5 ms·

I wonder where the top Speed Limit comes from, is it battery cooling or artificial limiting to proclaim a better Range? In Germany that number raised a few eyeb

I wonder where the top Speed Limit comes from, is it battery cooling or artificial limiting to proclaim a better Range? In Germany that number raised a few eyebrows.

Already__Taken 10y ago
I would assume it's to simplify the gearing.
ferongr 10y ago
Most electric cars lack a variable-ratio transmission. At some point and you only have the final drive to reduce the speed of the electric motor. The final drive has to strike a balance between torque for negotiating inclines and top speed, as the electric motor cannot rev to infinity, neither can the power systems handle infinite power though them. An electric car with a two-ratio gearbox would be ideal for both taking off from a standstill and top speed.
- Roritharr 10y ago
  What does the Model S use then?
  - ferongr 10y ago
    A way more powerful motor and power electronics that allow for a "taller" (smaller reduction ratio) final drive.
  - ahakki 10y ago
    IIRC the AWD versions (recognizable by a 'D' at the end of the model name, as in P90D) use 2 motors, one for the front and one for the rear. These two motors have different gear ratios, so one is more opimized for torque and the other for high speed. The rear wheel drive versions have a single ratio which has to strike a balance between speed and torque. This is also the reason that the AWD versions are faster than the RWD even though they are heavier due to the additional motors.
  - andruby 10y ago
    Tesla wanted to use a 2-gear transmission for the original roadster, but they couldn't find one that could handle the massive torque, so they ended up with a single ratio gearbox. Source: http://www.autoblog.com/2008/01/23/breaking-tesla-has-a-solution-for-their-transmission-woes-get/ http://www.autoblog.com/2008/01/23/breaking-tesla-has-a-solu... and Ashley Vance's biography of Elon Musk.
- awjr 10y ago
  One of the concerns I have is that many of the EVs that are being released are not capable of towing significantly. I own a horse and we need something that can tow a braked 3.5T trailer. This market in the EU is primarily serviced by diesel 4x4s. I can see a future where the diesel engine is banned and only at that point will something fall out of the woodwork to cope with that towing capacity.
  - ollie87 10y ago
    Extra battery in the floor of the trailer? Hooks into the car and gives it some extra juice, why wouldn't that work?
    - Klathmon 10y ago
      IIRC it's not the battery power as much as it is the cooling. Many of the parts on electric cars overheat when pushed pretty quickly, and while adding cooling similar to a traditional ICE seems to be the answer, it adds even more weight and complexity to a platform which is already fighting to save every oz. Even the Tesla Model S can't do more than a single lap around many tracks without overheating.
      - jgust 10y ago
        The diesel-electric locomotive comes to mind. The road has different challenges than rail, but the application is proven.
      - mikeash 10y ago
        I have heard that the more recent Model Ss (Models S?) are a lot better about being able to keep up with power output on tracks, although what you say was definitely true for a long time.
  - camiller 10y ago
    I think that there will always be some internal combustion driven vehicles for tasks such as these.
  - 10y ago
ygra 10y ago
A top speed around 150 km/h isn't unusual for smaller ICEs either. Unless you spend that much time on the Autobahn and do mostly city driving the speed limit isn't really a problem in my experience. But I guess with an electric vehicle you're unlikely to drive long distances at those speeds anyway.
- Roritharr 10y ago
  Not a big Problem, but being limited to 150 was one of the main things why my old ICE car to me felt unsafe to drive on the Autobahn. For short periods its just safer to be able use the most left lane at 160-180 than to go 120-130 in the Center and wait for a chance to overtake with not much headroom over the left at 140-150.
  - Xylakant 10y ago
    germany is one of the last countries where that may be perceived an issue. Literally all other countries in the world have a speed limit that is way below 150km/h. So top speeds above 150 are just there for germany and for bragging rights, not for any practical purpose. What's more interesting is the torque and acceleration available at higher speeds.
    - patrickk 10y ago
      Sluggish acceleration or below par top speed isn't a perception problem on the Autobahn, it's a real problem. It's completely practical to drive long distances at high speeds because of the well-maintained road surface and well-maintained cars. There are plenty of stretches of the Autobahn with no speed limits. This is a common scenario: you're in the middle lane, and a particularly slow driver is in the middle lane (who should be in the right lane), you attempt to overtake in your underpowered econobox, and out of nowhere a Porsche or high-end BMW is sitting on your tail, and you try to overtake asap to get back into the middle lane to let the other guy pass. This is doubly true if you're going up a slight hill, which exaggerates the lack of torque. Another common scenario is a truck driver, overtaking another truck driver in the middle lane, and you have to move to the left lane to avoid him (truck drivers are supposed to keep to the right lane). Source: someone who's driven plenty of shit, underpowered rentals in Germany, e.g. a Fiat 500X and a Renault Captur. Why anyone, apart from rental companies, would buy these ugly, overweight, underpowered shitboxes is beyond me.
phkahler 10y ago
>> I wonder where the top Speed Limit comes from... Gear ratio. It says the motor spins to 8800rpm which is high but not crazy high. Given a top speed of the motor the gear ratio will determine the top speed of the car. They may have optimized it for better acceleration at the expense of top speed. Many people in the US never drive that fast so it's not a bad tradeoff. If you're wondering about having multiple gear ratios, it doesn't make much sense in EVs. These motors already behave in a similar way. For a given (maximum rated) current they will produce constant torque from zero to what is called "base speed" which is the point where the back-emf matches the bus voltage. Beyond that speed it enters an operating region called "field weakening" where some of the current is used to oppose the magnets which is equivalent to reducing the back-emf so it can go faster. In this region it operates much like a constant power device so the torque drops with the inverse of rpm much like a CVT. There's another metric called the constant-power-speed-ratio (CPSR) which is the top speed divided by the base speed. In my experience, CPSR for an EV is likely to be somewhere between 3 and 5 though it can be much higher. Source: one of my specialties happens to be algorithms for very high CPSR. It's an odd place I never meant to go, but I would enjoy taking a stab at tuning the Bolt.
- mikeash 10y ago
  I believe that multiple gear ratios could make sense for high-end EVs. For example, the Model S P100D's top speed of 155MPH is pretty low for a car of such power and low-end acceleration. And its acceleration tends to fall off at higher speeds (if you watch drag races against high-end ICE cars, the Tesla tends to stomp them initially, then the ICE car starts to catch up after a few seconds) and I have heard that being able to change gear ratios would help with that, although I certainly could be misinformed.
  - phkahler 10y ago
    My colleagues looked into a 2-speed gearbox for EVs and they just couldn't make the case for it. Tesla was also going to use one early on and seems to have abandoned that idea. For the Model S (or any EV), increasing top speed would mean either increasing the motor RPM, or increasing the gear ratio which would reduce acceleration. I don't know what limits their top speed to 155 and don't want to guess.

I wonder where the top Speed Limit comes from, is it battery cooling or artificial limiting to proclaim a better Range? In Germany that number raised a few eyebrows.

Already__Taken 10y ago

I would assume it's to simplify the gearing.

ferongr 10y ago

Most electric cars lack a variable-ratio transmission. At some point and you only have the final drive to reduce the speed of the electric motor. The final drive has to strike a balance between torque for negotiating inclines and top speed, as the electric motor cannot rev to infinity, neither can the power systems handle infinite power though them. An electric car with a two-ratio gearbox would be ideal for both taking off from a standstill and top speed.

Roritharr 10y ago
What does the Model S use then?
- ferongr 10y ago
  A way more powerful motor and power electronics that allow for a "taller" (smaller reduction ratio) final drive.
- ahakki 10y ago
  IIRC the AWD versions (recognizable by a 'D' at the end of the model name, as in P90D) use 2 motors, one for the front and one for the rear. These two motors have different gear ratios, so one is more opimized for torque and the other for high speed. The rear wheel drive versions have a single ratio which has to strike a balance between speed and torque. This is also the reason that the AWD versions are faster than the RWD even though they are heavier due to the additional motors.
- andruby 10y ago
  Tesla wanted to use a 2-gear transmission for the original roadster, but they couldn't find one that could handle the massive torque, so they ended up with a single ratio gearbox. Source: http://www.autoblog.com/2008/01/23/breaking-tesla-has-a-solution-for-their-transmission-woes-get/ http://www.autoblog.com/2008/01/23/breaking-tesla-has-a-solu... and Ashley Vance's biography of Elon Musk.
awjr 10y ago
One of the concerns I have is that many of the EVs that are being released are not capable of towing significantly. I own a horse and we need something that can tow a braked 3.5T trailer. This market in the EU is primarily serviced by diesel 4x4s. I can see a future where the diesel engine is banned and only at that point will something fall out of the woodwork to cope with that towing capacity.
- ollie87 10y ago
  Extra battery in the floor of the trailer? Hooks into the car and gives it some extra juice, why wouldn't that work?
  - Klathmon 10y ago
    IIRC it's not the battery power as much as it is the cooling. Many of the parts on electric cars overheat when pushed pretty quickly, and while adding cooling similar to a traditional ICE seems to be the answer, it adds even more weight and complexity to a platform which is already fighting to save every oz. Even the Tesla Model S can't do more than a single lap around many tracks without overheating.
    - jgust 10y ago
      The diesel-electric locomotive comes to mind. The road has different challenges than rail, but the application is proven.
    - mikeash 10y ago
      I have heard that the more recent Model Ss (Models S?) are a lot better about being able to keep up with power output on tracks, although what you say was definitely true for a long time.
- camiller 10y ago
  I think that there will always be some internal combustion driven vehicles for tasks such as these.
- 10y ago

ygra 10y ago

A top speed around 150 km/h isn't unusual for smaller ICEs either. Unless you spend that much time on the Autobahn and do mostly city driving the speed limit isn't really a problem in my experience. But I guess with an electric vehicle you're unlikely to drive long distances at those speeds anyway.

Roritharr 10y ago
Not a big Problem, but being limited to 150 was one of the main things why my old ICE car to me felt unsafe to drive on the Autobahn. For short periods its just safer to be able use the most left lane at 160-180 than to go 120-130 in the Center and wait for a chance to overtake with not much headroom over the left at 140-150.
- Xylakant 10y ago
  germany is one of the last countries where that may be perceived an issue. Literally all other countries in the world have a speed limit that is way below 150km/h. So top speeds above 150 are just there for germany and for bragging rights, not for any practical purpose. What's more interesting is the torque and acceleration available at higher speeds.
  - patrickk 10y ago
    Sluggish acceleration or below par top speed isn't a perception problem on the Autobahn, it's a real problem. It's completely practical to drive long distances at high speeds because of the well-maintained road surface and well-maintained cars. There are plenty of stretches of the Autobahn with no speed limits. This is a common scenario: you're in the middle lane, and a particularly slow driver is in the middle lane (who should be in the right lane), you attempt to overtake in your underpowered econobox, and out of nowhere a Porsche or high-end BMW is sitting on your tail, and you try to overtake asap to get back into the middle lane to let the other guy pass. This is doubly true if you're going up a slight hill, which exaggerates the lack of torque. Another common scenario is a truck driver, overtaking another truck driver in the middle lane, and you have to move to the left lane to avoid him (truck drivers are supposed to keep to the right lane). Source: someone who's driven plenty of shit, underpowered rentals in Germany, e.g. a Fiat 500X and a Renault Captur. Why anyone, apart from rental companies, would buy these ugly, overweight, underpowered shitboxes is beyond me.

phkahler 10y ago

>> I wonder where the top Speed Limit comes from... Gear ratio. It says the motor spins to 8800rpm which is high but not crazy high. Given a top speed of the motor the gear ratio will determine the top speed of the car. They may have optimized it for better acceleration at the expense of top speed. Many people in the US never drive that fast so it's not a bad tradeoff. If you're wondering about having multiple gear ratios, it doesn't make much sense in EVs. These motors already behave in a similar way. For a given (maximum rated) current they will produce constant torque from zero to what is called "base speed" which is the point where the back-emf matches the bus voltage. Beyond that speed it enters an operating region called "field weakening" where some of the current is used to oppose the magnets which is equivalent to reducing the back-emf so it can go faster. In this region it operates much like a constant power device so the torque drops with the inverse of rpm much like a CVT. There's another metric called the constant-power-speed-ratio (CPSR) which is the top speed divided by the base speed. In my experience, CPSR for an EV is likely to be somewhere between 3 and 5 though it can be much higher. Source: one of my specialties happens to be algorithms for very high CPSR. It's an odd place I never meant to go, but I would enjoy taking a stab at tuning the Bolt.

mikeash 10y ago
I believe that multiple gear ratios could make sense for high-end EVs. For example, the Model S P100D's top speed of 155MPH is pretty low for a car of such power and low-end acceleration. And its acceleration tends to fall off at higher speeds (if you watch drag races against high-end ICE cars, the Tesla tends to stomp them initially, then the ICE car starts to catch up after a few seconds) and I have heard that being able to change gear ratios would help with that, although I certainly could be misinformed.
- phkahler 10y ago
  My colleagues looked into a 2-speed gearbox for EVs and they just couldn't make the case for it. Tesla was also going to use one early on and seems to have abandoned that idea. For the Model S (or any EV), increasing top speed would mean either increasing the motor RPM, or increasing the gear ratio which would reduce acceleration. I don't know what limits their top speed to 155 and don't want to guess.