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Aero Helps Performance, Not Just Range

Discussion in 'Model X: Driving Dynamics' started by gavine, Aug 19, 2018.

  1. gavine

    gavine Petrol Head turned EV Enthusiast

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    I have seen two racing videos which prove how important aero is for performance.

    First video below shows the i-Pace being faster than X100D in quarter mile (beginning of video) but the X pulls away (a lot) in the roll race (starts at 2:30). Even the presenter is surprised by it but he doesn’t contemplate aero.

    The second video shows the more powerful Jeep Trackhawk having the same off-the-line acceleration, but the X pulls away when typically ICE cars pull away (or catch up) on Teslas at higher speeds (race starts at 6:50). I really thought the i-Pace had good aero, but apparently not. The Jeep obviously doesn’t.



     
  2. Yinn

    Yinn Active Member

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    MX Drag Coefficient is 0.24
    iPace Drag Coefficient is 0.29
    Jeep Drag Coefficient is 0.34

    The other thing is Jaguar's marketting department has come out and stated the MX produces lift at speed, where as the iPace is focused on producing downforce at speed. I know the additional downforce adds traction and increases the drag coefficient, which I know is good for traction and cornering; but I don't know how that affects a straight line speed test.

    With that said, for comparison sake to counter the numbers above...
    Toyota Sienna has a drag coefficient of 0.30
    Ford Fiesta has a drag coefficient of 0.32
    Audi R8 drag coefficient is 0.36

    So while I understand the concept of dragging a parachute makes me slower, it seems with enough power traction takes over. Although I'm not sure why the focus; an economy focused vehicle only eakes out 0.32.
     
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  3. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Well-Known Member

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    Because
    1) manufacturing cost
    2) utility
     
  4. gavine

    gavine Petrol Head turned EV Enthusiast

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    Apparently .05 makes a big difference. In both cases above, the more powerful car lost to the more aerodynamic one. It can also be noted in the range. The heavier Model X 90D gets more range than the smaller iPace with the same battery size.
     
  5. ecarfan

    ecarfan Well-Known Member

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    Excuses, excuses...

    And that is a silly excuse. If by “at speed” they mean highway cruising speeds of 65-75mph, that “downforce” is going to decrease the iPace range, which is not what you want in an EV. The iPace is not a track car, it’s for use on public roads carrying multiple people and cargo.
     
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  6. Peteski

    Peteski Active Member

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    There are other factors besides aero to consider in these comparisons. You also need to compare the frontal area of the iPace vs MX to determine which one actually produces the most aero drag, not just the coefficient.

    It's quite possible that the iPace actually has similar or even possibly less drag than the MX due to its lower frontal area, but I don't have figures for their respective frontal areas to calculate. But we know the MX is wider and taller than the iPace, so it will definitely have a larger frontal area, which will offset against it's lower drag coefficient i.e. CdA

    A few other factors to consider when comparing acceleration at higher speeds are gearing and motor torque. The Tesla has more torque available and probably slightly taller gearing than the iPace, so not that surprising it hauls the iPace back in at higher speeds. Aero drag really has more effect on maximum speed than acceleration up to 100 mph at least.
     
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  7. gavine

    gavine Petrol Head turned EV Enthusiast

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    Agreed, but how about the Trackhawk? That has more power as well as gearing for top-end, but the X just pulls away as they gain speed (and wind resistance). That shows the aero difference in plain site. When the Model S and Hellcat race (same drive trains as their SUV counterparts), the Hellcat starts to reel-in the S, but the Trackhawk doesn't reel-in the X. Has to be aero.
     
  8. Peteski

    Peteski Active Member

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    #8 Peteski, Aug 23, 2018
    Last edited: Aug 23, 2018
    For sure the Trackhawk has much more aero drag (higher coefficient and higher frontal area), so that will be a major factor once the speed ramps up (power required to overcome drag is proportional to the cube of speed). It's not so clear cut with the iPace, where I think they are much closer matched on CdA.

    Also hard to compare peak ICE power figures vs EV as they are so different in their nature. ICE power is ramped up on a curve vs engine speed, EV much more instantaneous*. Trackhawk has to change gear multiple times too, so a little loss there too. Both look to be traction limited off the line as you would expect with such power. Tesla traction control is the best in the business too, again largely due to the fine control resolution of EV power.

    * Actually with EV you basically have a constant power motor (in practice it tails off somewhere above 120 mph due to back emf etc) and therefore torque drops off with speed (As power basically equals torque x speed and with an EV power is a constant). With an ICE you have more of a flat torque curve in each gear and so power increases with engine speed until you shift into a higher gear and therefore drop onto a lower torque level at the wheels and start the power ramping again vs engine speed. So in an ICE your acceleration rate is basically in discrete steps proportional to each gear ratio, while in an EV it's more of a flattening curve. So comparing peak power in an ICE vs EV is apples vs oranges and can be very misleading.
     

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