Eli5 why electric cars don’t have smthn like reverse thrust while braking (wheels spin backwards in hard braking situation)
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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.
What does it matter whether the car is electric or fuel powered in this case?
One way to think about why it wouldn’t work is: you’ve seen eg racing cars or motorbikes spin the rear wheels while keeping their front brakes on, right? This means that the brakes acting on stationary tyres are stronger than the motor spinning them on road surface.
The reason no cars do this is because things stick together better when they’re not moving past each other than when they’re sliding (in technical terms, the coefficient of static friction is usually higher than the coefficient of dynamic friction). For example, if you have something just staying put on a slope, then give it a nudge, it will keep sliding. This means that spinning the wheels backwards would increase the braking distance, and as an added bonus destroy the tyres.
What does it matter whether the car is electric or fuel powered in this case?
One way to think about why it wouldn’t work is: you’ve seen eg racing cars or motorbikes spin the rear wheels while keeping their front brakes on, right? This means that the brakes acting on stationary tyres are stronger than the motor spinning them on road surface.
The reason no cars do this is because things stick together better when they’re not moving past each other than when they’re sliding (in technical terms, the coefficient of static friction is usually higher than the coefficient of dynamic friction). For example, if you have something just staying put on a slope, then give it a nudge, it will keep sliding. This means that spinning the wheels backwards would increase the braking distance, and as an added bonus destroy the tyres.
Put simply, it doesn’t happen because it doesn’t work.
While it’s true that presenting an opposite force would, in theory, cause the car to slow down faster, this wouldn’t work in practice. That’s because the car needs friction to work. The wheels making contact with the road provides it’s movement.
If you tried to make the wheels spin the opposite way while the car was moving, it would begin to skid and you would lose control. Once you lose that contact with the road, the only thing moving the car is the speed it already had and you have no way to stop it.
Of course you *could* strap a jet engine to the front of your car, but they take so long to warm up and produce thrust that, by the time it’s ready, it’s providing thrust to a crashed wreck or, more likely, exploding. Same with rockets. In the time it takes to ignite the fuel source and begin providing thrust, a normal car would have either come to a stop or crashed and exploded due to the rockets onboard.
So, in short, car’s rely on friction to move, so it’s better to rely on friction to stop too. Any kind of emergency thruster is more likely to kill you than save you.
Put simply, it doesn’t happen because it doesn’t work.
While it’s true that presenting an opposite force would, in theory, cause the car to slow down faster, this wouldn’t work in practice. That’s because the car needs friction to work. The wheels making contact with the road provides it’s movement.
If you tried to make the wheels spin the opposite way while the car was moving, it would begin to skid and you would lose control. Once you lose that contact with the road, the only thing moving the car is the speed it already had and you have no way to stop it.
Of course you *could* strap a jet engine to the front of your car, but they take so long to warm up and produce thrust that, by the time it’s ready, it’s providing thrust to a crashed wreck or, more likely, exploding. Same with rockets. In the time it takes to ignite the fuel source and begin providing thrust, a normal car would have either come to a stop or crashed and exploded due to the rockets onboard.
So, in short, car’s rely on friction to move, so it’s better to rely on friction to stop too. Any kind of emergency thruster is more likely to kill you than save you.
There’s static friction, sliding friction, rolling friction, and fluid friction. Static friction is the strongest type, like trying to push a big box across the floor. Once that box starts moving, it gets easier, right? That’s sliding friction, weaker than static.
If your wheels lose grip with the ground, for example by spinning wheels too fast, (in either direction) you’re now sliding and have less friction overall. Which means you can’t stop or control yourself as well.
The same way that you don’t gain more speed by doing a burnout when accelerating compared to a good launch, you need traction for that to happen, and a wheel spinning faster or in reverse have less traction than the regular rotation.
Only way to maximize braking is increased surface area, not reverse speed.
There’s static friction, sliding friction, rolling friction, and fluid friction. Static friction is the strongest type, like trying to push a big box across the floor. Once that box starts moving, it gets easier, right? That’s sliding friction, weaker than static.
If your wheels lose grip with the ground, for example by spinning wheels too fast, (in either direction) you’re now sliding and have less friction overall. Which means you can’t stop or control yourself as well.
The same way that you don’t gain more speed by doing a burnout when accelerating compared to a good launch, you need traction for that to happen, and a wheel spinning faster or in reverse have less traction than the regular rotation.
Only way to maximize braking is increased surface area, not reverse speed.
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