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.23Cd ?

Discussion in 'Model 3' started by TEG, Jul 29, 2017.

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  1. TEG

    TEG TMC Moderator

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    It looks like the press kit shows Cd was .23

    As I recall they were hoping to get to .21, so it looks like they missed that target a bit.
    Is the Cd lower than .23 if you get the 18" Aero wheels?

    On the other hand, going above 300mile range on what may be a ~75kWh battery is mighty impressive.
    They seem to have made this car very efficient in multiple ways, not just low Cd.
     
  2. dhanson865

    dhanson865 Active Member

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    #2 dhanson865, Jul 29, 2017
    Last edited: Jul 29, 2017
    I would assume the .21 is if you delete the side mirrors and use cameras instead. They just couldn't get government approval to delete those yet.

    They've been trying to remove side mirrors since before the Model X prototype.

    Tesla Wants To Change Government Rules So It Can Replace The Rear View Mirror With A Camera (misleading headline as it also talks about side mirrors)

    Tesla And Lobby Group Ask Regulators To Lose Side Mirrors

    Why It'll Be a While Before We Can Replace Car Mirrors With Cameras
     
  3. JeffK

    JeffK Well-Known Member

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    I'm not sure about the Cd but I'm pretty sure the aero wheels are the only reason they got the base model to 220 mi of range. The base battery capacity is lower than many of us anticipated in order to increase margins.

    Add a roof rack or tow something and we might see that range drop dramatically... We'll have to wait for real world data.
     
  4. Az_Rael

    Az_Rael Supporting Member

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    Based on the great efficiency folks are getting out of the Bolt, it is true that Cd isn't everything. It is nice to see Tesla focus on efficiency more than performance. (Even with my personal disappointment with the 0-60 times)

    It's fine for the Model S to be a relative "electron guzzler" considering it's class, but the 3 should be a more efficient option.
     
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  5. McRat

    McRat Active Member

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    The proof is in the performance.

    When real people start reporting, and more extensive testing is done by auto journalists, we will know more.

    I was betting that the Cd (shape) is superior on the MS cars than the M3 cars, so I'm pleasantly surprised at the great number. .21 Cd was never a realistic goal for a 5 seater. The M3 is taller and shorter relatively to the shape to make room for passengers. Short is not good for Cd, especially when combined with tall.

    But in the end, what is important is how many Wh/mi it uses. This affects operational costs and the charging times required at a given kW rate.
     
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  6. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Aptera wanted to get rid of side mirrors too. And also failed.

    Side mirrors suck from an aero perspective.
     
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  7. JohnSnowNW

    JohnSnowNW Active Member

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    Right, it's a matter of driving under the EPA testing procedures...
     
  8. Stirfelt

    Stirfelt Member

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    Not an authority by any means .....

    The M3 has pretty good ground clearance .... which delights me. My 2010 Prius was always scraping slight bumps in the roadway, and the new Prius is even lower ...... but it has .21 drag coefficient.
    So ..... perhaps the higher grounds clearance is one reason the drag coefficient is higher.

    I'm thinking it is a fair trade (if true)
     
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  9. McRat

    McRat Active Member

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    Drag Queens: Aerodynamics Compared – Comparison Test – Car and Driver

    Toyota might have claimed .21 Cd but independent testing is wildly different.

    Just like they claimed 134 horsepower.

    Neither were true. It's why a 2011 Volt used to clean your clock, not only uphill, but in the corners too.
     
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  10. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Ground clearance is ostensibly the same as a Prius - 5,5".

    I'd prefer more, what with the snow and sometimes poor roads here. But, it is what it is :)
     
  11. brianman

    brianman Burrito Founder

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    My recollection is that they targeted 0.22 for the Model S, and achieved 0.24. So apparently removing the mirrors is worth 0.02?
     
  12. kbM3

    kbM3 Member

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    If the leak from earlier was true, it looks like the model three might be the most efficient car in the world? What exactly are you looking for?

    Plus it looks like it has better performance than any internal combustion engine car in its class.
     
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  13. SageBrush

    SageBrush 2018: Drain the Sewer

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    Prime is 0.24; IIRC Prius is 0.25
     
  14. SageBrush

    SageBrush 2018: Drain the Sewer

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    You forgot to append 'driving slowly' to your sentence.
     
  15. McRat

    McRat Active Member

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    #15 McRat, Aug 5, 2017
    Last edited: Aug 5, 2017
    It's not necessary. As you often quote, a mag turned on the AC to 72°F (frosty, eh? That draws 3-4 kW on our cars) and locked the cruise at 75 mph in California, which doesn't have a lot of flat areas, and recorded 190 miles of range in hot weather.

    If the aero was tragically poor, I'd think would lose a lot more than that. 217 vs 190 with the AC blasting at laboratory temperatures over hilly terrain?

    Let's look at the closest thing we can get to apples to apples:

    2017 Volt EREV final eng'g 2015.
    .28 Cd (Tested by Car & Driver)
    Height 56.4"
    Width 71.2"
    Height x Width 4015 sqin
    Weight 35xx lbs, tires 215/50-17 LRRs.

    2017 Bolt EV final eng'g 2016.
    .308 Cd (according to latest data reported by GM, but don't trust it, you'll see why shortly)
    Height 62.8"
    Width 69.5"
    Height x Width 4385 sq in
    Weight 35xx lbs, tires 215/50-17 LRRs.

    So the Bolt is about 9% more cross section give or take. Probably more due to the more boxy cross-section.
    And it has 10% greater Cd, or close to 19% more aerodynamic drag.

    The Bolt has a EPA hwy of 217 and city of 255 mi which is a 15% highway test hit.
    The Volt has a EPA hwy of 49 and city of 57 mi which is a 14% high test hit.

    Can you see the problem? Higher speeds with should favor the car with 19% better aero. But it doesn't have much effect.

    65 mph to 75 mph for a car like a Bolt, should show 32% more power required. 217 EPA hwy / 190 C&D at 75mph is only 14%. Why doesn't it line up? Only two possibilities for such a large error. Either the Bolt EV's technology jumped wildly in 1 year, or the numbers Chevrolet reported to the EPA were too low.
     
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  16. stopcrazypp

    stopcrazypp Well-Known Member

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    #16 stopcrazypp, Aug 5, 2017
    Last edited: Aug 5, 2017
    I hope you do realize that the EPA highway test (both versions) is skewed low speed (average speed 48mph; regular version never goes above 60mph, the "high speed" US06 only spends 92 seconds out of 600 seconds ~15% above 70mph). It's not a useful comparison for aero.

    Try a 75-80mph drive between the two and see which one does better.
     
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  17. McRat

    McRat Active Member

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    And if that doesn't work, bump the MPH up more? If the numbers are still too low for the aero drag, then deflate the tires, or just do all testing uphill? Or should you mount a drag chute?

    What those numbers say, is either Chevy derated the Bolt's range for the EPA, or the .308 Cd is not correct, or GM boosted EV efficiency like 18% in a year over a car (2016+ Volt) that routinely uses under 250 Wh/mi at 65 mph with the AC on in hilly terrain over a round trip.
     
  18. stopcrazypp

    stopcrazypp Well-Known Member

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    #18 stopcrazypp, Aug 5, 2017
    Last edited: Aug 5, 2017
    More hyperbole again instead of addressing the point?

    Here's a picture from the Roadster Efficiency and Range blog entry (original link to picture is dead, but this is from a google search). Individual vehicles will vary based on tire type, drivetrain differences, etc, but just for illustrative purposes.

    Notice how the consumption for aerodynamics does not even cross over tire or drivetrain consumption until starting at 45mph. Aero doesn't cross into half of total consumption until 70mph.

    A cycle that averages 48 mph simply will not show anywhere close to a proportional amount of aero contribution.

    If Aero takes up only half of consumption at 70mph, the 32% hit would end up being around 16%, so there is nothing out of the ordinary for a 14% hit.

    Again, a better comparison of aero drag would be doing steady state at higher speeds, so you reduce the tire/drivetrain contribution as much as possible. 75-80 mph is quite reasonable as that's the speed on most long distance routes (in California at least).

    PS: I should note just multiplying height and width is not an accurate way to get cross sectional area (I know because I tried doing that before when doing some aero comparisons in the past; you can be off by a lot esp comparing very different body types).
    [​IMG]
     
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  19. Jeff N

    Jeff N Active Member

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    I see A/C overhead closer to 1 kW after cabin temperatures have stabilized on a long road trip. That amounts to about 5% of the overall energy (or less) when outside temperatures are 90F or more.

    No, 32% more power required for 10 mph increase from 65 to 75 mph is too high. The Nissan LEAF has been fairly carefully measured by Tony Williams and he shows about a 9% energy increase per additional 5 mph increase at those highway speeds. The Bolt has roughly similar aero drag shows markedly better high speed efficiency (relative to low speed) versus the LEAF which implies no more and perhaps less than a 9% increase increments for the Bolt. It also implies powertrain differences that favor the Bolt at highway speeds.


    No, it doesn't necessarily say that. That Volt and Bolt have very different motor, gearing, and battery designs. The Bolt may very well be tuned to be more efficient at those highway speeds than the Volt. The Bolt's final drive hearing results in lower rpms at the same road speed and less internal gearing friction. The can be other reasons as well such as the Bolt operating the battery at a much lower C rate at highway speeds which will tend to allow the battery to release a greater total energy than if it is quickly discharged at a higher rate.
     
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  20. omgwtfbyobbq

    omgwtfbyobbq Member

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    #20 omgwtfbyobbq, Aug 5, 2017
    Last edited: Aug 5, 2017
    The .23 Cd is probably with the Sport wheels, unless Tesla was using the Cd from a car with Aero wheels and the fuel economy rating from a car with Sport wheels. I guess that's possible, but for something as big as the 3 I think they would dot their t's and cross their i's.

    Anyhoo, I'm fairly confident that the EPA rating at this point, assuming it's accurate, is with the Sport wheels. Tesla only has one test group and the only way they can use a car with Aero wheels for their EPA rating is if less than a third of the cars they sell under that test group have Sport wheels, which I think is unlikely given that they're only selling Long Range/Premium Model 3s this year.

    In that context, the Cd=.23 they're using should be for the Sport wheels, and with the Aero wheels the car might approach or hit .21 Cd.

    Edit - Ignore everything I just posted if Tesla can restrict Aero wheel purchases to less than a third of employee purchases. In that case, the .23 is for a car with Aero wheels, and they'll release updated range estimates once they start selling the Long Range/Loaded cars to current owners/line-waiters, who will presumably order the majority of cars with Sport wheels.
     
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