Eli5 why electric cars don’t have smthn like reverse thrust while braking (wheels spin backwards in hard braking situation)
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They do. It’s called regenerative braking. By sucking power out of the electric motor instead of putting it in, they can reverse the torque spinning the wheels; the motors push backwards. This stops the car from moving even faster than traditional friction brakes. And the big bonus is they can capture that energy. Put it back in the battery to use for moving forward again later. That’s why it’s called regenerative braking.
Electric cars have a combination of traditional breaks as well as energy recovery. The electric motor can harvest electricity from the wheels slowing down the car and this is actually more effective at slowing a car down than the breaks.
Spinning the wheels backwards is possible with the torque a car has, but it wouldn’t help. This would just cause the tires to lose traction and make the car impossible to control, increasing the changes of a big accident.
Two reasons
1 – Static friction (the kind when your tires aren’t skidding) is greater than kinetic friction (the kind when your tires are skidding). So wheels spinning backwards would provide worse braking than just braking.
2 – Skidding the wheels backwards will also cause a loss of control of the vehicle. This is the principle for drifting cars… when your rear tires are skidding, you can use this to make the car go sideways. This is intentional/wanted when you’re drifting, but losing control of the vehicle when coming to a stop would not be ideal. If you slam on the brakes, you still might want the ability to turn your car, and you’ll lose that ability if the tires are skidding.
Both of these reasons are also why all modern cars have anti-lock brakes.
Two reasons
1 – Static friction (the kind when your tires aren’t skidding) is greater than kinetic friction (the kind when your tires are skidding). So wheels spinning backwards would provide worse braking than just braking.
2 – Skidding the wheels backwards will also cause a loss of control of the vehicle. This is the principle for drifting cars… when your rear tires are skidding, you can use this to make the car go sideways. This is intentional/wanted when you’re drifting, but losing control of the vehicle when coming to a stop would not be ideal. If you slam on the brakes, you still might want the ability to turn your car, and you’ll lose that ability if the tires are skidding.
Both of these reasons are also why all modern cars have anti-lock brakes.
Because electric cars propulsion system is still attached to the same thing as the braking system. If you’re braking you are already doing the most effective thing to slow down using the wheels (if you weren’t you’d be doing the most effective thing instead). Technically cars have always had what you describe, you can slam the car into reverse and hit the pedal though I guarantee it won’t slow you down faster than the brakes.
In an aircraft the wheel brakes and the propulsion systems are working on entirely different systems and principles. You can have your anti-lock wheel brakes doing most of the work and also have the engines directing thrust forward to help because the engine isn’t applying thrust through the wheel action.
Edit: In a way I guess you could see regenerative braking as a type of “reverse thrust” in that it reverses the action of the wheel motors into generators, bleeding off the car’s momentum back into battery charge.
Because electric cars propulsion system is still attached to the same thing as the braking system. If you’re braking you are already doing the most effective thing to slow down using the wheels (if you weren’t you’d be doing the most effective thing instead). Technically cars have always had what you describe, you can slam the car into reverse and hit the pedal though I guarantee it won’t slow you down faster than the brakes.
In an aircraft the wheel brakes and the propulsion systems are working on entirely different systems and principles. You can have your anti-lock wheel brakes doing most of the work and also have the engines directing thrust forward to help because the engine isn’t applying thrust through the wheel action.
Edit: In a way I guess you could see regenerative braking as a type of “reverse thrust” in that it reverses the action of the wheel motors into generators, bleeding off the car’s momentum back into battery charge.
IF the car was in mud or sand or snow and had the appropriate lugged or paddled threaded tires, couldn’t reverse spinning tires generate more stopping power via thrust ? (assuming the driven in mud or sand or snow is getting thrown towards the front of the vehicle in a fairly immense amount or at a high velocity)
Wouldn’t this be following that guy’s 1st (or 2nd) law ?
(The one about motion)
IF the car was in mud or sand or snow and had the appropriate lugged or paddled threaded tires, couldn’t reverse spinning tires generate more stopping power via thrust ? (assuming the driven in mud or sand or snow is getting thrown towards the front of the vehicle in a fairly immense amount or at a high velocity)
Wouldn’t this be following that guy’s 1st (or 2nd) law ?
(The one about motion)
On top of everything that has been said about traction and control problems there are even more issues.
DC motors operate by pulsing electric current to achieve different rpm (rotations per minute). Faster pulses of electricity = faster motor spin. Reversing a DC motor’s spin by changing the current flow from positive to negative would cause havoc with sparks in the coils, sparks in the ball bearings of the motor, and more internal damage. The motor would burn itself up.
AC motors are controlled by limiting voltage and current. You can’t simply turn down the voltage. That causes a “brown out” condition that damages and burns the motor coils. To reverse the spin requires the equivalent of a transmission to change the gears in use. As with gas cars, a sudden shift from Drive to Reverse while traveling at high speed will damage these gears severely.
Friction increases until the wheels starts spinning/locking, then friction drops significantly.
So your assumption is wrong. Tires spinning backwards would actually decrease decelleration. Just like locking the wheels do – hence ABS breaking systems.
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