6

(This isn't a "maintenance & repair" question, I hope you'll forgive me.)

When powerful electric cars (such as, but not only, the Tesla) can accelerate from zero to ludicrous in half a jiffy, and can do regenerative braking, then why do they still use "old school" mechanical disc brakes?

Does acceleration ability not equate deceleration ability? Is the brake construction mandated by law (I know, this depends on location, but seems to be the case for all brands anyhow)? Or is it a safety feature (no power=no braking, oops)?

  • Are you suggesting using regenerative braking only? Or some other braking system? Hydraulic disk brakes are effective in converting rotational energy into heat and dissipating the heat. They are also reliable, inexpensive (relatively) and simple.What's not to like? – Tim Nevins Apr 17 '18 at 21:58
  • What's not to like is that it's wasteful of energy. The same applies on a Diesel-powered car, but with electric motors you could reclaim it -- so yes, regenerative. Note: I am aware that some cars implement 'regenerative braking' as what happens when you just release the gas pedal, but this question is more about why it's not the only implemented method of deceleration. – KlaymenDK Apr 18 '18 at 11:12
  • Ah, this is all about saving energy, eh? You should have said something. Your question didn't mention it at all. If you want answers that address your concerns, put those concerns in your question. It comes down to this: regenerative braking systems are built to a purpose (harvest energy) with the side effect of slowing the vehicle., hydraulic brakes are built to a different standard: they don't harvest energy, but they do stop the car in a controllable manner. How exactly do you control the stopping power of regenerative brakes? – Tim Nevins Apr 18 '18 at 14:39
  • As it turned out, the answer didn't have anything to do with physics, but with rules and regulations. So it's fair to say that the question was good enough to receive a meaningful response. – KlaymenDK Apr 18 '18 at 18:16
4

It's not so much that the motors can't brake like mad.

It's as you hint at yourself that any auto-mobile that is required to be fully road legal needs two completely separate systems for braking, in case one of them fails. It's why even the most automated gas-fuelled cars still had a hand-brake type system.

It originates from the perceived unreliability of hard to maintain systems, such as hidden cables or liquid braking systems, so that there was a way to stop the vehicle, be it slower, if one of them fails.

  • 1
    They are getting rid of the hand brakes now. Now they have those electronic "parking" brakes. I don't know how auto makers get away with this. – rana Oct 27 '15 at 15:00
5

In addition to Asmyldof answer, I would like to Highlight another point.

Magnetic braking (motor regenerative/passive braking on an electric car) torque are dependent of the speed. This means that theoretically, this type of brake can never stop a vehicle because as the speed decrease, the maximum braking torque also decrease. This is a bit like the Zeno's paradoxes.

Here's a plot of the speed vs. torque for friction(red), magnetic(green) and aero(blue, like parachute): speed vs. torque

And a plot of the time needed to stop (time vs speed). Note that the green and blue (magnetic and aero) never touch abscissa. time vs speed Ilustrations based on "Eléments de théorie des mécanismes et des machines, G.Spinnler, 1984"

  • Ooh, that is a very interesting point, and a good explanation. Thank you very much! – KlaymenDK Oct 27 '15 at 9:08
  • 3
    This is true for passive (i.e. regenerative) braking. You could actively apply a torque to the motor in the opposite direction to bring the vehicle to a complete stop. It might be difficult to accomplish without sending the vehicle into reverse after it stops though. – Chris Mueller Oct 27 '15 at 12:16
  • 7
    I tried reading Zeno's paradoxes, but couldn't get to the end. – Poisson Fish Oct 27 '15 at 15:29
  • 3
    Was it not a joke about never reaching the end like Achilles never reaches the tortoise? ;) – Tonio Oct 27 '15 at 16:16
  • 2
    @ChrisMueller: Plugging (trying to power a motor in reverse) is very hard on motors, but will allow a motor to produce peak braking torque which is double its starting torque [while producing four times as much heat as full-power operation when stalled]. Plugging at near-zero speeds, however, is far less problematical, though a mechanical holding brake will still be needed to avoid burning power when the system is supposed to be motionless. – supercat Mar 31 '16 at 17:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.