Towing - Weight Distribution Hitch compatibility

[ecarfan]

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.

Fascinating exercise, thanks for the analysis! It would be interesting to see how the numbers change with different air suspension settings.

 

[JimVandegriff]

An interesting, scary, and amusing thing happened to us today while pulling our trailer with our X which tangentially touches on tongue weight. We decided to check our battery fluid levels today. We have 2 lead acid batteries sitting just behind our propane tanks on the front of our Airstream. When we opened the lid to the second battery box we noted that the box was filled with water and the battery was sitting in this water. Illijana thought I should tell you that we emptied the battery case and reduced our tongue weight, ohmman. We have no clue how the water got in there. Our weight distribution hitch is working well - the trailer tracks beautifully. Did Tesla ever get you any further info, ohmman?

 

[ohmman]

An interesting, scary, and amusing thing happened to us today while pulling our trailer with our X which tangentially touches on tongue weight. We decided to check our battery fluid levels today. We have 2 lead acid batteries sitting just behind our propane tanks on the front of our Airstream. When we opened the lid to the second battery box we noted that the box was filled with water and the battery was sitting in this water. Illijana thought I should tell you that we emptied the battery case and reduced our tongue weight, ohmman. We have no clue how the water got in there. Our weight distribution hitch is working well - the trailer tracks beautifully. Did Tesla ever get you any further info, ohmman?

Tesla never got back to me. I haven't followed up, but will do so when I visit my Service Center next week for some parts.

The stock battery box on the 22' Sport doesn't drain. I still need to drill a drainage hole in mine. I've been busy working on other projects before our ~3500 mile trip in a month. I'm still trying to get the backup camera mounted so I can use the 17" screen to see out the back of the Airstream. There's an owner "SilverLife" over on Airforums who is doing very nice work on his Sport. You should check it out.

I'm glad you lowered the tongue weight.. ha. Did you have it measured with the trailer loaded? I never saw an update on that.

 

[vandacca]

Tesla never got back to me. I haven't followed up, but will do so when I visit my Service Center next week for some parts.

The stock battery box on the 22' Sport doesn't drain. I still need to drill a drainage hole in mine. I've been busy working on other projects before our ~3500 mile trip in a month. I'm still trying to get the backup camera mounted so I can use the 17" screen to see out the back of the Airstream. There's an owner "SilverLife" over on Airforums who is doing very nice work on his Sport. You should check it out.

I'm glad you lowered the tongue weight.. ha. Did you have it measured with the trailer loaded? I never saw an update on that.

What sort of sorcery will that involve? Are you going to somehow splice a 2nd camera feed into the standard rear camera? My curiousity is peaked.

 

[ohmman]

What sort of sorcery will that involve? Are you going to somehow splice a 2nd camera feed into the standard rear camera? My curiousity is peaked.

I'll post some photos when it's done, but so far I've run Tesla camera cables from the rear of my Airstream through the cabin to the wire harness at the front. I'm still unsure exactly how I'm going to mount the second camera on the back, but I believe I'm going to open up the existing hole for the license plate light and run the cable through, then create a mount for the camera just above that. The X portion will be a little more tricky. I have to take off the rear appliqué to get to the camera plug. I'd like to make it accessible from inside the liftgate so that I can disconnect the X rear view camera and connect the harness for the Airstream camera when I hitch/unhitch.

There is also a camera switch that people have used to add a front view camera to their Model S. @artsci has some some cool things with it. That may be the direction I have to go eventually, but since I'm hitching and unhitching cables it doesn't seem overly difficult to add one plug. We'll see.

 

[JimVandegriff]

Tesla never got back to me. I haven't followed up, but will do so when I visit my Service Center next week for some parts.

The stock battery box on the 22' Sport doesn't drain. I still need to drill a drainage hole in mine. I've been busy working on other projects before our ~3500 mile trip in a month. I'm still trying to get the backup camera mounted so I can use the 17" screen to see out the back of the Airstream. There's an owner "SilverLife" over on Airforums who is doing very nice work on his Sport. You should check it out.

I'm glad you lowered the tongue weight.. ha. Did you have it measured with the trailer loaded? I never saw an update on that.

I spent some time last night and today on the Airforums and was amazed at the work SilverLife is doing on his Airstream. You folks are inspiring! We haven't done the tongue weight weigh in yet but I intend to do it when we get home just for the piece of mind. Hole drilling is also on my agenda.
Where are you going on your 3500 mile trip?
We are planning a 4 month trip to Michigan and back probably through Nevada, Colorado, Nebraska, Iowa, Michigan, Kentucky, Missouri, Oklahoma, Texas, New Mexico, Utah, Arizona, and SoCal starting in July. Jim

 

[ohmman]

Where are you going on your 3500 mile trip?

We are going up to Crater Lake, to Bend, into the Gorge (and hopefully will see @bonnie at The Dalles), northeast into Glacier NP, through Calgary (and again, hopefully a few more TMCers), west to Banff NP, then through BC with a visit to @Canuck, and eventually down through Seattle and the West Coast. I'm very excited but have a lot to do before we go.

 

[mongo]

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.


View attachment 213415

Hey ohmman,

Bumped into this post today, I know its a little late since your trip trip has concluded, but wanted to add to the weight distribution pool of knowledge, and the potential receiver failure mode.

I ran the numbers a different way, take a look and let me know if this makes sense:.

So starting with a 500 lb tongue weight.
Weight at rear axle is 500*(13.5+41.5+117)/(13.5+41.5) = 735 lbs
Weight at front axle is 500 - 735 = -235 lbs (front + back = tongue) also 500*(13.5+41.5) = 235*(117)
so this is what we are trying to balance out.
The WD hitch adds a torque/ preload, but after that it can be modeled as a rigid system, so I solved for the trailer loading needed to equalize the front and rear axle loads.
System of equations:
A-235 = B+735 Goal: same load on each axle
A-235 +B + 735 +C = 500 Total weight does not change (ignoring vehicle and trailer weight since those are only offsets)
B = - C * (117+41.5+13.5+156)/(117) Lift on rear axle is leverage from trailer load with front axle as pivot, is negative for uplift
So after mathing, the forces due to WD loading are:
A = 380
B = -590
C = 210
Final loads are
Front: 145 (-235+380)
Rear: 145 (735 - 590)
Trailer: 210 (210 + 0)
Total: 500

Now that we have trailer load we can calc torque at the receiver adapter:
210*(13.5+156)/12 =35595 inlbs
= 2966 ftlbs

Loading spec: 13.5*500/12 = 562.5 ftlbs

So a little over 5X the design spec, if this approach is correct.

 

[ohmman]

Hey ohmman,

Bumped into this post today, I know its a little late since your trip trip has concluded, but wanted to add to the weight distribution pool of knowledge, and the potential receiver failure mode.

I ran the numbers a different way, take a look and let me know if this makes sense:.

So starting with a 500 lb tongue weight.
Weight at rear axle is 500*(13.5+41.5+117)/(13.5+41.5) = 735 lbs
Weight at front axle is 500 - 735 = -235 lbs (front + back = tongue) also 500*(13.5+41.5) = 235*(117)
so this is what we are trying to balance out.
The WD hitch adds a torque/ preload, but after that it can be modeled as a rigid system, so I solved for the trailer loading needed to equalize the front and rear axle loads.
System of equations:
A-235 = B+735 Goal: same load on each axle
A-235 +B + 735 +C = 500 Total weight does not change (ignoring vehicle and trailer weight since those are only offsets)
B = - C * (117+41.5+13.5+156)/(117) Lift on rear axle is leverage from trailer load with front axle as pivot, is negative for uplift
So after mathing, the forces due to WD loading are:
A = 380
B = -590
C = 210
Final loads are
Front: 145 (-235+380)
Rear: 145 (735 - 590)
Trailer: 210 (210 + 0)
Total: 500

Now that we have trailer load we can calc torque at the receiver adapter:
210*(13.5+156)/12 =35595 inlbs
= 2966 ftlbs

Loading spec: 13.5*500/12 = 562.5 ftlbs

So a little over 5X the design spec, if this approach is correct.

Hey @mongo, thanks for the contribution.

I'm following your initial calculations OK but I start to get lost where you take the load on the trailer axle (210) and assume that's a downward force acting at the distance to the trailer axle. That's only the case if it's cantilevered, yes? The ground is providing a reciprocal force via the trailer wheels. Your torque calculations don't seem to include the offset from the other axle points.

 

[mongo]

Hey @mongo, thanks for the contribution.

I'm following your initial calculations OK but I start to get lost where you take the load on the trailer axle (210) and assume that's a downward force acting at the distance to the trailer axle. That's only the case if it's cantilevered, yes? The ground is providing a reciprocal force via the trailer wheels. Your torque calculations don't seem to include the offset from the other axle points.

It is sort of a reverse cantilever. The WD shifts the front/rear axle weights by pushing down on the trailer to unload the rear axle. The 210 pounds is the additional load on the trailer wheels and also the reciprocal ground induced upward force applied to the trailer axle.
(With the extreme case having the rear tires off the ground)

The way I was looking at it, the arms on the WD system place a torque hinged at the ball that is trying to bend the receiver and trailer downward (move pivot upward), this places a downward force on the trailer. Once the arms are locked, the system is rigid in the static case. So I ran calcs as if we were already rigid and solved for the force that needed to be applied to/by the trailer to shift the force distribution on the front and rear axes to be the same.
This Calc is resulting front = resulting rear and the rear is calculated as a force at the rear axle position produced by a lever the distance from front to trailer and the front as fulcrum. Total resulting force total stays at 500 pounds.

Then I applied the trailer force to the adapter connection with the adapter+trailer length as the distance.

 

[ohmman]

It is sort of a reverse cantilever. The WD shifts the front/rear axle weights by pushing down on the trailer to unload the rear axle. The 210 pounds is the additional load on the trailer wheels and also the reciprocal ground induced upward force applied to the trailer axle.
(With the extreme case having the rear tires off the ground)

The way I was looking at it, the arms on the WD system place a torque hinged at the ball that is trying to bend the receiver and trailer downward (move pivot upward), this places a downward force on the trailer. Once the arms are locked, the system is rigid in the static case. So I ran calcs as if we were already rigid and solved for the force that needed to be applied to/by the trailer to shift the force distribution on the front and rear axes to be the same.
This Calc is resulting front = resulting rear and the rear is calculated as a force at the rear axle position produced by a lever the distance from front to trailer and the front as fulcrum. Total resulting force total stays at 500 pounds.

Then I applied the trailer force to the adapter connection with the adapter+trailer length as the distance.

I have to try to back out your method because it's not adding up to me. It has pointed out an error in my calculations (and yours), though. It'll be worthwhile to weigh each axle at some point out of curiosity, but the WD hitch is supposed to reload only the lost vehicle weight back to the front axle. In your calculations, that would be 235 lbs, not the full 380. The rear axle should still carry the tongue weight along with some reloading back to the trailer axle. I imagine this is the reason people say that WD hitches don't change the tongue weight requirements of a vehicle.

Thanks again.

 

[mongo]

I have to try to back out your method because it's not adding up to me. It has pointed out an error in my calculations (and yours), though. It'll be worthwhile to weigh each axle at some point out of curiosity, but the WD hitch is supposed to reload only the lost vehicle weight back to the front axle. In your calculations, that would be 235 lbs, not the full 380. The rear axle should still carry the tongue weight along with some reloading back to the trailer axle. I imagine this is the reason people say that WD hitches don't change the tongue weight requirements of a vehicle.

Thanks again.

I can rerun with a different weight distribution, but from reading the Reese setup guide, they are trying to get front and rear vehicle axle compressions identical (within1/2 inch)

I'm pretty sure the 3 axles need to add to the 500 lb tongue weight, since the WD system torque isn't creating mass. For the front axle to gain weight, the WD has to lever down on the trailer. This also unloads the rear axle.

 

[ohmman]

I can rerun with a different weight distribution, but from reading the Reese setup guide, they are trying to get front and rear vehicle axle compressions identical (within1/2 inch)

Collected from famed RV guy Les Adams:

Ford specs for 2011 and newer trucks say the w/d should be adjusted to eliminate about 50% of the front end RISE... IOW, if the front rises 1.5" initially, the RISE should be reduced to around 3/4"...
Ford specs for SUVs say the w/d should be adjusted to return the front bumper to within 1/2" of the unhitched height...
Equal-i-zer specs, beginning in 2010, say the w/d should be adjusted to eliminate at least 50% of the rise...
GMC specifies that 100% of the rise should be eliminated...
Curt Mfg says, "A weight distributing system is properly set up and coupled when the tow vehicle has settled with the front wheel opening at the original uncoupled dimension measured and slightly lower in the rear. ~ This will assure the front wheel load remains unchanged. This results in good handling and the desired load on the rear axle."...
Reese 2009 specs say the final front end height must be less than or equal to the unhitched height...
Eaz-lift, reported to have invented the w/d hitch in 1952, says, "Remeasure front and back of towing vehicle and have both settle the same amount or up to an inch lower in back by adjusting chain on spring bars."
From The Toyota manual..
"If using a weight distribution hitch when towing, return the front axle to the same weight as before the trailer connection.
If front axle weight cannot be measured directly, measure the front fender height above the front axle before connection. Adjust weight distribution hitch torque until front fender is returned to the same height as before connection.
Do not reduce front fender height below original measurement."

In almost all cases, the guidance appears to be to return the front end to no further than the unhitched height. My Equal-i-zer manual states (as he notes above) that I should measure the original front wheel well clearance, hitch up the trailer, measure the new clearance, divide the difference by two, and adjust to that difference.

Once I get my new receiver bar on (maybe today), I will have to readjust my WD setup for the different offsets. Hopefully that'll give me a better feel, and then we can revisit your calculations.

 

[bikeandsail]

Ohmman,

Don't you have to disable the air suspension for the your method to work?

 

[mongo]

Collected from famed RV guy Les Adams:

In almost all cases, the guidance appears to be to return the front end to no further than the unhitched height. My Equal-i-zer manual states (as he notes above) that I should measure the original front wheel well clearance, hitch up the trailer, measure the new clearance, divide the difference by two, and adjust to that difference.

Once I get my new receiver bar on (maybe today), I will have to readjust my WD setup for the different offsets. Hopefully that'll give me a better feel, and then we can revisit your calculations.

Ok, I'd started with 50/50 to match your force diagram, but 0 net in the front prevents exceeding the front axle weight rating.

 

[ohmman]

Ohmman,

Don't you have to disable the air suspension for the your method to work?

Yes. Jack mode disables auto leveling.

 

[mongo]

Ohmman,

Don't you have to disable the air suspension for the your method to work?

I was thinking through this too. With the WD system, the vehicle and trailer almost become one structure (slight compliance at hitch ball). The air suspension tries to hit a specific height, but can't normally change loading. With the WD, an increase in either axle height also raises the other height and changes the axle loading.

Due to compliance, and the weight of an X, I think the suspension has enough force to work with to achieve its desired final height, but the more it adjusts, the further off the weight distribution will be. So it is important to be at the driving height before attaching/ adjusting the WD system.

 

[mongo]

Collected from famed RV guy Les Adams:

In almost all cases, the guidance appears to be to return the front end to no further than the unhitched height. My Equal-i-zer manual states (as he notes above) that I should measure the original front wheel well clearance, hitch up the trailer, measure the new clearance, divide the difference by two, and adjust to that difference.

Once I get my new receiver bar on (maybe today), I will have to readjust my WD setup for the different offsets. Hopefully that'll give me a better feel, and then we can revisit your calculations.

With a target of 0 for front axle I get:
A=0 (-235 + 235)
B = 370 (735 - 365)
C = 130 (0 + 130)
Torque at adapter = 1,836 lb-ft

With a target of half the non WD front load I get:
A= -117.5 (-235 + 117.5)
B = 552.4 (735 - 182.6)
C = 65.1 (0 + 65.1)
Torque at adapter = 921 lb-ft

and the linear system is scaling linearly, that's always nice

I think the net torque at the adapter connection is the trailer WD loading only, and does not get an offset due to the original tongue weight induced torque.
Once the arms are locked, the system is rigid and all the torque is applied to/at the connection. The tongue weight at the ball is no longer free moving, and any weight becomes a vertical vector at the adapter.
Mental picture: if the hitch ball were jacked up to the point where there was no torque on the adapter and the welded to the trailer, when the jack is removed, the adapter would have 0 torque, and the trailer would have 0 extra loading, but the adapter would be supporting the tongue load.

The other thing these calcs are showing me is the range of loading the adapter can go through. If the vehicle tilts relative to the trailer, it is the same as changing the preload (arm angle vs delta ground plane angle). So the adapter can be exposed to high loads depending on the arm spring rate and length.

 

[ohmman]

The other thing these calcs are showing me is the range of loading the adapter can go through. If the vehicle tilts relative to the trailer, it is the same as changing the preload (arm angle vs delta ground plane angle). So the adapter can be exposed to high loads depending on the arm spring rate and length.

Thanks again. Yes, I think the loading range is definitely eye opening. This is relieved to a great degree by using a weight bearing hitch since it can articulate around the ball. That said, the dynamics of the system still will result in a 500 lb tongue weight bouncing down and inducing higher torques than the 562.5 lb-ft number.

I believe this can explain why I approached a form of failure on my hitch adapter. I've already submitted to Tesla that they need to adjust guidance and recommend against WD hitches in the manual. I think others should consider the same caveat.

This begs the question of whether the 235 lb unloading at the front axle is enough on a 5200 lb vehicle to justify the weight distribution aspect of these hitches, disregarding anti-sway.

 

[mongo]

Thanks again. Yes, I think the loading range is definitely eye opening. This is relieved to a great degree by using a weight bearing hitch since it can articulate around the ball. That said, the dynamics of the system still will result in a 500 lb tongue weight bouncing down and inducing higher torques than the 562.5 lb-ft number.

I believe this can explain why I approached a form of failure on my hitch adapter. I've already submitted to Tesla that they need to adjust guidance and recommend against WD hitches in the manual. I think others should consider the same caveat.

This begs the question of whether the 235 lb unloading at the front axle is enough on a 5200 lb vehicle to justify the weight distribution aspect of these hitches, disregarding anti-sway.

Total agreement here.
Disregarding antisway as you mentioned, one could just add 235 pounds of luggage/ ballast to the frunk. (Or a tungsten California front license plate holder) Even a front passenger would go a long way to canceling out the effect. (Heck I'd probably cancel it all myself)

On the other hand, loads with a high CG produce a large downward moment at the hitch ball when decelerated. With the WD system, this effect is reduced by the the increased bending of the load arms. Again, more a use case for a one ton truck with heavy equipment, vs a camper.