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Theory of Roadster w/ better range & less perforance?

Discussion in 'Future Cars' started by NEWDL, Jan 26, 2009.

  1. NEWDL

    NEWDL R#350 R#1323 Sig23 8136

    Jan 5, 2009
    Would it be possible for Telsa to sell roadsters at a later date with the same battery pack but longer range and less performance?

    I am no Mechanical Engineer but was thinking that most of us will enjoy the 0-60 in 3.9 (or 3.75 with Sport) but do we really need it? If Tesla built a roadster with less performance and longer range it could increase economies of scale and lessen cost...then again if most of the cost is in the battery pack then you would not be able to charge much less for a longer range/slower car...

    Then again with a 0-60 of 6.0 and a range of 300+ you may not have to lessen cost by too much....maybe it would just lessen the fear of range restriction...

    Just thinking out loud...feel free to do the same.
  2. WarpedOne

    WarpedOne Supreme Premier

    Aug 17, 2006
    Slovenia, Europe
    #2 WarpedOne, Jan 26, 2009
    Last edited: Jan 26, 2009
    Short answer: No

    You need bigger ICE engine to get more max power.
    You need bigger electric motor to get more max power.

    Bigger ICE engine running at low load is LESS efficient than smaller ICE engine running at that same load.

    Bigger electric motor running at low load is MORE efficient than smaller electric motor running at that same load.

    So if Tesla installed weaker electric motor, the max range would DROP.

    Ofcourse there are some diminishing returns with increasing the size of electric motors but those are completely different from bigger ICE problems (mainly weight and rotating mass).

    Better acceleration comes from higher gear ratios. Higher gear ratios mean electric motor spins faster than with lower ratios. Faster spinning electric motor is again more efficient than slower spinning one while outputing the same power.

    So, there are no benefits in reducing max possible acceleration. Though it could be done artificialy in software to make it "safer" to handle.

    What would somewhat increase the max range is increase in the gear ratio. If you would except max speed of say 90mph then the ratio could be 1/3 higher than it is now. This would translate into 1/3 higher average motor rpm meaning higher average efficiency and longer range with same battery pack.

    But ofcourse, this would mean the acceleration would be even more "dangerous". Think around 3s.
  3. TEG

    TEG TMC Moderator

    Aug 20, 2006
    Silicon Valley
    Short answer: sort-of

    If longer range was the only goal, and performance (and style) could be tossed you could do some drastic things. They would primarily be to cut down on wind resistance and rolling resistance.

    #1: Reduced top speed. If you only let someone ever go 55mph then they wouldn't have to fight as much wind.
    #2: Make the car more aerodynamic. Make it look more like an Aptera.
    #3: Skinnier, harder, higher pressure lower rolling resistance tires (but the car would look silly and not corner as well).
    #4: (Very drastic) switch to front wheel drive with better brake blended regen.

    So I don't see any of the above really happening to the Roadster. They have already worked very hard to optimize efficiency without compromising performance or style. They may be able to eek out some minor gains with regen tuning and minor eMotor efficiency improvements, but I think the big range gains will likely have to wait for higher capacity batteries.
  4. AntronX

    AntronX Member

    Nov 17, 2008
    Miami, FL, USA

    It can be done with second identical battery pack towed behind the car in a sleek aerodynamic trailer. Towing extra 1000 pounds should reduce acceleration by a few seconds. If latest higher capacity cells used, then maybe this combination could get 500 - 600 miles of range at 60 mph on the highway. Main pack would supply most of the current for acceleration and second pack would only assist in maintaining speed.
  5. DRM

    DRM Roadster #619

    Mar 10, 2008
    San Diego, California, United States
    I don't expect there is a lot of weight in the power electronics package, but it could be substantially downsized if the peak torque requirement was reduced. Additionally, reducing peak battery draw could potentially lead to a simpler cooling system....
  6. Tim

    Tim Member

    Dec 15, 2008
    #6 Tim, Jan 26, 2009
    Last edited: Jan 26, 2009
    Maybe a little, but not much. Range is determined primarily by energy density of the battery. Without changing that engineers are limited to maximizing regen, reducing weight, parasitic loads/drivetrain efficiency, rolling resistance and CdA(drag coefficient times frontal area). While those can all be optimized further to increase range, there is no simple tradeoff between range and acceleration like you're used to with gas powered cars. As other posters mentioned the motor weight and efficiency are marginal to the entire car weight and power train efficiency and downsizing them would yield very little in the way of range improvements. Battery development is by far the limiting factor regarding range. Why do you think you need a range over 240 miles per charge?

    As far as they go:

    Weight - the roadster is a VERY light car. The easiest way to cut weight is to reduce the number of cells in the battery pack. This helps range(with higher cap cells) and accel.

    Regen - Front wheel drive, cap pack, better battery management all will help regen. Helps range, but not accel.

    Rolling resistance - easy change, but reduces handling. Increases range, reduces accel.

    Parasitic loads. More efficient motor, inverter and battery. Increases range and probably accel.

    CdA. Roadster frontal area is small. Cd can be improved. It's is currently .365 which is high relative to a prius(.25), but is pretty low for a high end sports car. Would require extensive wind tunnel testing and redesigned body. Increases accel and range.

    Smaller motor/inverter - reduces accel - marginal effect on range

    Bigger battery. Increases range, reduces accel.
  7. mt2

    mt2 Member

    Aug 29, 2007
    Chicago Area, Northwest Burbs
    So, there you go. "No, sort of, yes, maybe".

    TEG nailed it. The Tesla blogs have on several occasions noted that the Roadster is a compromise (despite the No Compromises slogan). It was developed as a high end sports car to bury the cost of the batteries. As such, they promised over 200 miles range, under 4 acceleration, and an insane top speed. When the original design goals could not be met, they reduced the top speed from 135 to 125 to 120. They determined that their customers find acceleration most important, range probably the next most important, and top speed the least. Oh, and drop-dead sexy good looks are on the top of anyone's list.

    The Roadster is a balance of these things. So while it is possible to increase range by redesigning the car, it wouldn't be the Roadster when you got done.
  8. mpt

    mpt Electrics are back

    Oct 15, 2008
    Warren, New Jersey, United States
    I guess that the Aptera 2e is the answer to "Design me the most efficient electric car", fill the back of that with batteries.

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