Clarity Hybrid's Cd (Coefficent of Drag)

Discussion in 'Clarity' started by Alantn, Jul 23, 2018.

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  1. Alantn

    Alantn Member

    Couldn't find this number anywhere? Wonder why Honda does not show it. Prius is .25
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  3. Atkinson

    Atkinson Active Member

    I've seen numerous write-ups where Honda refused to state Cd.
    Not sure why.
    Maybe it's not a selling point?
  4. PHEV Newbie

    PHEV Newbie Well-Known Member

    My suspicion is that the drag coefficient is very low. The "odd" styling of the Clarity is due to optimizing aerodynamics even at high cost. For example, rear end is higher for aerodynamics and Honda put in those two windows to improve the rearward view at great expense (Car manufacturers, including Honda, will even use cheaper hard plastics in the rear doors compared to the front just to save a few pennies--BTW, that wasn't done in the Clarity). The list of aerodynamic enhancements is huge and a lot aren't even visible (like the smooth underside cladding).
    Last edited: Jul 24, 2018
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  5. Kranberry

    Kranberry Member

  6. Nemesis

    Nemesis Active Member

    I agree and wondered why the Coefficient of drag is not published - the exact definitive number, not the guesses. I started a thread on this a few months back and no one had the exact number. Glad you're interested in finding this information out as well.
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  8. JCEV

    JCEV Active Member

    My guess is the 235 tires killed the drag and it wasn't anything special so they didn't want to be embarrassed having a similar drag coefficient with the need for the wheel cover. I'm sticking to that but it's probably around .25
  9. insightman

    insightman Well-Known Member Subscriber

  10. Nemesis

    Nemesis Active Member

    JCEV, you know what. You are probably correct about the tire width of 235 bringing down the coefficient drag. When I was doing my pre buying research, I was surprised at the tire size of the Clarity rims and tires, I love it l, just was surprised based on the plug in hybrid vehicle.
  11. Odobo

    Odobo Active Member

    235 on clarity probably due to the weight and they need the extra surface to help stop the car
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  13. Probably Honda realized it's not something worth mentioning for their target audience. Only the end result MPG and eMPG might matter to their customers.

    The Hyundai Ioniq has a CD of 0.24, they did try to emphasize that in publication for it's as good as Tesla Model S and I've seen that many times in reviews. But does that really help selling their cars? (It's not even mentioned in the quoted article)
  14. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    One of the auto review magazines, I forget which one, does their own wind tunnel tests and reports on coefficient of drag, but I guess they haven't tested the Honda Clarity yet. If they had, then a Google search should find it, and it doesn't.

  15. insightman

    insightman Well-Known Member Subscriber

    Car & Driver's editorial offices are in my town, Ann Arbor. Six or seven years ago, they got in touch with me through my local Honda dealer. They were looking for a gen 1 Insight to calculate its coefficient of drag. I agreed and visited their offices.

    It turns out that instead of a wind tunnel, the magazine has an alternative way to perform the calculation. They had a set of precisely measured distances marked in the large parking lot of the complex where their offices are. They set up a fancy camera on a tripod about 1/8 mile away from where they had me park my car. The various long-distance photos they took evidently gave them accurate frontal and side cross-section profiles. They then used those photos with some kind of software to calculate the coefficient of drag.

    I doubt Car & Driver's photographic technique is as accurate as a wind tunnel test (eg. their photos don't take into account the car's underbody aerodynamics), but I've read that different wind tunnels provide different results, too. At least their technique gives them apples-to-apples comparisons among the different cars they photograph. They said they'd email me the final number, but that never happened. So I'm continuing to believe Honda's claimed cD of 0.25 (yes, Honda proudly announced this number back in 2000).

    While coefficient of drag is certainly important when it comes to improving gas mileage, the frontal area is another important factor. The coefficient of drag is just a ratio. The coefficient of drag combined with the measure of a car's frontal area provides a more concrete measure of wind resistance.

    Still, I wish Honda hadn't discontinued revealing the coefficient of drag on their fuel-efficient cars. It would be a small way to reward the anonymous engineers who work so hard to minimize this number. Here's a photo Honda released nearly 20 years ago.

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  16. Viking79

    Viking79 Well-Known Member

    Is it possible to coast in the Clarity? I don't recall off hand. If you can truly coast with no regen you should be able to derive Cd and Crr by how long the car takes to coast down. However, I think even in neutral the car might regen or apply some power? Might be worth doing, probably take a pretty long flat stretch of road to test though, not sure I have a spare runway to test on ;)
  17. KentuckyKen

    KentuckyKen Well-Known Member

    From my driving experience (on the same incline), you are always in one chevron of regen when in D and foot off the go pedal even if the chevron is not present.
    In N, it felt like true coasting with no resistance of regen felt. So Vikings test is theoretically possible and perhaps the only use of N outside of a car wash.
  18. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Yeah, I've read that we should not take too seriously the claims from an auto maker regarding its aero drag or Cd ratings, because there apparently isn't any standardization for wind tunnel tests. What one auto maker reports may be using a slightly different metric than another. I treat all such specs as merely an estimate, not a scientifically accurate number.

    I didn't know that Car & Driver was just using cameras (and presumably 3D modeling based on those image) to calculate Cd. Thanks for a very interesting story! And as you say, that can't possibly be as accurate as real wind tunnel tests.

    Also, thanks for highlighting the fact that frontal area is just as important as Cd, when it comes to the amount of aero drag a vehicle has. That's something a lot of comments and articles on the subject ignore.
    Last edited: Jul 25, 2018
  19. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Apologies for nit-picking, but actually, lower Cd is better, so "bringing down" the Cd would be an improvement.

    And I could be wrong, but I think wider tires would have more of an impact on frontal area than on Cd. But either way, wider tires are certainly going to increase aero drag. It's interesting that the BMW i3 uses special narrow tires to reduce drag, but obviously other EV makers didn't think that was a good solution because newer EVs have not followed BMW's lead.

  20. Viking79

    Viking79 Well-Known Member

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