internalaudit

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Currently, AWD systems do not help stopping but my guess is with negative torque from electric motors, it should help shorten the braking distance. Isn't it just like treading on snow or ice in the opposite direction?

It's likely not going to be as effective as moving forward since winter tires are typically unidirectional but every inch of shortened stopping distance is welcome.

Four electric motors will make the Macan BEV pricey but I think with this, it will be well worth it.

Toyota/Lexus, hope you are keeping abreast of the competition.
 
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ssun30

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Stopping in snow (or in any road condition) only depends on tyre grip. It doesn't matter what kind of braking system is used. No AWD system is going to help either.
 

internalaudit

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If wheels spin the other way, of course the stopping distance will be shorter because Forward Force - Reverse Force will likely be less than the original Forward Force (I'm not pretending I'm a scientist or engineer or even good at math) since some of the forward force gets cancelled out. It's like saying putting a car in reverse (negative torque in this example ) will not drive the car backward.

Spinning in reverse isn't possible in ICEVs without putting the gear in reverse which is why ICEVs equipped with AWD will not have better stopping distance because it's all about the tire/surface friction.

With electric motors, it's possible. Acura's Sports Hybrid SH-AWD can already apply negative torque:

 
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ssun30

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If wheels spin the other way, of course the stopping distance will be shorter because Forward Force - Reverse Force will likely be less than the original Forward Force (I'm not pretending I'm a scientist or engineer or even good at math) since some of the forward force gets cancelled out. It's like saying putting a car in reverse (negative torque in this example ) will not drive the car backward.

Spinning in reverse isn't possible in ICEVs without putting the gear in reverse which is why ICEVs equipped with AWD will not have better stopping distance because it's all about the tire/surface friction.

With electric motors, it's possible. Acura's Sports Hybrid SH-AWD can already apply negative torque:

You've just described how any friction brake works.

Negative torque is not negative angular velocity, that's your physics lesson right here. You can have negative torque while having positive wheel speed, that's what happens when a car brakes i.e. friction brakes already are negative-torque generators. Motor-driven negative torque works the exact same way as a brake-based system, except it's proactive instead of reactive plus you get some energy back, that's why it's better.

Actually, friction brakes can create a lot more braking power than motors. Your average commuter car can dissipate all of the kinetic energy moving at 100km/h in about 3 seconds; they take over twice that amount of time to accelerate to that speed using the prime mover.

In order to turn wheels backwards, you need negative wheel speed. The problem with reversing while moving forward is the same for both ICEV and BEV. Doing so would destroy the reduction gear on a good surface (ICEV actually deals with it better since a hydraulic torque converter can slip instead of destroying the gears), or just create wheel spin on low-grip surface, neither of which is safe.

The only 'practical' way to stop cars faster than traction allows is aerodynamic drag or reverse rocket thrusters.
 

internalaudit

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You've just described how any friction brake works.

Negative torque is not negative angular velocity, that's your physics lesson right here. You can have negative torque while having positive wheel speed, that's what happens when a car brakes i.e. friction brakes already are negative-torque generators. Motor-driven negative torque works the exact same way as a brake-based system, except it's proactive instead of reactive plus you get some energy back, that's why it's better.

Actually, friction brakes can create a lot more braking power than motors. Your average commuter car can dissipate all of the kinetic energy moving at 100km/h in about 3 seconds; they take over twice that amount of time to accelerate to that speed using the prime mover.

In order to turn wheels backwards, you need negative wheel speed. The problem with reversing while moving forward is the same for both ICEV and BEV. Doing so would destroy the reduction gear on a good surface (ICEV actually deals with it better since a hydraulic torque converter can slip instead of destroying the gears), or just create wheel spin on low-grip surface, neither of which is safe.

The only 'practical' way to stop cars faster than traction allows is aerodynamic drag or reverse rocket thrusters.
Thank you for your comprehensive explanation.

So the following explanation doesn't apply? You seem right though that it can cause damage but so can a collision that could have been avoided.


For AC induction motors, the stator voltage is reversed by interchanging any two of the supply leads. The field then rotates in the opposite direction and the motor’s slip (the difference between the speed of the stator’s rotating magnetic field and the speed of the rotor) becomes greater than unity (s > 1). In other words, the rotor spins faster than the rotating magnetic field in the stator. Torque is developed in the opposite direction of the motor’s rotation, which produces a strong braking effect.

When the motor speed reaches zero, if it is not disconnected from the supply, it will begin to reverse, or rotate in the opposite direction. In some applications, reversal of the motor’s direction is the goal. But when plugging is used to brake the motor, a zero-speed switch or plugging contactor is used to disconnect the motor from the supply when its speed reaches zero.

One of the potential problems with plugging as a braking method (especially when the braking time is short) is that it can be difficult to brake the motor at exactly zero speed. Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes. Plugging is also a very inefficient method of stopping and, therefore, generates significant heat.

Despite these drawbacks, plugging is used in equipment such as elevators, cranes, presses, and mills, where a rapid stop of the motor (with or without reversal) is required.
 

ssun30

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Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes. Plugging is also a very inefficient method of stopping and, therefore, generates significant heat.
In a car moving at 100km/h this basically means broken gears and fried motor.
 

internalaudit

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^ Why would the independent motor go in reverse at 100 km/h when like you said, there's the friction brake to do the work more effectively?

Good to know you couldn't rebut that article, which means the motors can spin in reverse, maybe when in panic mode. :)

I love the internet! :)
 

internalaudit

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Isn't that what you want?
No, I wanted better stopping distance overall.

I don't think they make BEVs with no friction brakes and as you aptly pointed out, they're better until maybe the last mile or two (just like with regular HEV regen), when reverse wheel spin could actually help. :)
 
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Sulu

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When braking, no wheels are being driven, so it does not matter whether your vehicle has FWD, RWD, 4WD or AWD, all vehicles have No-wheel-drive when braking. To stop in the minimum distance then, you need traction -- you need tires that will provide you with the traction at the weather and road conditions at that time.

And we have to remember what we learned from ABS and ESC (electronic stability control), which is that to remain in control of the vehicle, the tires must have traction, and not sliding or skidding (i.e. the wheel must be turning in the direction of travel). That would rule out putting the vehicle into reverse (by shifting the transmission into reverse gear or applying "negative torque" to the electric traction motor) in order to slow down the vehicle even more quickly.

(In the Cybertruck vs Ford F-150 tug-of-war, we can see that the Ford was out of control when the Cybertruck won the contest and the Ford was effectively travelling in reverse by being pulled (sliding / skidding) in the opposite direction to the transmission setting.)

If and when a vehicle is travelling in the direction opposite to the intended direction (before you burn out the transmission or the drive motor), the wheels are sliding. When wheels are sliding, the vehicle cannot be controlled (cannot be steered) by the driver, and is completely at the whim of outside forces (that may be driving your vehicle into the ditch). Sliding out of control is what happens when a driver in an AWD vehicle going too fast for the conditions, tries to brake.
 

internalaudit

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^ We are talking four independent motors here, not one or two. :)

Read the article I shared and tell me what they wrote there that's inaccurate or misinformation. We are talking last mile of stopping power, not when the car is travelling at 100, like ssun suggested I was alluding to LOL. He himself already stated friction braking will be much more effective for hard braking.

If an AWD system can move the car forward or backward relatively easy on snow/icy road conditions, then reversing the wheel spin (via electric motor) when the car is sliding say at 20-30 km/h will intuitively mean stopping distance will be shorter thanks to the counterforce.
 

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^ We are talking four independent motors here, not one or two. :)


Read the article I shared and tell me what they wrote there that's inaccurate or misinformation. We are talking last mile of stopping power, not when the car is travelling at 100, like ssun suggested I was alluding to LOL. He himself already stated friction braking will be much more effective for hard braking.


If an AWD system can move the car forward or backward relatively easy on snow/icy road conditions, then reversing the wheel spin (via electric motor) when the car is sliding say at 20-30 km/h will intuitively mean stopping distance will be shorter thanks to the counterforce.
The problems with reversing the polarity and forcing the traction motor into reverse were described in the article:

One of the potential problems with plugging as a braking method (especially when the braking time is short) is that it can be difficult to brake the motor at exactly zero speed.
If you want to use reverse torque to brake a vehicle, you want precise control of all motors (fine if you have just one drive motor) at precisely the same time to prevent loss of control of the vehicle (i.e. skidding); you need to be able stop all motors precisely at zero speed (to prevent one or more of the wheels from actually spinning in reverse) at the same time, which is difficult to do.

Suddenly going in reverse is dangerous, especially in heavy traffic situations (which is probably when you would want quick braking). Also dangerous is loss of directional control due to one wheel skidding and sliding (or actually going in reverse) when others continue to have forward traction.

Reversing torque on one wheel (and one wheel only, while the others remain in forward motion) in an AWD vehicle, in order to sharpen the turn, is using slipping and dynamic motor braking under a controlled situation. Not being able to precisely control all motors at precisely the same moment is an uncontrolled situation, which is NOT what you want while driving, especially in heavy traffic.

Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes.
High mechanical shock loads can damage the drivetrain (motor(s) and any axles connecting motor(s) to wheels). Fixing the drivetrain is much more difficult and much more expensive than fixing worn mechanical brake components.

Additionally, damaging the drivetrain in the middle of the road in the middle of a trip means you are dead in the water -- you are going nowhere -- whereas placing a lot of load (and a lot of wear) on mechanical brakes in the middle of a trip may not prevent you from proceeding, you may just have to proceed slower.

Plugging is also a very inefficient method of stopping and, therefore, generates significant heat.
Significant heat can damage other components on the vehicle, even cause a fire.
 

internalaudit

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The problems with reversing the polarity and forcing the traction motor into reverse were described in the article:



If you want to use reverse torque to brake a vehicle, you want precise control of all motors (fine if you have just one drive motor) at precisely the same time to prevent loss of control of the vehicle (i.e. skidding); you need to be able stop all motors precisely at zero speed (to prevent one or more of the wheels from actually spinning in reverse) at the same time, which is difficult to do.

Suddenly going in reverse is dangerous, especially in heavy traffic situations (which is probably when you would want quick braking). Also dangerous is loss of directional control due to one wheel skidding and sliding (or actually going in reverse) when others continue to have forward traction.

Reversing torque on one wheel (and one wheel only, while the others remain in forward motion) in an AWD vehicle, in order to sharpen the turn, is using slipping and dynamic motor braking under a controlled situation. Not being able to precisely control all motors at precisely the same moment is an uncontrolled situation, which is NOT what you want while driving, especially in heavy traffic.



High mechanical shock loads can damage the drivetrain (motor(s) and any axles connecting motor(s) to wheels). Fixing the drivetrain is much more difficult and much more expensive than fixing worn mechanical brake components.

Additionally, damaging the drivetrain in the middle of the road in the middle of a trip means you are dead in the water -- you are going nowhere -- whereas placing a lot of load (and a lot of wear) on mechanical brakes in the middle of a trip may not prevent you from proceeding, you may just have to proceed slower.



Significant heat can damage other components on the vehicle, even cause a fire.
I think damaging the drivetrain or generating significant heat is better than falling off a cliff or ramming into an inanimate object like a barricade or median, but maybe that's just me. That's what automobile insurance is for, to cover the value of the vehicle.

That's what the computer is for to monitor the three or four independent motors and other things like yaw, brake pedal, vehicle speed to determine the best course of action. To suggest reverse wheel spin at low enough car speed is impossible after traction is lost and after which power to that wheel is cut off is at best just an educated guess. It's no different from my educated guess that will be possible with multiple electric motors.

I'm not talking efficiency, I'm talking potential life or death scenarios.
 
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ssun30

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stopping distance will be shorter thanks to the counterforce.
You didn't understand what I said. Reversing the angular velocity of the wheel does NOT increase the force that slows down the car (it actually decreases it because of wheel spin), because the maximum braking force is determined by the traction of the tire, period. There is zero way to stop a car faster than traction allows. A car moving too fast and heading to a cliff can only be saved by a reverse rocket/jet engine.

AWD makes cars easier to move in snow because there are two extra driven wheels putting down forward moving force so there is a higher chance of one wheel getting at least some traction. Cars always have four wheel braking (if not you shouldn't drive one), so they always use however much traction there is available to slow down. Also modern cars already have full independent braking force control to maintain attitude, it's called EBD. You don't need a four-wheel torque vectoring system to brake on surfaces with uneven traction.

It's no different from my educated guess that will be possible with multiple electric motors.

I'm not talking efficiency, I'm talking potential life or death scenarios.
You are not making 'educated' guess because it's against the laws of physics.

On one hand you say it's for 'last mile stopping' (there is no such concept, by the way), and on the other hand you say it saves life in emergency scenarios? I'm very confused.
 

internalaudit

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You didn't understand what I said. Reversing the angular velocity of the wheel does NOT increase the force that slows down the car (it actually decreases it because of wheel spin), because the maximum braking force is determined by the traction of the tire, period. There is zero way to stop a car faster than traction allows. A car moving too fast and heading to a cliff can only be saved by a reverse rocket/jet engine.

AWD makes cars easier to move in snow because there are two extra driven wheels putting down forward moving force so there is a higher chance of one wheel getting at least some traction. Cars always have four wheel braking (if not you shouldn't drive one), so they always use however much traction there is available to slow down. Also modern cars already have full independent braking force control to maintain attitude, it's called EBD. You don't need a four-wheel torque vectoring system to brake on surfaces with uneven traction.


You are not making 'educated' guess because it's against the laws of physics.

On one hand you say it's for 'last mile stopping' (there is no such concept, by the way), and on the other hand you say it saves life in emergency scenarios? I'm very confused.
You already responded to me with your angular velocity talk but I never used that with any of my statement. I initially used negative torque and you had corrected me. Since your helpful post, I have started using your more apt terminology -- wheel spin.

EBD is 100% braking or stopping wheel spin, it has nothing to do with reversing the wheel spin direction.

You and Sulu seem to be suggesting that if I was driving an AWD car sliding/slipping at 10-20 km/h, I can't put the car in reverse and attempt to have the car move backward. Of course the transmission can get badly damaged but that's besides the point since we are talking about independent electric motors. If from a standstill the car will reverse fine, why wouldn't it move in reverse, if I engaged it from D to R? I'm not getting all your explanation about rocket/jet engine. I think you are overthinking and over explaining things. :)

Update:
Oh it seems putting the car in R while sliding doesn't help haha:
 
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Sulu

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You and Sulu seem to be suggesting that if I was driving an AWD car sliding/slipping at 10-20 km/h, I can't put the car in reverse and attempt to have the car move backward. Of course the transmission can get badly damaged but that's besides the point since we are talking about independent electric motors. If from a standstill the car will reverse fine, why wouldn't it move in reverse, if I engaged it from D to R? I'm not getting all your explanation about rocket/jet engine. I think you are overthinking and over explaining things. :)
Assuming you could slam a car into reverse while travelling forward, without damaging the transmission or electric drive motors, the car will likely continue to slide/skid forward (and because the car is sliding, you lose traction and you lose steering control). This is because of the great forward momentum of the car.

It would take immense amounts of traction force of the tires and incredibly strong torque of the reverse gear or drive moto(s) to overcome the forward momentum. Assuming there is enough reverse torque to suddenly overcome the forward momentum, I would not be surprised if the tires burst.

And assuming there is enough reverse torque to suddenly overcome the forward momentum, the passengers in the car would be slammed around, running the risk of injuring the passengers.

Do you really want to risk the expensive damage to the car and the risk of injury to passengers, just to stop a bit sooner?
 

internalaudit

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Assuming you could slam a car into reverse while travelling forward, without damaging the transmission or electric drive motors, the car will likely continue to slide/skid forward (and because the car is sliding, you lose traction and you lose steering control). This is because of the great forward momentum of the car.

It would take immense amounts of traction force of the tires and incredibly strong torque of the reverse gear or drive moto(s) to overcome the forward momentum. Assuming there is enough reverse torque to suddenly overcome the forward momentum, I would not be surprised if the tires burst.

And assuming there is enough reverse torque to suddenly overcome the forward momentum, the passengers in the car would be slammed around, running the risk of injuring the passengers.

Do you really want to risk the expensive damage to the car and the risk of injury to passengers, just to stop a bit sooner?
This could happen between 10-20 km/h?

I mentioned computers doing the calculation, so if sensors don't detect the right ingredients, reversing the wheel spin will not happen.
 

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This could happen between 10-20 km/h?


I mentioned computers doing the calculation, so if sensors don't detect the right ingredients, reversing the wheel spin will not happen.
If you are going that slow (about the speed of racewalking), why not just continue to use the mechanical brakes? There is nothing as effective and efficient at bleeding off and transferring the energy of forward momentum as mechanical brakes. It won't stop you on a dime but to stop that suddenly (transfer that much forward momentum energy that suddenly) is quite damaging to the car and its occupants.

Crashing into a brick wall or stopped transport truck will stop you on a dime but all that forward momentum energy is transferred into incredible damage to the car and possibly fatal injuries to the passengers.
 

ssun30

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@Sulu is missing the point. The point here is that reversing while moving forward not only damages the drivetrain, but it also doesn't reduce stopping distance, in fact it increases it.

TL;DR for @internalaudit. I'm going to say this one last time: there is no way to brake a car in a shorter distance than traction allows. Trying to break this rule by attempting to make the car move backwards does not work.

I tried to make this easier to understand for everyone but it seems for clarity I have to introduce one more terminology: optimal slip ratio.

When the car is moving forwards there are two velocities: the linear velocity of the car itself and the tangential velocity of the tyre (which is angular velocity multiplied by tyre radius), and the difference between these two is called slip. The ratio of this difference vs. linear velocity is called slip ratio.

When the car is cruising there is no slip, the car and the contact pitch of the tyre are moving at the same speed. When the car is accelerating the slip is positive (i.e. tyres spin faster than the car travels) and vice versa. It is this slip that generates the force that changes the velocity of the car.

There is an optimal slip ratio that maximizes this force, above this point traction decreases significantly. Depending on the surface the optimal slip ratio ranges from near zero (on dry paved road) to about 0.2 (on ice). That's why wheel spin during acceleration and locking the brakes during deceleration are so bad: the slip ratio is massive (well above one) so the car loses tons of traction. Things get even worse when you put the car into reverse while moving forward: the slip ratio is twice as high (since now you are subtracting a negative value instead of zero) compared to just locking the brakes so the traction loss is even worse. The reason we invented TC and ABS is to try to keep slip ratio to the optimum.

The true holy grail in braking is a perfectly adaptive linear ABS that will achieve the theoretical minimum stopping distance on any surface. Traditional ABS avoids lock-up by pulsing the brake fluid pressure so it's like those 'on-and-off' thermostats. An ideal ABS would be like a highly skilled race driver who knows exactly how much pedal input is needed for the surface condition. We are still a few years away from a near-ideal ABS but it has nothing to do with reversing an electric motor.
 
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