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Towing - Weight Distribution Hitch compatibility

Discussion in 'Model X: Driving Dynamics' started by ohmman, Dec 8, 2016.

  1. ohmman

    ohmman Maximum Plaid Member

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    The Model X is capable of towing up to 5000 pounds with 500 pounds of tongue weight (19-20" wheels). When considering towing anything more than half of that maximum, the subject of WD (weight distribution) hitches comes up. I've seen a number of rules of thumb, but generally what I see is that if the TT (travel trailer) is more than 50% of the TV (towing vehicle)'s weight, a WD hitch is recommended. The Model X weighs ~5300 lbs, so any trailer over 2650 lbs would qualify.

    First, it will be helpful to describe a typical (non-WD) towing configuration. The Model X has two axles on the ground, and the travel trailer, for ease of discussion, has a single axle on the ground. The two vehicles are connected solely by the ball hitch and safety chains. The tongue weight is the weight pushing downward on the ball hitch at rest. That weight pushes downward on the back of the Model X, and therefore pushes down the back axle while taking weight off of the front axle. As the tongue weight increases, this scenario can create instability in the form of lack of maneuverability. It can also create a situation where the trailer tilts downward at the front significantly, with the towing vehicle tipping downward at the rear and upward at the front. A tiny amount of trailer front-tilt is desired for stability reasons, but too much will also cause issues.

    Weighting the trailer behind the axles can alleviate some of this and balance the load. However, too much in the back and the trailer will tilt backward. From what I understand, this is the most unstable configuration, as air under the trailer can cause lift and sway.

    In comes the WD hitch. The best description I've read is that this is like standing on the front of the trailer and lifting upward on a stiff pipe inserted into the receiver on the towing vehicle. What happens when you do that is that you take weight off of the rear axle of the tow vehicle, and distribute that weight between the trailer axle and the front axle of the towing vehicle. In doing so, you create a bit of vertical plane rigidity between the two, and they act almost as one single mass.

    So, that's good enough and makes plenty of sense, except my online research shows that there are some problems with this. One problem is that the Model X has a unitary body construction, and there are varying opinions on whether or not WD hitches are appropriate for that style of frame. If you imagine that rigid rod going through the receiver, and remember that it's also square, you can imagine the torque that is placed on the hitch and the vehicle whenever the road is not similar for the trailer axles and the vehicle axles. A bumpy road will transfer a lot of stress into the unibody of the Model X. The reason that's considered detrimental is that the entire body will bend and twist repeatedly, and could eventually fail. Opinions vary on whether or not this is a serious problem, and I can find no empirical data one way or the other. Tesla's use of aluminum in lieu of steel is also a big question mark in this realm.

    Another problem appears to be the auto-leveling mechanism built into the Model X. This is set up so that when you attach a tow vehicle, the air suspension levels the X for you. Similar products from Mercedes and BMW suggest completely disabling this when a weight distribution hitch is attached, and most manufacturers of unibody vehicles suggest against using a WD hitch at all.

    Where does that leave us? On one hand, the literature says that a travel trailer approaching more than half of our vehicle weight should use a weight distribution hitch for safety and stability. On the other hand, there is advice against it because of the unibody construction and air suspension of the X.

    Are there any knowledgeable opinions on this?

    *Also, any corrections to erroneous statements I've made are welcome.
     
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  2. Az_Rael

    Az_Rael Supporting Member

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    I am not super knowledgable about unibody designs and weight distribution hitches, but I do tow with one with my Tacoma (and I stayed at a Holiday Inn Express!). It does distribute the weight forward so that I get less lift on my front suspension (that's actually how I set the tension on it - by measuring the amount of lift on the front suspension). By lowering the front suspension lift, the vehicle is much safer to drive and towing is much smoother. So you got all that part right based on my experiences "in the field" so to say.

    The other big advantage to WD hitches is you can add sway control to them. I don't currently run with a sway control setup since I don't tow all that often, but it is a huge advantage when towing at highway speeds (with the one downside that it has to be removed prior to backing) .

    As far as the Model X and the combination of unibody design plus air suspension.... that's tough. Its possible that if the air suspension can compensate for the trailer on the rear axle by leveling the car, then a WD hitch might not strictly be required. Now, I have no idea how much something like that would stress the air suspension, but if it is designed to be on while towing...then it SHOULD be designed for that. A lot of big trailer guys end up putting "airbags" on their tow vehicles, so I would assume that is emulating the effects of an air suspension.
     
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  3. ohmman

    ohmman Maximum Plaid Member

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    Thanks. Regarding the quoted note above, the Model X auto-leveling will not redistribute weight to the front axle, it just will make sure the vehicle is level with the ground. I'm sure you can picture why if you think about the loading. There's no way for the X to shift that load forward. Other manufacturers suggest disabling air suspension, managing your weight distribution (in the way you mention - measure the lift on the front suspension), then enabling air suspension again. That makes sense to me, and is probably appropriate with the X.

    I have made contact with Tesla and they're looking into this on their end. It will be comforting to have some guidance from the people who engineered the vehicle. :)
     
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  4. ohmman

    ohmman Maximum Plaid Member

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    An update. Tesla did not get back to me yesterday, so I'll follow up Monday.

    I did, however, speak with Can-Am RV Centre in London, Ontario. The principal, Andy Thomson, is known in the RV/trailering community as the preeminent guru of hitching and towing. Can-Am is known for modifying sedans, adding hitches capable of towing large 25-30 foot Airstreams. They have some critics, specifically those who say that Can-Am is able to structurally support these heavy trailers but they can at times ignore the transmission/engine capabilities of the towing vehicle. Overall, though, they are well respected as knowing their stuff when it comes to towing and unibody construction.

    I was told that it is quite unlikely that a lightweight and aerodynamic trailer like the Sport 22 would cause any vehicular stress with a WD hitch. In fact, I was told that they would absolutely recommend towing with one with the hitch on the Model X. I am going to follow up with them (per their request) in email, and they said they'd provide me with some documentation that they supply their customers.

    So far, that's positive news.
     
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  5. Evbwcaer

    Evbwcaer Member

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    In my opinion, the S and X are not unibody construction. Look at photos of the skateboard, you'll get a sense that the vehicle is more like a ladder frame constructed vehicle.

    The answer is probably some where between the traditional unibody/ladder frame, but the S and X surely have major structural integrity coming from elements that are not part of the body.
     
  6. JimVandegriff

    JimVandegriff Member

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    This thread has caused some serious digging into manuals, books, and articles about towing! I found myself reading the Trailer Life supplement on towing yesterday. Phew. I really don't know enough about the science to knowledgeably comment, but my experience (for what that is worth) is that the weight distributing, anti-sway hitch we have (made by Equal-i-zer and installed by Airstream) works extremely well to safely pull our 22 ft Airstream Bambi Sport with our X 90D. I'm hoping ohmman's red santa cap can get Tesla to reply with more concrete info. I also spoke with Can-Am before we purchased our rig, and was impressed with their knowledge.
     
  7. qadaemon

    qadaemon Member

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    Thanks for contacting Tesla and all the other legwork you've done getting information on camper towing. It is much appreciated.
     
  8. ohmman

    ohmman Maximum Plaid Member

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    Sure, my pleasure. I did get a chance to speak with Tesla today. They have added notes to the engineering file, and it can take some time to get an answer. How much time? An indefinite amount of time. :)

    I did get a hint that since we have the skateboard, as @Evbwcaer mentioned, the car isn't truly unibody and should likely support a WD hitch safely.
     
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  9. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    A great thread. From my long experience in hauling trailers of all sizes, I admit never thinking a weight distributing hitch could be of use for pulls of less than the 5,000lbs limit of the Model X, but with the modest size* of that car, I suppose it makes all sorts of sense. I certainly think the battery pack-stiffened frame of Teslas only enhances, never denigrates, the benefits of such a hitch. By the way, I also use the Equal-i-zer hitch that Jim V mentioned.

    *as opposed to full-sized pickups
     
  10. ecarfan

    ecarfan Well-Known Member

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    I assume there has been no response from Tesla yet. I remain very interested in this topic. As @ohmman pointed out in the OP, the widely published general rule of thumb is a WD is not needed if the TT (Travel Trailer) weight is no more than half the TV (Tow Vehicle) weight. But as has been discussed, it's not really that simple, and when using an X as the TV further complications ensue because of the air suspension and how to classify the X frame/body construction.

    The Airstream Sport 22 that @JimVandegriff and @ohmman are towing has a "Unit Base Weight" of 3,634 lbs ( 2017 Airstream Sport Floorplans & Specs | Airstream.com ) which is well over 50% of their TV weight. So I can understand why they would want to use a WD hitch.

    The TT I plan to get in 2018 is a Safari Condo Alto F1743 which has a dry weight (without options) of just 1,592 lbs. Even loaded up with water, various options, kitchen gear, food, etc., I expect it to weigh less than 2,200 lbs. My TV will also be an X (to be purchased later this year, I plan on getting a 100D when it is released) and with my wife and I and gear in the car I estimate it will weigh about 5,800 lbs.

    So my TT will weigh only about 38% of my TV (an unusually low percentage for a 17 ft camper trailer). I anticipate initially towing without a WD hitch and see how it feels.
     
  11. ohmman

    ohmman Maximum Plaid Member

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    Nothing yet.
    I think it should be fine for two-lane driving, but on highways it's quite possible you'll find that you want one for sway control, as @Az_Rael noted above. With hard edges and a lightweight frame, the Alto could be susceptible to sway from passing trucks or cross-winds. Only your experience will tell you, though.
     
  12. ohmman

    ohmman Maximum Plaid Member

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    I've spent some time solving for the static forces exerted on the receiver with a weight distribution hitch. Note that these measurements are approximate, and that I haven't been able to ensure equal 1/3 distribution of the tongue weight, as I assume in my diagram and calculations. Also, the measurements are specific to my modified hitch and trailer combination.

    That said, I solved the attached free body diagram for the torque on the weight distribution arms t(WD) and came up with approximately 11900 in-lbs (just about 1000 ft-lbs) in order to get the distribution of weight on the axles as shown. Assuming that's where it is adjusted, I am able to solve for the moment forces created by the trailer tongue at point R, which is the connection point of the receiver into the Model X. Those forces are the 500# "max" tongue at the ball and the ~1000 ft-lbs of torque from the ball in the opposing direction. They net to -4100 in-lbs, which means approximately 342 ft-lbs of torque in the CCW direction.

    So what does that mean relative to the rating of the stock Bosal hitch receiver? Well, the Bosal hitch receiver is stated to support 500lbs of downforce at 8" from the hitch pin. That equates to 500 lbs at approximately 13.5" from my location R, or 562.5 ft-lbs of CCW torque. So, given that my calculations have any merit at all, and noting that they do not include any dynamic effect, it appears that the WD hitch relieves not only the rear axle, but also the receiver from torque that would normally be applied, allowing for a larger factor of safety around tongue weight fluctuations.

    I welcome (and request) any feedback on my calculations, especially if anyone sees something I've done wrong. I recognize I didn't include some of the vertical offsets, but I think the effect is minor in the statics problem. I may revisit and try to tweak it a bit.


    tongue-statics.png
     
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  13. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    You do know how to determine that with a bathroom scale and a lever, don't you? I'll not proceed unless you tell me "no"....
     
  14. vandacca

    vandacca ReActive Member

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    I'm sure @ohmman knows, but maybe you can proceed for the rest of us. :)
     
  15. ohmman

    ohmman Maximum Plaid Member

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    I can measure tongue weight that way (or, more directly, with my tongue scale), but I cannot measure axle weights to determine tongue weight distribution in that way. The only way to know whether the tongue weight has been distributed equally is to go to a scale and measure the rig's axle weights before and after engaging the torsion bars.
     
  16. ohmman

    ohmman Maximum Plaid Member

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    This may help.

    faq118_dd.jpg
     
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  17. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    #17 AudubonB, Feb 5, 2017
    Last edited: Feb 5, 2017
    You're conflict is with left-right distribution, then? If it's pre- versus post-hook-up distribution, I'm pretty sure the scale/lever method also would work - position same under the vehicle's hitch then learn how much deflection (weight, of course) occurs.

    OR: approximate all through the other end of the vehicle. Measure deflection of the front headlights against a vertical wall. I vaguely remember my Equal-i-zer Hitch had some such reference....??????

    On edit: I'm sorry; it's obvious only the second paragraph is of interest to you. And there, the headlight trick will do exactly what you want; that is, what the aid of an equalizing hitch is: to distribute evenly the weight between the vehicle's two axles. Aligning the post-hookup headlight pattern exactly to where your previously-marked pre-hookup pattern was provides you just what you're desiring.
     
  18. ohmman

    ohmman Maximum Plaid Member

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    It will approximate it. One issue is that the air suspension on the X complicates leveling a bit. As an example, look at what airbags do on trucks that are towing - they actually lower the weight on the front axle, despite the vehicle appearing level.

    I plan to get things loaded up similarly to my plans for our summer month-long trip and taking it to a local scale. At that point, I'll have good numbers to plug back in and should feel pretty good about the whole rig.
     
  19. K-MTG

    K-MTG Sunshade Captain of TMC

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    @ohmman, what % loss do you see with your trailer compared to driving without it.
     
  20. ohmman

    ohmman Maximum Plaid Member

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    Strongly depends on the speed and terrain. At 55mph, it's about 180% of the consumption without the trailer.
     
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