-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
Fisker Karma
- Thread starter vfx
- Start date
-
- Tags
- Electric Vehicles fail Fisker karma
Yeah, it depends where that torque is measured... e.g., at the motor, after the gear box, or at the wheels. I think what really matters is power (and the distribution of it) since Torque is just Power/RPM (with appropriate units) and Power is also a function of RPM.
Anyhow, I think that Karma torque number is inflated since it's probably "measured" past the gear box. This is similar to how that initial Audi E-tron torque seemed so huge since it was measured at the wheels (IIRC).
Anyhow, I think that Karma torque number is inflated since it's probably "measured" past the gear box. This is similar to how that initial Audi E-tron torque seemed so huge since it was measured at the wheels (IIRC).
stopcrazypp
Well-Known Member
The age old torque vs horsepower/weight ratio debate about which is more important to acceleration. I have yet to hear a good physics based answer that explains why which one is a better indicator. This is complicated (on the torque side) by the gearing question and where it is measured. You can get better acceleration just by changing your gears (a frequent modification to Mustangs is to swap in 3.73 gears which can net you a couple hundred milliseconds off the quarter mile), regardless of crank numbers.
Plus these torque/horsepower numbers are measured at the crank based on a peak number at a certain RPM. That may not be a good indication of its performance throughout the rev range.
Last edited:
ljbad4life
Member
Is it odd that everyone in last couple of posts regarding Karma specs is being reasonable, logical and IMHO right. I thought I'd never see the day a forum would be reasonable :smile:
I find it odd that Fisker sandwiched the two motors in one casing (similar to how the prius cvt is). I don't see a point to having it this way for a pure Electric drivetrain. Each motor could have been mounted on the axle closer to each wheel (providing power to each wheel individually). I do think heat dissipation is an issue. having two large motors close together with a differential in the middle (to me atleast) screams copious amounts of heat generation during operation.
There is no way the torque is 900lbs unless the torque drop off of the Karma is significantly sharper than other EVs. Weigh does effect 0-60 times. 5,000+lbs is a lot of weight to move. For 100k Fisker should have used more weight saving materials. The only parts that aren't aluminum are the trunk lid and maybe the hood. Using composites or carbon fiber on the roof (under the solar panels), doors and body panels could have save them a ton of weight (pun intended :biggrin: )
I find it odd that Fisker sandwiched the two motors in one casing (similar to how the prius cvt is). I don't see a point to having it this way for a pure Electric drivetrain. Each motor could have been mounted on the axle closer to each wheel (providing power to each wheel individually). I do think heat dissipation is an issue. having two large motors close together with a differential in the middle (to me atleast) screams copious amounts of heat generation during operation.
There is no way the torque is 900lbs unless the torque drop off of the Karma is significantly sharper than other EVs. Weigh does effect 0-60 times. 5,000+lbs is a lot of weight to move. For 100k Fisker should have used more weight saving materials. The only parts that aren't aluminum are the trunk lid and maybe the hood. Using composites or carbon fiber on the roof (under the solar panels), doors and body panels could have save them a ton of weight (pun intended :biggrin: )
Last edited:
It seems to me that from a physics perspective, the only two things that matter for acceleration are the force delivered to the pavement and the mass of the vehicle. Force means torque at the wheels divided by the radius of the wheel. You should subtract slippage, of course, but that's probably minor if TC is operating.
Of course you can't use a single number. You'd have to average the actual torque applied over the whole 0-60 run, including any backing-off due to TC. A narrow peak isn't going to make much difference.
Of course you can't use a single number. You'd have to average the actual torque applied over the whole 0-60 run, including any backing-off due to TC. A narrow peak isn't going to make much difference.
Just a random thought - dual motors sharing a common differential will split the power to the back wheels evenly even if one of the motors is a laggard.
Perhaps they did it that way to avoid concerns that handling could get questionable if one motor was making more power than the other?
Perhaps they did it that way to avoid concerns that handling could get questionable if one motor was making more power than the other?
ljbad4life
Member
That's a distinct possibility, but that effect could be used for supreme traction control and stability control. I don't think power output differences would be a problem if both motors are the same and are monitored by the computer to stay with in certain variances (less power to one when going around a corner for example).
I would suspect that it could be a real regulatory nightmare to bring a vehicle to market with independent drive motors for each side. Probably all kinds of issues with proving safety in all potential failure modes involving full power on one side and zero power or even worse yet full regen on the other side.
strider
Active Member
But the historical use of a "single number" should be in the Karma's favor as the peak should be available at or very near 0 rpm vs a much higher rpm for an ICE. That 922 ft-lb peak number for the Veyron is not at 0 rpm as it would be for the Karma.
Thinking aloud I'm wondering if we in the EV community need to think about how to portray this stuff in the same way we struggle w/ mpg comparisons? As I said in my previous post, any gearhead will see 900 ft-lbs of torque next to the Karma and question why the 0-60 is so slow. If someone is comparing Model S to a BMW or Mercedes there could be similar confusion.
Hmm... may have to split off this power/torque discussion.
Anyhow, perhaps a better figure of merit would be something like the integrated power over the rev range.
Anyhow, perhaps a better figure of merit would be something like the integrated power over the rev range.
the advantage of a split drive is, that you need two independent motor-controller for each motor one. then you can apply torque control as well. in the worst case, if one of the controller/motor may fail, you still have an emergency mode with one wheel drive to reach the next workshop.
As Doug_G stated up thread, acceleration is really dictated by the force applied to the pavement and mass of the vehicle. Force to the pavement is torque to the drive axle divided by the tire radius.
The Karma (as the Tesla) does not have an selectable inline torque multiplier (transmission) like ICE vehicles have. For example say the Yeyron applies 1x unit of toque to the wheels in its top (7th) gear. In this case it will supply almost 4x units of torque in first gear, 2.84x units in second, 2.09x in third, etc. It also has full torque available from 2,200 - 5,500 rpm. The electric motor torque from 0 rpm is critical to allowing clutch-less single gear operation but doesn't help against the Veyron which will take only a tiny fraction of a second to reach its torque band in first gear and then has 100% torque available from there on out combined with < 150ms shift times.
Last edited:
Motor Trend review 2011-Motor Trend-Fisker Karma (May)
Power consumption is 50KWhr/100, that's twice the roadster as a datapoint. MPG: 67 combined but really, what the hell is that number?
Power consumption is 50KWhr/100, that's twice the roadster as a datapoint. MPG: 67 combined but really, what the hell is that number?
ljbad4life
Member
well I would expect a huge demand for power when the karma is north of 5k lbs. I have no idea how motor trend got 4k lbs when from any other publication has the weight estimated at 5k.
Then the CD on the Karma is not that great. it looks slick, but looks doesn't mean it's aerodynamically slick.
Fisker is definitely more about form than function. I think he's talented enough to have gone after both though...
Tommy
Member
There's a lot of drag when your mouth is open; and a big one at that:tongue:
And to be sure there is no misunderstanding, I am referring to the Karma, never met the OP.
ljbad4life
Member
He could have kept the same design and used more composites and carbon fiber. A 1k savings in weight could easily boosted performance (0-60 times) and extended range. a 100k car using steel and aluminum is disingenuous when weight matters.
I think Fisker wants a large margin on the Karma and a short turn around time to profitability.
If there is enough demand at that price, then that's just good business.
The Panamera gets about 24 mpg on the highway and (IIRC) I think we figured that the Karma would get about 25 mpg after it's Stealth Mode is depleted. So on a long trip, they get similar fuel economy as well.
Similar threads
