How much will the Cybertruck weigh?

[Bill906]

It comes down to the different technologies used to maintain traction at the limits of braking vs. acceleration. Braking uses ancient friction and hydraulics technology pulsed by digital traction controls at the limits of traction while acceleration uses electromagnetic propulsion modulated by digital traction controls. While the former has more power at it's disposal and is able to lock up the wheel at will, the latter is more granular and faster to respond than hydraulic valves operating hydraulic cylinders for friction. The result is the motors can modulate power faster and more accurately to keep the tires closer to the limits of traction than the ABS can.

Thank you for that answer. It makes sense. With that said, it makes me think that if regen could handle the power, regen braking from 60-0 would be better than friction braking. (Better in the sense of stopping distance, not looking at the gained efficency) Yes, I realize that is a big if. In industrial 3-phase motors controlled by a VFD, you can decelerate a motor/system as fast as you can accelerate it as long as you have a means to get rid of the regenerative energy. There are a few ways of doing that. Most common is to send the energy to a large power resistor and burn the energy off as heat. Another method is to have an active front end on the VFD. Active front ends have the ability to send the excess energy back onto the supplying AC line.

If regen braking isn't used for fast deceleration (think emergency stopping) because the battery cannot handle the large regen power surge, but would be significantly better at stopping the car than ABS and mechanical brakes, I would think they would at least look into adding a braking resistor to dump the excess power the battery cannot absorb.

IF this is all true. (Yes I am armchair engineering this). My guess is in the future, if/when they perfect brake by wire, they will have virtually all stopping being done by motor regeneration and only use friction brakes in the event of a failure or corner case issue. Maybe by then the batteries will be more acceptable to the power surge.

Sponsored

 

[HaulingAss]

Thank you for that answer. It makes sense. With that said, it makes me think that if regen could handle the power, regen braking from 60-0 would be better than friction braking. (Better in the sense of stopping distance, not looking at the gained efficency) Yes, I realize that is a big if. In industrial 3-phase motors controlled by a VFD, you can decelerate a motor/system as fast as you can accelerate it as long as you have a means to get rid of the regenerative energy. There are a few ways of doing that. Most common is to send the energy to a large power resistor and burn the energy off as heat. Another method is to have an active front end on the VFD. Active front ends have the ability to send the excess energy back onto the supplying AC line.

If regen braking isn't used for fast deceleration (think emergency stopping) because the battery cannot handle the large regen power surge, but would be significantly better at stopping the car than ABS and mechanical brakes, I would think they would at least look into adding a braking resistor to dump the excess power the battery cannot absorb.

IF this is all true. (Yes I am armchair engineering this). My guess is in the future, if/when they perfect brake by wire, they will have virtually all stopping being done by motor regeneration and only use friction brakes in the event of a failure or corner case issue. Maybe by then the batteries will be more acceptable to the power surge.

Motors could be designed to brake as effectively as they currently drive the car but there are always tradeoffs. The motor needs to be designed with efficiency in mind. Because motors are made up of a discrete number of fields, and these fields work using angular offsets, the very design of the motor determines whether it will be most efficient in drive mode or in regen mode. This is due to the asymmetrical shape of the magnetic fields generated.

From a practical standpoint, almost all braking is already done by regen which is why brake pads last so long. The most common reason regen is limited for normal braking is a cold battery or a battery near a full state of charge.

While it would be nice to have full regen, up to the limits of traction, at all times, it's not really as important as having maximum drive efficiency and long battery life. Also, when a car is used in a sporty manner, heat is generated that must be carried away by the cooling system. The more heat generated, the larger and more power intensive the cooling system must be. Handing off hard braking to friction brakes reduces the amount of heat that must be shed by active means. Cars are designed to use the available technology in a manner that makes economic sense and is practical. Certainly, advances in technology will increase what is possible in the future but, in the end, it comes down to how well does it work and how much does it cost. And for that, drive efficiency will always be more important than being able to regen more at the cost of drive efficiency or higher new car prices.

 

[Bill906]

Motors could be designed to brake as effectively as they currently drive the car but there are always tradeoffs. The motor needs to be designed with efficiency in mind. Because motors are made up of a discrete number of fields, and these fields work using angular offsets, the very design of the motor determines whether it will be most efficient in drive mode or in regen mode. This is due to the asymmetrical shape of the magnetic fields generated.

From a practical standpoint, almost all braking is already done by regen which is why brake pads last so long. The most common reason regen is limited for normal braking is a cold battery or a battery near a full state of charge.

While it would be nice to have full regen, up to the limits of traction, at all times, it's not really as important as having maximum drive efficiency and long battery life. Also, when a car is used in a sporty manner, heat is generated that must be carried away by the cooling system. The more heat generated, the larger and more power intensive the cooling system must be. Handing off hard braking to friction brakes reduces the amount of heat that must be shed by active means. Cars are designed to use the available technology in a manner that makes economic sense and is practical. Certainly, advances in technology will increase what is possible in the future but, in the end, it comes down to how well does it work and how much does it cost. And for that, drive efficiency will always be more important than being able to regen more at the cost of drive efficiency or higher new car prices.

Agreed. I guess my point was, if you can accelerate a car significantly faster using the motor than you can decelerate same car with friction brakes and ABS, I'd think they'd at least look into using the motor for fast stops. Be interesting to see how different the 0-60 time is compared to the ABS braking's 60-0 time.

 

[Crissa]

It's not possible to regen brake as fast as acceleration because the battery cannot absorb electron-state as quickly as it can emit it.

tl,dr? Battery doesn't charge as fast as it discharges. The ratio can be like 10:1.

That's why lots of manufacturers were looking at super-capacitor banks, but so far it hasn't worked out. Taycan is rumored to have banks of resistors to mimic regen so that excess power is shed when the battery is heated to capacity.

It's possible to have bigger brakes and stop more quickly, but that costs money, weighs more, and so that needs to be balanced in considerations as well.

-Crissa

 

[Bill906]

It's not possible to regen brake as fast as acceleration because the battery cannot absorb electron-state as quickly as it can emit it.

tl,dr? Battery doesn't charge as fast as it discharges. The ratio can be like 10:1.

That's why lots of manufacturers were looking at super-capacitor banks, but so far it hasn't worked out. Taycan is rumored to have banks of resistors to mimic regen so that excess power is shed when the battery is heated to capacity.

It's possible to have bigger brakes and stop more quickly, but that costs money, weighs more, and so that needs to be balanced in considerations as well.

-Crissa

Completely agree the battery most likely cannot take that large of a power surge. If regen braking is significantly better than mechanical braking, they could add a few components to the inverter that would dump the excess energy into a power resistor. Admittedly, I don't know how significant motor regen braking would be compared to mechanical ABS. If regen could slow the car from 60-0 in 2 seconds where mechanical ABS took 10 seconds on the same surface, I think they'd seriously look into it. If regen is 2 seconds and ABS mechanical is 2.3 seconds, maybe not.

Keep in mind, my first question on this issue is why can't a car slow down as fast as it accelerates. The answer given, and I accepted that answer, was because the motor and inverter have far better control over the tires than mechanical brakes with ABS. This begged the question (at least for me), if motor/inverter controlled stopping is significantly better than mechanical, why don't we use it. We then got into discussion about the regen power surge, can the battery take it, and if not, could we do something else with it.

 

[Crissa]

I think you answered your own question.

But, larger brakes could have better thermal properties and stop the car faster - the Taycan can stop better than it starts, and better than the Model S Plaid.

And yes, you could regen as much as you accelerate, but that's again extra weight and heat for resistance.

-Crissa

 

[HaulingAss]

I think you answered your own question.

But, larger brakes could have better thermal properties and stop the car faster - the Taycan can stop better than it starts, and better than the Model S Plaid.

And yes, you could regen as much as you accelerate, but that's again extra weight and heat for resistance.

-Crissa

I think that is likely the tires. Or the tester. Or the suspension. The difference is probably small. Maybe the brakes work slightly better, I really don't know. What people like to gloss over is the biggest deficiency when it comes to stopping is not how good the brakes are but how good the driver is. There are also the variables of traces of oil and anti-freeze on the road, leaves and needles, algae, etc.

We could ask why don't municipalities reduce accidents by cleaning the roads better of oils and other slippery contaminants? Why aren't drivers given reaction time tests when renewing their licenses? Why do regulations allow tires that last a long time but don't stop very quickly?

 

[Deleted member 11233]

2001 Model Y?

I fixed it to 2021

Sponsored