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X air suspension setting while towing: could higher be more energy efficient?

Discussion in 'Model X: Driving Dynamics' started by ecarfan, Jan 31, 2017.

  1. ecarfan

    ecarfan Well-Known Member

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    We know that at highway speeds energy efficiency is improved when the air suspension is at the lowest setting, thereby reducing the overall frontal area of the vehicle as measured from the road surface up. What follows is my speculation:

    When towing a trailer that is taller than the X, raising the air suspension to the highest setting may improve energy efficiency because then the very aerodynamically shaped X will more effectively push the air away from the top of the trailer. In contrast, having the air suspension at the lowest height when at highway speeds will result in more of the upper body of the trailer being exposed directly to the oncoming airstream.

    I realize that setting the X air suspension on high allows more air to flow underneath the car. But if the trailer underbody is higher off the road than the X underbody, and if by raising the air suspension setting so that the X underbody matches the trailer underbody, would that improve X energy efficiency while towing?

    It would be relatively easy for an X owner with a camper trailer to test that proposition. I hope someone does.
     
  2. McManX

    McManX Member

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    Per the manual aren't you supposed to leave suspension set to standard and trailer mode ensures it doesn't change with speed/location/etc?

    Can you override or would you have to forgo trailer mode? Also I remember reading the car sensing a trailer and auto enabling trailer mode, would this even change the suspension back to standard?
     
  3. Zaphod

    Zaphod Galaxy President (former)

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    Raising the suspension will also increase the pitch of the trailer, which could increase (or decrease) the frontal area. The air between the X and trailer will still predominantly be turbulent. I think any efficiency improvements would be so minimal it would be lost in the noise and would be hard to attribute the improvements specifically to that one thing.

    Keep in mind as well, just because you are raising or lowering the X suspension, the height differences are not independent of one another. The trailer is still connected to the hitch, so raising or lowering will cause the hitch to raise and lower as well and potentially tilt the trailer forward or backwards. It's not such that the relative height of the X roof and trailer will change significantly.
     
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  4. BerTX

    BerTX Active Member

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    I suspect it would have little, if any affect on the aerodynamics. Studies I have seen testing drag related to trailer towing indicate that, unless the tow vehicle is within 24 inches of the back of the tow vehicle, there is little effect. Couple that with the slippery body of the MX maximizing air flow attachment, the very back area of the car is the only area with any drag effect. Moving that relatively small shadow up or down a couple of inches on the aerodynamic brick that is a travel trailer will have little or no benefit.

    It is the shape of the rear of the trailer that makes all the difference in drag. Making the front of the trailer pointy is of very limited value, that's just for sales purposes. In theory a teardrop trailer would be best, but unfortunately manufacturers have to build them so that people can get inside. They modify the shape to increase interior room, and poof, it isn't any better aerodynamically than a square box. That's why some manufactures make a teardrop trailer that pops up, so it has good aerodynamics on the road, but transforms into a usable interior space.
     
  5. ecarfan

    ecarfan Well-Known Member

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    I assume you mean "unless the towed vehicle is within..."
    In another thread (which I just spent 15 minutes searching for but could not find, sorry) there has been some interesting informed speculation that a retractable roof teardrop trailer is in fact not necessarily more aerodynamic than a more conventional trailer shape. The steep downward slope of the teardrop trailer can cause airflow separation as it passes beyond the roof peak resulting in turbulence which then interacts with turbulent air coming off the top of the sidewalls behind the roof peak resulting in increased drag. In contrast, a trailer roof shape that is flatter will not produce the same turbulent airflow patterns. Anecdotal reports that have been conveyed to me from Safari Condo Alto R1723 owners (teardrop retractable roof, 83" high) and Alto R1743 owners (fixed roof, 96.5" high) shows no difference in MPG numbers between the two trailer models for the same tow vehicle model. And I have information from a company that regularly tows those two trailer models stating that, using the same tow vehicle, they see lower MPG numbers towing the fixed roof R1743 model compared to the retractable roof R1723 model. They say this is because at highway speeds (over 35mph) teardrop trailer aerodynamics are poor.
     
  6. ecarfan

    ecarfan Well-Known Member

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    I assume you mean "unless the towed vehicle is within..."
    In another thread (which I just spent 15 minutes searching for but could not find, sorry) there has been some interesting informed speculation that a retractable roof teardrop trailer is in fact not necessarily more aerodynamic than a more conventional trailer shape. The steep downward slope of the teardrop trailer can cause airflow separation as it passes beyond the roof peak resulting in turbulence which then interacts with turbulent air coming off the top of the sidewalls behind the roof peak resulting in increased drag. In contrast, a trailer roof shape that is flatter will not produce the same turbulent airflow patterns. Anecdotal reports that have been conveyed to me from Safari Condo Alto R1723 owners (teardrop retractable roof, 83" high) and Alto R1743 owners (fixed roof, 96.5" high) shows no difference in MPG numbers between the two trailer models for the same tow vehicle model. And I have information from a company that regularly tows those two trailer models stating that, using the same tow vehicle, they see slightly lower MPG numbers towing the fixed roof R1743 model compared to the retractable roof R1723 model. They say this is because at highway speeds (over 35mph) teardrop trailer aerodynamics are poor.
     
  7. BerTX

    BerTX Active Member

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    Yes, towed, thanks.

    The point I was trying to make is exactly your point -- teardrop trailers are not really aerodynamic. Certainly not worth the interior room compromises. The idea behind the Safari Condo is good, but again, compromises were made somewhere along the line.
     
  8. ohmman

    ohmman Maximum Plaid Member

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    The X suspension is fixed in "standard" when in tow mode. Unfortunately, this isn't testable. I like the idea, but would be surprised if it made a difference. The underbody of trailers is also not very aerodynamic (fresh/grey/black water tanks are mounted there), so allowing more air under there could offset any potential gains by raising the X.
     
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  9. ecarfan

    ecarfan Well-Known Member

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    Thanks. So much for my idea.
     
  10. ohmman

    ohmman Maximum Plaid Member

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    Just to close out the idea - the reason it's fixed is because once you set up and hook up your hitch, the last thing you want to do is move the towing vehicle's height relative to the trailer. The goal is to keep them both level.

    The air suspension does come in handy for lowering the X all the way to get the ball hitch under the trailer, though. At that point, I return it to standard and do the rest of my hitch routine.
     
  11. ecarfan

    ecarfan Well-Known Member

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    Does that negate the need to raise the trailer up to get it above the ball? That is, if you set the X suspension to the lowest point, can you then move the X so the hitch ball is in position below the trailer frame and then raise the suspension up so the ball fits into position?

    I asssume not since I have not seen anything posted by you or @JimVandegriff about hitching up using that approach. But it would be really cool if that was possible.
     
  12. ohmman

    ohmman Maximum Plaid Member

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    I guess in theory this would work to some degree, but the trailer still needs to be up on the tongue jack foot quite a ways. You wouldn't be saving any time, especially if you have an electric tongue jack. I don't see it being useful.
     
  13. JimVandegriff

    JimVandegriff Member

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    When we travel and have unhitched the X from the trailer, we do move the air suspension to low. This allows the ball to disengage from its receiver and eases the unhitching. We do the same when hitching back up, backing up in low, then raising the air suspension to standard as we hitch up. We are looking forward to our next trip to practice said maneuvers!
     
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