Weight Distribution and Sway Control -Obsolete for a Self Propelled Trailers?

[turbopilot]

A provocative title for a provocative subject. Nothing gets the Airstream Forums more excited than another thread about Weight Distribution systems and Sway Control. Year after year we see these megathreads like the latest one, Swaying to the (music?). These threads always invite a variety of informed and not so informed opinion. Everyone has a favorite solution that they think will work but what drives all the opinions and controversy is that nothing scares a trailer owner more than a potential catastrophic trailer sway event.

What is seldom discussed is the idea that you can design a trailer that will be susceptible to sway and does not require special equipment to avoid a catastrophic sway event. That day may have arrived. We will soon find out.

A few weeks ago I was invited to take a LightShip AE.1 Cosmos on a combination four hour drive and overnight camping trip. I used my own 2023 F-150 PowerBoost pickup and NO WEIGHT DISTRIBUTION device. I “pulled” that LightShip through all highway conditions: windy mountain roads, stop and go city traffic and interstate highways. At all times I had a stiff cross wind out of the West primarily driving North and South along the front range of Colorado. The LightShip was absolutely stable behind the PowerBoost, without any WDH.

Here is a short time lapse video documenting part of my first "tow" of the LightShip AE.1 Cosmos.



My adventure started out with some worry. All of my trailer towing experience has been with my F-150 PowerBoost, my 25’ highly modified 7,300 lb Globetrotter and initially a Blue Ox “bar” WDH then followed by a B&W Continuum WDH. I have never experienced even a hint of trailer sway with either WDH. But those WDH’s are a PITA and heavy. When I pulled into the LightShip production facility to hook up the brand new LightShip production prototype I realized neither LightShip or I had a WDH. The LightShip weight is 1,000 lbs more that my Globetrotter. But they are about the same length. We considered how to do a test drive without a WDH but the LightShip folks said to give it a try without a WDH. The purpose of my trip was to test the LightShip, so off I went without a WDH.

Is it possible the LightShip does not need a WDH? I don’t think LightShip is comfortable making that claim but I experienced enough testing of the LightShip to suspect that such a device may not be necessary with this unique new “trailer”. It is easy to understand as you tease apart all the factors that contribute to trailer sway. It all boils down to three design elements associated with the LightShip that may make a WDH unnecessary. Those elements are:

1. Very low center of gravity
2. Very low aerodynamic profile
3. Self propelled with kinetic regenerative braking

Let’s tease out each of those attributes then analyze why they all contribute to a very stable trailer.

Very Low Center of Gravity - The LightShip has a very low center of gravity. In addition to the water, gray and black tanks hanging from the frame of the LightShip, there is also a 77 kWh LFP battery, electric motor, gear box and inverter ( weighing around 1,500 lbs) buried in the the frame. The LightShip HVAC system is not on the roof like a typical RV, it is located on tongue of the LightShip. So at least 300 lbs of equipment that typically rides 10’ to 11’ off the highway now rides low on the hitch just above the frame. This configuration has a significant impact on lowering the LightShip vertical center of gravity to a point very low just above the axles. The net impact of this is significantly lowering center of gravity that creates an inherent resistance to a rolling moment that is so important in a catastrophic sway event.

Very Low Aerodynamic Profile - The LightShip is configured to operate in two modes: camping mode and drive mode. The LightShip is designed as a “canopy and tub”. The canopy raises from the drive mode to the camping mode with four electric operated jack screws. In the camping mode the LightShip is 10’ 1” tall. In the drive mode the LightShip is 6’ 11” tall. The LightShip is 8’ 5” wide. The result of the drive mode conversion from an aerodynamic standpoint is that a flat plate equivalent to a 3’ 2” by 8’ 5’” (27 square feet) is not being dragged against the air in motion. Imagine the energy saved by not pushing air against a 27 sq ft flat plate. But there is more aerodynamic improvement as the entire LightShip in the drive mode is aerodynamically refined. There is no obstructions on the roof and the belly is flat with the axles recessed into the chassis not in the slip stream. One more very important aerodynamic trick. The most turbulent area to airflow (ie drag) is found in the area between the tow vehicle rear and the front of the trailer. LightShip reduced drag significantly by designing a stream lined AeroHub that reduces drag while providing a home for the HVAC system.

All of these aerodynamic improvements come into play with for sway control. The flat plate area along the sides of the trailer is significantly reduced. Compare this to a tall slab sided trailer and consider an incipient sway event. As a slab sided trailer begins to sway it presents one side to a high pressure push of air in the slip stream but at the same time it presents the opposite side of the slab trailer to a big low pressure area created by a lift effect. Think of a high slab sided trailer as an aircraft wing flying a 90 degrees to the surface into the wind. When the trailer begins to sway in one direction air pressure builds on one side of the trailer and rapidly declines on the opposite side of the trailer. All this results in the typical whip back to the opposite direction always overshooting a neutral position. This aerodynamic whipping is what ultimately leads to a catastrophic sway event. With the surface area is significantly reduced combined with aerodynamically rounded edges, the Lightship presents a much smaller surface area to the prevailing wind, potentially significantly reducing this lift induced oscillation.

Self Propelled with Kinetic Regenerative Braking - The most significant deterrence to the onset of a sway event is driver initiated trailer braking (trailer only, not tow vehicle). It is common knowledge that nothing aborts an incipient sway event like applying trailer brakes only when sway is detected together with acceleration of the tow vehicle, if conditions allow. The absolutely worst thing you can do is apply tow vehicle brakes at the beginning of a sway event. Tow vehicle braking as sway starts will increase the oscillation of an incipient sway event.

Self propelled trailers using electric motors also incorporate a kinetic regeneration braking mode. When a self powered trailer like the LightShip senses the tow vehicle slowing down the force sensing hitch will automatically switch the electric motor to a generator. In generator mode the electric motor will start charging the battery but more importantly for sway control the electric motor will exert a braking effect for only the trailer, not the tow vehicle. This is exactly the reaction you get by aborting an incipient sway event by manually activating the trailer friction brakes alone.

Here is an example about how kinetic regenerative braking can abort an incipient sway event. Imagine you have just topped a hill with a trailer in tow on an interstate highway. You have a stiff crosswind and about the same time a large semi truck over takes you in the left lane while passing under an overpass. The hill is steep so you initiate tow vehicle braking just as the semi passes you going under the overpass that was partially blocking the wind. All at once the trailer is buffeted by a strong cross wind and a reverse cross wind induced by the semi truck. Your instinct is to apply more tow vehicle braking as the a sway event begins. That is a bad decision. See this video.

Here is a video I borrowed from YouTube then slowed it down with annotations of the various activities leading up to a catastrophic sway event.



Now replay this event with a low profile aerodynamic trailer with dynamic automatic regenerative braking. As the tow vehicle and LightShip enter the same scenario described above there are two big differences. Very reduced aerodynamic surfaces to the prevailing wind but just as important as soon as the LightShip tops that hill it will sense automatically that it should convert to kinetic regenerative braking. That means the LightShip will be initiating trailer braking way before the sway event would have initiated in the scenario described above. In essence the LightShip has a very effective automatic anti sway system that just happens as part of the energy conservation system associated with an EV.

In my four hours “towing” the LightShip with my F-150 the trailer was absolutely stable. The current LightShip firmware will currently support trailer braking using either the electric friction brakes or kinetic regenerative braking. LightShip is targeting to develop a blended braking mode where both regenerative and friction braking can be used at the same time. Right now as soon as the tow vehicle brakes are applied the 12 volt trailer brakes will engage while simultaneously disengaging kinetic regenerative braking. I suspect the automatic onset of kinetic regenerative braking going down hill is the most important consideration to abort an incipient sway event.

So what does this all mean? My initial driving experience with the LightShip suggests to me that it is very possible the LightShip does not need a supplemental sway control device. But what about weight distribution?

This is where it gets interesting. The LightShip has a tongue weight of 820 lbs. The LightShip GVWR is 8,300 lbs. That means the LightShip in tow places just 10% of the trailer GVWR on the hitch of the tow vehicle. Even with my “heavy” F-150 Platinum PowerBoost the addition 820 lbs on the hitch does not exceed the GVWR of 7,350 lbs for two people towing the LightShip. But what about returning the front axle load restoration to the Ford recommended 50% if you don’t have a WDH? Remember the goal of the LightShip TrekDrive self propelled motor is to place virtually no load on the ball by using computer control via the hitch mounted force sensors.

Ford says that you should use a WDH with a hitch weight above 500 lbs and/or a trailer that weighs 5,000 lbs GVWR. On the other hand, I can load up the bed of my F-150 with cargo to the GVWR without a trailer and Ford does not require me to somehow restore weight to the front axle to achieve the FALR recommendation. So the truck has obviously been tested to operate properly with less weight on the front axles up to GVWR, not towing a trailer.

So the suggestion to use a WDH for a light tongue weight LightShip is an open question in my mind, if the trailer is self powered. Obviously a powered trailer is a new type of vehicle. It was just invented. So there is no chance for any SAE guidelines concerning tongue weight and WDH to be developed without a test vehicle.

Based on my first experience I think the advent of aerodynamic, powered trailers will require rewriting the “rules of the road” for safe operation for this new category of RV. It is possible the new guidelines for self propelled trailers will not require weight distribution and sway control.

There is only one other self propelled trailer coming to the market, the Pebble Flow. There are some important differences between the two. The Pebble Flow has no road mode, thus it presents a significantly increased flat plate surface area to the prevailing wind and has a higher center of gravity. It is likely the Pebble Flow will need some kind of augmented device for sway control. But the arguments in favor of specific WDH should be the same.

 

[Kevin D Pem]

First I must thank the moderators for deleting my first response on this, Truly my anger came through in my post. So Now with clearer head I will start again.

Realizing that our 3D Matrix has rules defined in physics. I will try to stick with the facts.

As a one time design house owner, I look at many things differently than most. First and foremost I look at legal issues. This is what I see in this case study.

1. A fulcrum, is a fulcrum, no matter what weight you lift with it, or how the device was designed. The size of the device is only important when considering the percentages. No one knows this better than auto manufacturers. this is why tow weights are established on hitches installed at the factory. they figure in a fudge factor because humans are, well human! I believe that of all the manufacturers, Ford has the lowest fudge factor. At the end of the fulcrum are the front wheels. The ratio is set in stone, and reflects the weight the hitch can handle before under steer becomes a possibility.

That label that tells you what you can tow, and what tong weight your vehicle can accommodate is not just an information label, it is a manufacturer disclaimer first. The law will let you do what you want to. But if your negligence results in harm, or death, the courts will not be on your side. Your insurance company can refuse your claims, and any company that is party to the negligence, can bare the financial burden for their part. For a what? crowd funded or startup, knowingly allowing an ill equipped setup out of it's lot puts the whole operation at risk. So I ask. have you invested in this management?

2. Hi-lows have increased weight over standard! why? because both halves must be stronger to handle the function. yes they have an advantage in reduced frontal area reducing the power to move it through the air. But that is where it ends. The complexity involved is enormous! linear slides wear more, and faster than radial bearing surfaces. Now armed with more information, you figure it out!

3. aerodynamics! according to my calculations a 6 to 7 deg angle per side, produces the least drag, therefore the least resistance, at highway speeds. This lesser drag presents a problem. Look at the rig as a control paddle. The surface presented to the wind, be it frontal motion or side, this surface acts with these forces, some to the betterment of control, some to the worse. to fully comprehend what I am talking about, ex-sailors have an advantage. I am talking about wind vectors, and how the side wind reacts to forward motion. Just remember that flat is not always a bad thing! And any reduction in frontal area is good!

4. Center of gravity! No question center of gravity will increase the possibility of a roll over, however if you're taking corners fast enough to have this issue, you really need to slow down. At the same time if you are in a jackknife scenario, gravity, in any form high or low, will have a lousy outcome.

5. Tail wagging the dog! weight distribution is the key to safe towing. pull trailers need 10 to 15% tong weight. This is a measure to overcome the pendulum effect. This alternative drive system if sensitive enough may overcome much of this effect, however a glitch is always a possibility, so I wouldn't want to rely on it. All trailer manufactures make sure they print the disclosure about distributing weight properly someplace to lessen their chance of liability! They don't need to explain to you why that number is important, they just need to tell you how to load the toys. It's a legal issue!!!

In closing! This tread will go down in history! Yes, anyone doing a search will find it. It could lead to an unexpected circumstance. Not only for Lightship, but for this forum, the poster or anyone in the way of a grieving family member.

Morally I needed to point out what is needed to put this thread in perspective! Hope I didn't offend anyone with my comment.

 

[redbaron]

@turbopilot have you any information on what a “dead” trailer would act like while towing?

I ask as I am sure this will be part of the testing and has been planned for.

I would expect that a tow vehicle would be matched to the trailer in its battery off mode or as I called it “dead” mode.

Going with that line of thought, I would expect that certain vehicles would handle better with a WDH.

Knowing that the trailer handles well without one makes sense, given the fact that it is very close to a self propelled unit on its own.

 

[turbopilot]

First I must thank the moderators for deleting my first response on this, Truly my anger came through in my post. So Now with clearer head I will start again.

Good idea and a much better approach to thoughtful discussion.

The LightShip is a new category of RV. The product has not shipped. I am the first customer allowed to "pull" the LightShip and actually spend a night camping in it. I was given the opportunity to "pull" (really not the right term for a powered trailer) a LightShip prototype. Since the product is still in testing there was a test engineer in the back seat on my trip watching trailer dynamics with a "kill switch" should there be any unusual deviations. I actually signed a release in order to test the prototype with my own F-150 HEV PowerBoost. This was the first time an HEV had been used to test the LightShip.

What I learned is that the handling qualities of the LightShip in this configuration is much different than my experience towing a similar sized Airstream trailer. This is true when the TrekDrive system is off but much more evident when the TrekDrive system is active. I also learned that the dynamics of an HEV tow vehicle interacts with the inputs of the TrekDrive trailer system in different ways depending on whether the F-150 PowerBoost was in NORMAL or TOW/HAUL mode.

But given all of the variations, it was very clear to me that the LightShip in DRIVE MODE is much more stable riding in back of my F-150 in any high profile trailer I have pulled.

I made no attempt during my test drives to induce a sway event. I did test the stability of the LightShip moving at high speeds on the interstate highways to steering wheel and wind gust input. The LightShip was rock solid without a WDH. I experienced large semi trucks passing me at high speed with strong cross wind components. The LightShip was steady with little or no "semi push" typically felt by high profile trailers.

LightShip is doing extensive testing of this new RV vehicle class at their San Francisco test facility and the Denver production facility. Three vehicles are in test soon to be joined by two more production intent vehicles in the coming weeks.

LightShip in the FAQ's is recommending the use of a WDH. They are still testing various WDH's. They have found some WDH's can interfere with the TrekDrive sensor module on the hitch by transferring loads around the sensor. So the final conclusions on what WDH may be needed is still to be determined.

Bottom line, LightShip is a new RV type that is currently in testing. I reported my impressions.

 

[turbopilot]

@turbopilot have you any information on what a “dead” trailer would act like while towing?

I ask as I am sure this will be part of the testing and has been planned for.

I would expect that a tow vehicle would be matched to the trailer in its battery off mode or as I called it “dead” mode.

Going with that line of thought, I would expect that certain vehicles would handle better with a WDH.

Knowing that the trailer handles well without one makes sense, given the fact that it is very close to a self propelled unit on its own.

In LightShip parlance this would be "pulling" the trailer with TrekDrive off. The majority of my test drive was actually in that mode. In my test I did not notice any difference in stability with TrekDrive ON or TrekDrive OFF but I did notice very different interactions between the truck and trailer with TrekDrive ON and OFF.

I was very interested in the TV/LightShip performance with TrekDrive ON or OFF. Here is why.

The drag profile of the LightShip in the wake of a pickup tow vehicle is very similar to a Tesla Model 3. That is because the canopy retracts to 6'11" in Drive Mode significantly reducing the flat plate area of the trailer to the wind. The LightShip rides in the wake of the F-150.

Pulling my high profile Airstream I will typically see around 11.5 MPG at 62 MPH. Pulling the LightShip with the same truck with TrekDrive OFF I was seeing about 15.5 MPG at 62 MPH. "Pulling" the LightShip with TrekDrive ON I was seeing 22 MPG with the PowerBoost. The PowerBoost was achieving similar fuel efficiencies "pulling" the LightShip with TrekDrive ON as it experiences not pulling a trailer.