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Top Speed. Governed or maxed out?

Discussion in 'Model S: Driving Dynamics' started by spatterso911, Mar 10, 2012.

  1. spatterso911

    spatterso911 MSP#7577 **--** MX#1891

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    The top speed of each iteration changes based upon battery choice (110 mph to 130 mph). Are these the maximum attainable speeds for each battery/motor combination or are they governed electronically to limit wear and tear or maintain a temp or ... ???

    My current car is governed to a max speed electronically, but will go at least 40 mph faster if the governor is removed. What about the Model S?? The Performance version?? The 40 kWh??
     
  2. AnOutsider

    AnOutsider S532 # XS27

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    No factual evidence, but I'd vote it's electronically limited (as well as there being physical limitations with pack sizes). Tesla controls the battery through software, and I'd imagine there's also software to keep the car's operation within acceptable ranges.

    Again, just my assumption.
     
  3. rabar10

    rabar10 FFE until Model 3

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    I suppose it would be possible that the gear ratios are subtly different between the models with different-sized packs, but that would undermine consistency between cars of different pack sizes. Pretty sure this isn't it...

    Much more likely that the different max power levels (constant C rates with different pack sizes) allow different top speeds to be obtained or sustained.

    An earlier power curve for the Model S showed largely constant power output like the Roadster, but with power dropping sharply as the motor reached max RPM. This means even if max battery power isn't needed to maintain top speed, the smaller packs may not allow the car to hit max RPM based on that top-end power curve...
     
  4. bonnie

    bonnie Oil is for sissies.

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    I believe it's limited for safety reasons. The larger battery weighs more, so the max speed can be a bit higher.
     
  5. stopcrazypp

    stopcrazypp Well-Known Member

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    The only situation where it is really directly physically limited is different gearing for every battery size (to make up for lower power in the smaller packs). But like rabar10, I find that unlikely since it probably costs more money (and makes the production less efficient) to have all those different gears rather than keeping it all the same.

    I also agree with rabar10, it's most likely battery power. Tesla probably wants to keep the C-rate reasonable so they limit the top speed in the smaller packs to prevent overheating/damaging the cells.

    Only in the 85kWh vs the Performance (with the 125mph vs 130mph) difference, is it clearly not a battery difference. Perhaps it is the PEM difference in that case (or they are squeezing a bit more rpms out of the motor).
     
  6. TEG

    TEG TMC Moderator

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    I doubt that is the reason.

    That would be my guess/assumption as well.
     
  7. bonnie

    bonnie Oil is for sissies.

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    I doubt your doubting :). If it were because of battery power, there would be three different top speeds (for three different range options). But there are only two different top speeds. Which just happen to correlate to two different battery weights.

    But I'm only going on a casual conversation I heard at the X event. I don't know this for a fact.
     
  8. TEG

    TEG TMC Moderator

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    #8 TEG, Mar 11, 2012
    Last edited: Mar 11, 2012
    In my experience, you don't need weight to justify top speed.
    Just as an extreme example, the McLaren F1 weighed only ~2500lbs, yet went over 230mph.
    I think top speed limitations tend to be more about aerodynamics (avoiding lift) and tire capabilities.
    Also, not sure why more battery pack weight would be equated to more safety. To the contrary, you have more kinetic energy there, so it is harder to slow down a heavier vehicle.
     
  9. TEG

    TEG TMC Moderator

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    There do seem to be 3 different top speeds:
    speeds1.jpg

    The reduced 0-60 performance numbers also suggest that the smaller packs could have less maximum power output.
     
  10. spatterso911

    spatterso911 MSP#7577 **--** MX#1891

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    Wouldn't the ratio of horsepower to weight be the main factor, with limitations set by aerodynamics, gear ratios and tire performance? The McLaren has a phenomenal power/weight ratio. If this is the case, then the different battery sizes have proportionally different power outputs, given that each weighs differently, but installed in the same chassis. That must be the major difference in their purported top speeds. It likely explains the difference between performance and standard 85 kWh models. Perhaps this is really an issue of programming for the power output.
     
  11. Adm

    Adm Active Member

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    Another option is that Tesla wants to differentiate more between the different battery levels so people are tempted to go for a bigger battery pack.
     
  12. dsm363

    dsm363 Roadster + Sig Model S

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    Although range will be greatly decreased at top speed, maybe they also don't want a certain TV show taking it out on the track at 155mph and saying the range is 1/10th advertised.

    I'm sure it's an actually technical reason that people have stated above though.
     
  13. Doug_G

    Doug_G Lead Moderator

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    I'm sure the speed is limited to protect the motor. If you spin the shaft fast enough, it will disintegrate due to centripetal acceleration (colloquially "centrifugal force").
     
  14. stopcrazypp

    stopcrazypp Well-Known Member

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    The 85kWh and 60kWh packs will have very little weight difference (under 100lbs) because Tesla has indicated they will use the same amount of cells. The 40kWh will be significantly lighter (about 70% the weight of the 60kWh pack) since it uses 5500 vs 7800 cells in the other two packs.

    However, during top speed, it is the aerodynamics that dominates (and the rolling resistance becomes less important; it's almost flat throughout anyways, so it doesn't seem to increase that much based on speed). So I doubt weight is a huge factor in the top speed.
    http://www.teslamotors.com/blog/roadster-efficiency-and-range
     
  15. stopcrazypp

    stopcrazypp Well-Known Member

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    Again, unless each car had subtly different gearing, like rabar10 suggested, that shouldn't result in a different top speed for every version.
     
  16. Doug_G

    Doug_G Lead Moderator

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    I doubt they have different gearing. The power is probably limited on smaller packs to keep the C rate to a safe level.
     
  17. spatterso911

    spatterso911 MSP#7577 **--** MX#1891

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    Can you post an image of that power curve??
     
  18. NigelM

    NigelM Recovering Member

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    I doubt you're doubting his doubting anymore! :smile:
     
  19. bonnie

    bonnie Oil is for sissies.

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    No doubt. :)
     
  20. richkae

    richkae VIN587

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    The performance version lists 130 mph as the top speed - 5 more than the regular 85kWh car.
     

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