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Model S don't like big booty and I cannot lie

Discussion in 'Model S: Driving Dynamics' started by PhatCat, Aug 30, 2017.

  1. PhatCat

    PhatCat Kisco Kid

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    Here's a puzzler that has stumped this chump. I have a facelift 2016 S75D fully optioned, including air suspension. My normal daily commute is a 14 mile one way mix of 65 mph highway, 50 mph state road and a little stop and go with moderate change in elevation. In clear temperate weather i normally average from 280-300 kWh/mi for the most recent 30 mi, depending on traffic and how exuberant I am with the accelerator.

    While dong my Costco run last weekend i noticed that my usage was over 300 kWh/mi and climbing even though the ambient temp was about 80 and dropping. This continued during my commute Monday and when i returned home the display showed 350 kWh/mi. I have never seen usage this high.

    All Monday night i racked my brain to understand this. No indication of low tire pressure. I had turned off AC and heat. No jack rabbit starts. And then i remembered that I had left two 40 lb bags of cat litter in the trunk. So I removed them yesterday morning and, presto, by the time i returned home my usage had dropped below 280 kWh/mi and it continued this morning.

    Can this really be the cause of such a significant increase in consumption? i assumed that the air suspension would level the car.and 80 lb wouldn't matter. The only thing i can think of is the weight was all the way in the rear on the passenger side. Perhaps the air suspension could not level the offset weight and the asymmetry destroyed the laminar flow.

    Any ideas?
     
  2. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    That should have an insignificant impact on your performance.
     
  3. Az_Rael

    Az_Rael Supporting Member

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    I know that when I took a trip with 4 boxes of books in the trunk (~150lbs), my usage was much higher. The projected line on the trip planner was waaaay below what the computer projected. On the way home, without the books, the projected line was exactly with the computer projection and our usage was much lower.

    So, there must be a reason that EV Trip planner accounts for weight. I am guessing 80lbs of cat litter was the answer to your usage.
     
  4. scaesare

    scaesare Well-Known Member

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    Hmm... a ~25% increase in energy usage due to 80 lbs additional load seems odd based on my experience.

    I don't see nearly that kind of difference when I have a couple hundred additional pounds of family in my car as opposed to driving solo on several mixed city/hwy driving routes that I commonly take. Nor do I notice that kinds of difference when loaded up with people/luggage for road trips.

    80 lbs is something like 1.6% additional weight. And given that many other factors contributing to energy usage remain constant (aero, car systems, etc...), that seems like an awfully large additional usage....

    (and here I was thinking your thread title had something to do with your user name)...
     
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  5. u00mem9

    u00mem9 Member

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    Bumpsteer.
     
  6. Doug_G

    Doug_G Lead Moderator

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    Weight is insignificant unless you are climbing hills.
     
  7. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Also affects rolling drag and accel/decel losses, but yeah, that's such a tiny amount of weight compared to the total vehicle weight that it shouldn't have any meaningful direct impacts of its own, nor offbakabce the vehicle (even without automatic suspension balancing) in a way that would cause any meaningful decrease in range.

    I think it's just random factors and the fact that people's memories naturally try to make associations between random events. In practice, the reason for the performance difference was probably something else. But if anyone wants to know for sure, they could certainly experiment - driving around with a weight in their vehicle every other day for a couple months and then comparing the results ;)
     
  8. PhatCat

    PhatCat Kisco Kid

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    OK folks, here's an update. I performed the following experiment. After removing the cat sand, i drove my normal routine for 3 days and usage dropped under 280 wh/mi. I then put the two bags of cat sand back into the trunk in the original location and drove for three days. Usage zoomed over 300, peaking at 370. I have never seen anything remotely close to that in the year I have owned the car. I then took the sand out and drove another three days and usage dropped back below 280.

    Make of this what you will.
     
  9. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    It's interesting, but hardly conclusive, because weather and driving conditions could have changed in those times.

    If it actually would be true, something would be very wrong.
     
  10. u00mem9

    u00mem9 Member

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    This is the effect of the alignment change with the rear suspension compressed and possibly front raised. Bump toe / bump steer.
     
  11. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Except that should be compensated for, and even if it wasn't, it should have an insignificant effect on consumption.
     
  12. u00mem9

    u00mem9 Member

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    Air suspension is pretty crude in its method for maintaining ride height AND at the extremes of alignment things can change rapidly. I would get the alignment checked and a decent shop will even indulge your experiment with the added weight.

    I owned an original Honda Insight for a time...1mm of toe was worth about 3mpg on highway with that. Eye opener
     
  13. thecloud

    thecloud As rhythm raced inside, the ship came alive

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    I wonder if you would get the same results with the bags of cat litter riding in the rear passenger seats...
     
  14. PhatCat

    PhatCat Kisco Kid

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    I have driven this same route every week day for the last year and the consumption never varies by more than 15 wh/mi. There was no appreciable climate change during the period and i drove the speed limit as always.

    I'll ask the SvC when I take it in next.
     
  15. OilSucks

    OilSucks Member

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    Was it by any chance hot outside? I had a similar experience in the Bay Area this past weekend where the temperatures were in the high 90's to 100's and my Kwh/Mi reading was at 489! Which is much higher than my average of 325 to 280. Temps have gone down and the car seems to be back to normal.
     
  16. Ardie

    Ardie Member

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    Okay, it seems that there *is* something going on here.

    I propose that you put the bags of sand back in the car -- but in the frunk this time -- and repeat your experiment.
    Let's see if your power consumption:
    [ goes up ],
    [ stays the same ], or
    [ goes down ].

    I'm gonna guess that it [goes down] due to a sublime difference in aerodynamics, or maybe a sublime difference in weight distribution that the car's computer perceives, and overcorrects. Because the absolute difference in weight shouldn't change the game so much.

    -- Ardie.
    "No, you can't sit back there. It will ruin my mileage. Sit up front with me."
     
  17. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Determining whether it's an aero issue should be easy. Take consumption datapoints on flat ground going the same direction in the same weather conditions after the tired are heated up - do the run at a number of different speeds. Now do the same with the weight in the trunk. You can also do rolldown tests. Bring the numbers back here and we can figure out whether your drag coefficient is changing. :)
     
  18. P85DEE

    P85DEE Active Member

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    #18 P85DEE, Sep 7, 2017
    Last edited: Sep 7, 2017
  19. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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  20. sandpiper

    sandpiper Active Member

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