck of a stop where "hiking wasn’t an option as it was pretty hot and we had no water", lol. Does Tesla make a motocross bike?

Lack of cell signal is still a problem in some parts of the U.S. I can think of a few places within a couple hundred miles where I can drive 45 minutes or so without any signal. It's also an area where triple digit temperatures are common in the summer.

You can push a tesla in most typical push situations. Sure its heavy, but its in the ballpark of pushing any large sedan or SUV, not just electric ones. You can enable free-rolling wheels in the car and push away, especially if you have luxury of two people pushing.

The weight of a Tesla is not some unprecedented figure for a passenger car - a model 3 is no heavier than many BMW 5 series configurations etc.

The far bigger problem for pushing EVs in my experience is blown pyro-fuses that lock the axle after crashes preventing the wheels from turning - a common issue after an EV shunt. This is why you will often see EVs "dragged" out of shunts by the recovery vehicle until they can get the locked wheels onto dollies. This is only an issue if the EV being pushed has been in an impact generally, but the bar for a pyrofuse blow seems to be low too - it will trip even if airbags haven't deployed. The wheels on many EV motor axles wont turn again until the pyrofuse is replaced.

I think the earlier example that required a full recovery truck - that sounds exactly like a blown pyrofuse, which is by far the most common reason an EV ends up requiring a full lift/dollies rather than a simple tow on its own axles.

This is also why you generally always smell burning in the cabin after a tesla shunt too - nothing to do with a battery fire, its the smell of the pyrofuse burning out under the rear passenger seat.

> https://www.wired.com/story/evs-fire-pyroswitches-cut-risk-s...

> You can enable free-roling wheels in the car and push away

This was mentioned a little earlier and further down in the comment tree but many thanks to you too for clearing this up.

I'm racking my brain on this. Why is this not just called exactly what it is, "Neutral"?

I just read about this feature which is advertised purely for car washes, and can be gotten into with a wierd combination of actions or "putting the car into neutral".

Is it...not just going into neutral via disconecting a drive shaft?

> Is it...not just going into neutral via disconnecting a drive shaft?

Conceptually you are correct its like neutral, but almost no EV from any manufacturer has disconnecting drive shafts. Every Tesla model except the original roadster is locked in a single gear transmission that never disconnects - one of the big reasons EV drivetrains can be built so reliable with virtually no maintenance requirements (there is no service items on most EV drivetrains until a quick coolant flush in ~10th year of car's life span).

The Model 3/Y just dump the output of the rotor into a permanently locked 9:1 trans. There is no need for a reverse gear either as unlike a gas engine the electric motor can easily rotate in both directions.

An electric motor cant "stall" like a gas engine and generally never has to change gear, so there is usually no real benefit to fitting the cost and complexity of clutching driveshafts you typically need when you have a gearbox or gas engine.

Someone will correct me if I'm wrong, but to best of my memory the only "mainstream" EV with a gearbox on sale today is the Porche Taycan/Audi E-Tron GT (they are both the same chasis/drivetrain underneath) with its pretty unusual 2-speed setup. I wouldn't be surprised at all if the next generation of that car eliminates the gearbox.

If the drivetrain never disengages, how does “freewheeling mode” (neutral) work? Spinning the wheels manually makes them a generator, and that causes resistance from the magnetic flux (hence regenerative braking). How do cars disable that?

It disconnects the excitation current.

> Is it...not just going into neutral via disconecting a drive shaft?

Very good point and although IDK how it's done I completely agree with you regarding the terminology. In other words, I don't see new internal combustion cars renaming neutral to be "car wash mode"... lol.

Cars should have neutral. And, if yours does then it should be named as such.

Making sure I've got "car was mode" is not what I want to do when I'm in the market for a new car. Nor do I want to start reading through the owners manual should I have to push my car onto the shoulder of the road in an emergency.

Perhaps my age and crotchety grumpiness is starting to show.

> You can enable free-roling wheels in the car and push away

Is that possible when the car is persumed dead? Or why would you be trying to push it otherwise? If you can enable free-rolling wheels, why not just drive away?

Right - pyrofuse blow has prevented this in my experience. That's why I've specifically called out shunts. If there is no impact, you would be pretty unlucky to not be able to do it.

Most cars have one or more clutches to put the car in neutral. This is required so the engine can idle while the car stands still, among other reasons. Electric cars don't need to idle and therefore don't have any clutches in the drivetrain. Adding clutches just for allowing the user to push around the car seems needlessly complicated and over optimizing for a scenario that will not occur for the majority of cars.

> don't have any clutches in the drivetrain

Wouldn't a clutch be unnecessary in this case... a complete disconnect from the drive train should suffice, no?

That's exactly what a clutch does. They show up in all sorts of places besides the third pedal "clutch" drivers of standard transmissions are familiar with, e.g. in part-time AWD systems, traditional automatic transmissions with planetary gears, torque converters and hybrid transmissions with lockup clutches for efficient cruising, and so on.

I understand that... so you're saying something like "unlocking hubs" would not suffice? Or they'd add too much cost to the vehicle?

Locking (wheel) hubs could certainly be used to disconnect the wheels from whatever drivetrain trouble lurks within, but they come with a whole lot of compromises: cost, weight, complexity, space, etc. Standard industry practice for vehicles that can't be towed with only two wheels immobilized is (and has been for decades iiuc) to flatbed tow them, so Tesla isn't really going against the grain here. I think it's a fine choice.

I would have thought regenerative braking could be disabled by simply electrically isolating the motors. You still have the mechanical resistance of turning the rotor, but I would also think that is easier than turning over an IC engine.

Others have mentioned that there is indeed a "free roll" mode and I'm prone to think your hypothesis regarding the implementation may be correct. The specifics, however, do not matter to me.

What I'm wondering now is, if "free roll" mode is activated while I'm at a red light... is there a chance I start rolling as I do in my manual when I take my foot off the brake?

Maybe I should just reach out to Tesla at this point though (-:

>> You still have the mechanical resistance of turning the rotor,

Isn't this exactly what regenerative braking is? There's no mechanical disconnect between the wheels and the motor (i.e. a clutch) so pushing the car essentially turns it into a generator.

Yes and no. Spinning the motor will generate a voltage but if it's not connected to anything there's no current flow and so the only resistance is friction. For it to act as a brake, something has to use the current - for example, charging the battery or more simply just shorting the motor (try turning an efficient DC motor with the wires open and the wires shorted - even with a stepper motor, you'll find a huge difference).

Your second sentence is correct, but it takes a lot more torque to turn a generator at a given speed when it is driving current through a load, than it does when it is open-circuit.

Inside the motor/generator, what happens is that if there's current flowing in the windings as a result of the EMF induced by turning the rotor, then, loosely speaking, it creates a magnetic field which opposes the rotation - but if it is not connected to some load (or a short-circuit) there's no current, even though there is still an induced EMF.

It's a standard case of the conservation of energy - if it were not the case, you could put an arbitrary number of generators on the same shaft, all supplying power to external loads, and then use the output of a couple of them to power a motor spinning the assembly...

If you are turning a motor/generator slowly, then the difference in torque is not much, but I am responding to the implication that the motor would have to be mechanically disengaged in order to disable the braking effect.

Correct me if I'm wrong but, if I understand things correctly... at this point we're wondering if manual locking hubs are a better (or worse) solution than electrically isolating the motor(s). Right?

And, off the top of my head I'm going to say the manual locking hubs will be better overall. Better from an energy efficiency perspective as well as overall convenience and simplicity of design. They might be worse for Tesla's P&L though since anyone with a tire jack and a standard parts dealer nearby could work on the car.

Then again, besides the fact that I'm a cynic... what the heck do I really know!??!

I don't think it would be useful for me to speculate about that. I'm just attempting to clear up some apparent misunderstandings about regenerative braking.

> Adding clutches just for allowing the user to push around the car seems needlessly complicated and over optimizing for a scenario that will not occur for the majority of cars.

The ability and need to push a car is incredibly common, especially for emergency or non-optimal scenarios. Cars often stop working, be it dead battery, out of gas, malfunction, broken axle(which teslas have).

"a scenario that will not occur for the majority of cars" isn't the standard, otherwise there would be no need to require most safety features, as the majority of cars never get into a major crash.

If a car runs out of charge or malfunctions, it would help everyone if it is easier to move it so it doesn't block traffic. It might be wise to make this a requirement. Seems to me that a clutch that simply switches between connected and neutral and that only operates when the car isn't moving would be far simpler and cheaper than a clutch that handles shifting between different gears while the engine is turning.

If you put the car in car wash mode, you would be able to push the car:

Controls > Service > Car Wash Mode And then "Enable Free Roll"

thank you

... assuming you have battery (which he didn't)

Yeah, that's a fair point but isn't it true that a mostly discharged battery will still have enough charge to perform some limited functions. E.g. even in a normal car I think a dead battery can power the lights even though the starter only "clicks".

Nonetheless, I'm a rather conservative type of person and see your point so in my "dream car" I might want a way to manually put the car in neutral (or free-roll mode, whatever you want to call it). That's why locking hubs came to my mind in a prior reply.

For the Tesla, it shut itself off in this story. Regardless of whether it may have had some juice left

Yeah when I lived in South Africa, I saw people towing each other all the time with just a rope. Sure it was one of those flat threaded ropes, but it works.

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