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Comment by e136 | Hacker News Reader

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.

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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.

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.

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).

"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.

> 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.

> 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).

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