My fellow Autopians. Welcome to another edition of Ask An Engineer. This time we are going to look into a question sent to me by one of you astute readers relating to towing a trailer. The question is: Why do some trailers sway back and forth as they are being towed while most others track straight behind their tow vehicle? A lot has been written and many videos have been posted online trying to explain this phenomenon, but in my opinion, none really explain it fully. Let’s see if we can’t put it all together and get to the bottom of it.
Most of the time, when you see someone towing a trailer down the road, everything looks fine. The tow vehicle and the trailer are driving nicely straight down the road. But every now and then, something goes horribly wrong, with the end result all too often being a trailer and tow vehicle laying on its side, or worse.
What’s happening here is called trailer sway, and it can be triggered by many different things. A cross wind can start it, a bump in the road can start it — even just passing a truck can set it off. Anything that causes the trailer to be temporarily pushed to one side can start the sway motion and then the dynamics of the particular trailer and tow vehicle take over from there.
I personally experienced this about 10 years ago when I was moving a lot of heavy tools in a rental trailer. While driving on the highway on a long downhill, the trailer started to sway back and forth. It got progressively worse, and fortunately, on this particular stretch of road, the left side emergency lane was exceptionally wide so I had plenty of room to allow the trailer to move back and forth without hitting other traffic.
My only concern then was to keep it from jack-knifing and causing a massive wreck. Let’s just say that my arms got a work-out counter-steering with each sway of the trailer. As it turned out, I made it to the bottom of the hill, and once the road flattened out again, the sway stopped, and I was able to keep going. Let me tell you: It was one of the most frightening moments I have ever had on the road.
So, what was it that caused the trailer to not only sway but also get progressively worse? The answer is all about the trailer weight and the weight distribution. This video does a great job of explaining how the way the weight of a trailer is distributed affects its stability:
Here is another one that comes to the same conclusion:
The problem with both these videos is that neither explains what is really going on. Why does a trailer sway when too much weight is located towards the rear? Why does having weight pushed forward keep the trailer from swaying? And why does keeping the weight close together help?
To understand the answers to these questions we need to look at the configuration of an imaginary trailer, and do a little math. Let’s look at a tow vehicle and trailer from above. I have included a box representing the load the trailer is carrying and a symbol showing where the center of gravity of the load and trailer combination would be.
Centered Load
In the first case, we’ve loaded the trailer in such a way that the center of gravity is directly over the axle:
Let’s say we are pulling this trailer down the road, and it encounters a cross wind or a bump in the road that shoves the trailer to the left. The situation would look like this:
The shove that the trailer got has caused it to rotate to the left. Once the trailer is moving to the left, it has inertia that wants to keep it moving left. At the same time, the rotation means the tires are no longer facing in the direction of travel and they are generating a side force in the direction opposite to the rotation. The force picture looks like this, with the inertia force and the tire forces inline with each other:
Since the lateral inertia and tire forces are inline, they will partially cancel each other out, depending on how large each one is.
Let’s look at the sequence of events that occur when a trailer starts to sway. At first, the trailer is traveling straight down the road. The lateral inertia force is zero and the tire reaction forces are also zero since the trailer is not yet moving sideways.
Once some force, like a cross wind, is encountered, the trailer moves to the side. With this lateral movement, the inertial force is no longer zero, but as the trailer moves sideways, the tire forces start to grow. At some point, the angle of the trailer will be enough that the tire forces become equal to and then exceed the inertia force, which is the point at which the trailer starts to swing in the opposite direction. Once it starts moving in the opposite direction, the inertia force grows and once the trailer moves past the straight ahead position and swings to the right, tire forces start to build in the opposite direction until they are high enough to overcome the inertia force and the whole cycle repeats itself. The trailer will act very much like a pendulum and just swing back and forth.
Rear Load
But what would happen if we moved that big box to the rear of the trailer?
Looking at this from the side, we can see that the location of the weight means the trailer will be pulling up on the hitch, giving us a negative tongue load:
Let’s start the trailer sway the same way we did before:
Looking at the inertia and tire forces under this condition we can see that they are no longer inline with each other but are offset:
This offset means the two forces act together to create a moment, or torque:
Notice that this torque and the rotation of the trailer are both in the same direction. The torque created by having the center of gravity behind the axle is in the same direction as the rotation of the trailer and helps to increase, or exaggerate, the motion of the trailer. Now, in order for the tire force to stop the rotation of the trailer, it not only has to overcome the lateral inertia force, but also the moment created by the force offset, which means the angle of the trailer has to be greater before the tire force is large enough to overcome both.
Front Load
Now let’s look at what happens when we move the box forward in the trailer:
Looking at it from the side, we can see that moving the weight forward causes a downward force on the hitch:
In this case, if we start our sway the same was as before, we can see the lateral inertia force and the tire forces are again not inline with each other, but now they create a moment in the opposite direction as the motion of the trailer:
With the moment in the opposite direction to the motion of the trailer, it has the effect of counteracting the sway and stopping or reducing it once it gets started. This is why you should always have a downward load on the hitch of a trailer. The downward load on the hitch means the center of gravity of the trailer and load combination is forward of the axle which will help to create a moment that stops or reduces trailer sway. Of course, the farther forward the load is placed, the bigger the moment will be and the more stability the trailer will have.
But that’s not what the videos above say. They say you need to centralize the load as much as possible. Why? The reason is that you not only want a downward tongue load, you also want the right amount of tongue load.
The Right Tongue Load
The rule of thumb for trailer tongue load seems to be that you want between 10% and 15% of the trailers weight pushing down on the hitch. Less than that, and you run the risk that the moment created by the inertial force and the tire forces will not be enough to stop sway, and too much puts a strain on the towing vehicle.
Since standard trailers are towed with a hitch attached at or near the bumper of the towing vehicle, the downward load on the hitch pushes the back of the vehicle down and lifts the front. Too much downward force on the hitch can create excessive lift at the front. Let’s suppose we designed our trailer to have the axle all the way at the back. That way, no matter how we load the trailer, the center of gravity will always be ahead of the axle:
You can imagine what would happen to the steering and controllability of the tow vehicle in a situation like this (there’s so little traction up front, your steering inputs won’t do much to actually turn the vehicle). There are of course ways of dealing with this, such as using a weight distribution hitch, but those can only do so much. In addition to the issue of front end lift, the tow vehicle also has to be designed to handle all that extra load. Most vehicles are only designed to handle the usual 10-15% tongue load and any more would put an undue strain on the structure and could cause damage to the vehicle.
Here is an example showing how trailer tow affects vehicle loading. We’ll use a 2023 Lexus RX 350 as our example vehicle. This car can tow a trailer up to 3500 lbs. and has a max tongue load of 350 lbs. which is exactly 10% of the max trailer weight. The maximum allowable weight on the rear axle (RGAWR) is 3260 lbs. The curb weight is 4155 lbs. and I will assume the weight distribution for the FWD version has 60% weight on the front axle. The front and rear axle weights are then:
If we now add a 3500 lbs. trailer with 10% tongue weight, we will get:
The rear axle weight is still well below the 3,260 lbs. maximum, but keep in mind that these numbers do not include any occupants or luggage. You can see how even a 350 lbs tongue load adds significantly to the weight on the rear axle and makes the front axle lighter.
If on the other hand we used our trailer with the axle way at the back, then the load situation might look like this:
Now, however, we have exceeded the maximum allowable rear axle load and we have reduced the front axle load by over 700 lbs. While this trailer configuration may be more stable and resistant to sway, we have put an enormous strain on our tow vehicle, and it is completely unnecessary. 10-15% tongue load is enough to prevent sway and anything more just adds extra load to our vehicle.
Load Distribution On The Trailer
Now that we understand the need for a downward load on the hitch, we need to understand the impact of how the load in the trailer is distributed. So far, we’ve looked at a load concentrated in one place. But what if the load was actually two large boxes and we placed them apart like this:
The overall center of gravity is still in front of the axle so we shouldn’t have any sway issues, right? But what would be the effect of placing part of the weight forward and the other part rearward? To understand this, we need to introduce a concept we haven’t talked about yet, which is rotational inertia.
Rotational Inertia
When a trailer is swaying, it isn’t just moving side to side, it is also rotating. An object that is rotating has, like all other objects that are in motion, an inertia that wants to keep it moving. Isaac Newton’s first law of motion states that an object in motion remains in motion at constant speed unless acted upon by some external force. And that motion can be either in a straight line, like the truck going down the road, or rotational like the trailer swaying. In addition, an object that is in motion has a certain amount of energy, called kinetic energy which is related to the speed at which it is traveling and its mass. For an object moving in a straight line, this is equal to:
E = ½ x Mass x Velocity²
For an object that is spinning, however, it is a little more complicated. The energy of a spinning object is:
E = ½ x I x Omega²
Where Omega is the speed at which the object is spinning and I is the moment of inertia of the object. The moment of inertia is a characteristic of any object and depends on its shape, its mass, and how that mass is distributed.
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A perfect example of this principle at work is a flywheel. A flywheel is meant to store rotational energy so that it can be used to power some machine, or to fill in the power gaps between cylinder firings of an engine. All ICE engines have a flywheel of some sort but a classic example are the flywheels used on steam engines.
Notice how this flywheel is shaped. It has a large heavy ring supported by relatively thin spokes. Most of its mass is concentrated as far from the pivot point as possible.
To better illustrate my point, I have created two rotating objects of equal mass but different shapes to see how their rotational energies compare. The first is a simple cylinder and the second is a flywheel like the one in the picture above. They both weigh exactly the same.
First we will look at a round cylinder weighing 0.210 Kg. It’s diameter is 41 mm:
The formula for moment of inertia here is:
I = ½ x Mass x (Inner Radius + Outer Radius)
For this object, it would be ½ x 0.210 x (0 + 41/2) = 2.1525. Don’t worry about what this number actually means, we just need it to compare with the flywheel.
Now let’s look at the flywheel:
In this case, we have an outer diameter of 80 mm and an inner diameter of 77 mm so the moment of inertia is now:
I = ½ x 0.210 x (77 + 80) = 16.485. Notice that even though the weight of both objects is the same, the moment of inertia for the flywheel is almost 8 times bigger simply by virtue of its shape.
Putting these numbers back into our formula for rotational energy, we can see that for any rotational speed, the flywheel will have a much higher rotational energy than the cylinder.
The same is true for a trailer. If we divide the load between the front and rear of the trailer, it will act much more like the flywheel than if we concentrate the load towards the middle. And, like the flywheel, once a trailer loaded like this gets spinning in a sway event, it will take a lot more energy from the tires to stop it.
Stopping Sway
So far, we’ve talked about the causes of trailer sway and how you can prevent it with proper loading. But what happens if you make a mistake and suddenly you are faced with a swaying trailer, like I was all those years ago. In that situation you can’t very well get out and repack your trailer. You have to deal with the event right then and there.
Looking at a trailer that is swaying, you can see that it is following a sinusoidal path while the tow vehicle is essentially going in a straight line:
You can see that the path the trailer is following is longer than the tow vehicle path. But since both arrive at the end of the path at the same time, it means the trailer must have been going faster than the tow vehicle. And that is the crux of the problem. The trailer is traveling faster than the tow vehicle. It is trying to overtake the tow vehicle, but because it is attached at the hitch, it can’t. All it can do it travel back and forth to “use up” its speed. What we need to do is bring the trailer and tow vehicle speeds back in sync with each other.
There are two ways we can do this: we can speed the tow vehicle up, or we can slow the trailer down. Both will work, but only one is desirable. If we speed up, we can bring the tow vehicle and trailer speeds together again, but if you’re on a downhill road, like I was, the trailer just wants to go faster and faster and you will end up going way too fast in the end. Also, everything just happens quicker at higher speeds. The sway happens quicker so the rotational inertia of the trailer gets higher and higher and eventually you still end up upside down on the side of the road. There is also very little time available to react when a sway condition starts before catastrophe sets in and that time is not enough for meaningfully speed up the tow vehicle.
Slowing down
The only really viable option is to slow the trailer down, and this is where trailer brakes come in. If you have a tow vehicle with a trailer brake controller, you can quickly activate the trailer brakes independently of the tow vehicle and this will snap the trailer back into line. Of course, not all tow vehicles have a trailer brake controller and most rental trailers don’t have externally controllable brakes anyway so in those cases, sway may not be stoppable. All you can do is try your best to slow everything down by lightly tapping the brakes.
All this means it is critical that you load your trailers properly and make sure there is a downward force on the hitch when you’re done.
Anyway, that’s a bit of the physics behind why some trailers sway and some don’t. If you have any interesting vehicle-dynamics questions, please email me at askanengineer@theautopian.com!
Top graphic image credit: New Jersey 101.5/YouTube
Amazing. I’ve never towed anything but now I know how in case the need arises. Thanks Huibert!!
I worked as an R&D engineer for nearly a decade at a premier weight distribution hitch company. I appreciate this article as it includes more information as to why things are the way they are. Adding in a weight distribution hitch does add a bit more math to it, but many (at least the quality ones) weight distribution hitches include sway control features as well.
This is awesome.Thankyou Huibert and The Autopian!You answered several things i never fully understood.
I’m happy to say our country has good trailer design rules which pretty much eliminate sway issues. That said we have one bizarre flaw.
For some reason we’re allowed three different types of braking systems yet only one really works safely.Kinda bizarre that
Regarding the trailer’s moment of inertia I’ve found that spreading the mass out is more stable. The higher moment of inertia makes it less likely that the trailer will start to rotate around its axle, both in sway and in up and down oscillations, that porpoising of the towing vehicle, which is another thing that causes instability and control issues as the loading on the steer axle changes when there is not a weight distributing hitch.
During my Vehicle Stability class at UC Davis, we rode a bicycle towing a trailer that had a weight that could be moved fore and aft. We got to experience how trailer loading and CG location affected the towing vehicle. Those affects are heightened due to the immediacy of riding the bike. Great times!
This is awesome, thanks. Question: What is inertial force? I’m accustomed to inertia being the thing upon which forces act, but in a different category, F=Ma and all that. I’m sure you’re using it correctly, as you’re a formalist, but can I have a technical definition? Something that will allow me to identify some quantity of inertia as equivalent to a force in a particular reference frame or something?
I probably should have used a different term but I thought it would be the easiest to understand. It’s really the momentum of the moving mass.
A quick discussion of sway controllers might have embellished the “how to correct it” part. They are similar visually to a shock absorber mounted between the hitch and trailer, converting sway into heat via friction.
Overkill for most properly configured trailers, but very beneficial when sway is expected, such as trailers that outweigh the tow vehicle.
That seems rather unnecessary, ever. Even when towing trailers that outweigh the tow vehicle considerably I have never ever had sway issues.
When I say outweigh considerably I mean 6500lb trailer behind a pickup that’s maybe 4000lb.
I’ve towed a 6500lb trailer with a 5000lb truck and a load distributing hitch. at moderate speeds it tracked fine and never swayed. It helps that my truck has a long wheelbase and and I tow at 60mph, not 75 like the brodozer crew.
Yeah I don’t even have a weight distributing hitch or long wheelbase on my side.
Also it’s not like 6500lb is a crazy amount of weight, that’s a Honda Accord on a trailer.
It really depends on a lot of factors. A flat bed with low center of gravity will be much more stable than an ultra light (for it’s size) travel trailer that is like a billboard for cross winds. I was an R&D engineer for a hitch company for nearly a decade. We had to purposely load our flat beds with very low or negative tongue weight to induce sway on our testing. With our travel trailer, it was actually pretty easy to make that thing sway.
Personally, I’m a fan of the “belt and suspenders” approach where practical. Load and size things so that stability’s OK, then add the friction sway control for good measure so you’ve got extra margin if you get hit with a surprise gusty crosswind while cresting a hill and passing a semi.
I’m curious about the impact of weight in the rear of the tow vehicle, both on vehicle/trailer system moment of inertia, and as it impacts the unweighting of the front end. While I’m usually pretty good about balancing out my camper, I’ve noticed some ‘tail wagging the dog’ when the back of my truck is loaded with a generator, firewood, extra water, etc.
Depends on where the weight is. Try to move the heaviest items up against the cab, that should promote stability.
Great write-up, and it really helps me understand the physics without getting too math-y.
Doesn’t help me at all, however, with backing the damn things up, which I still struggle with.
I swear my dad could back a trailer at speed through an autocross course. He makes it look easy enough that it convinces anyone they can do it, including me, but it doesn’t work that way. Instead I have to go slow, be methodical, try and picture it all out in my head, then pull forward and try again anyhow.
Your dad could be winning money with his skills!
https://www.youtube.com/watch?v=dbR5-g2NL0M
https://www.youtube.com/watch?v=MN_AxxHFRU0
I know! If they ever hold one of these events in Phoenix he’d be a shoe-in!
The one weird old trick that helps with backing a trailer is put your hand at 6 o’clock on the steering wheel. Then moving your in the direction you want the trailer to go produces the necessary counter steering. Also longer trailers are easier to back up than short single axle trailers.
Back when I worked at a rental yard I could turn off the main road and set up so I could back a brush chipper into the gate right in front of the gas pump to refuel.
I was taught this same hand at 6 o’clock trick by a fire fighter friend who steered the back of the ladder truck. Works great.
You should include a graphic or explanation on the implications of towing from a lifted truck, without an adjustable hitch. How does raising the hitch until the trailer is doing the Carolina Squat impact tongue weight and stability. I’ve seen some ridiculous trucks towing ridiculously unbalanced trailers… of course doing 90+.
It doesn’t really. Tongue height does not affect tongue weight much and doesn’t really change the loading on the tires or the stability of the trailer.
Trailer height looks like it should be a big deal but it kind of just isn’t. If the trailer is still loaded correctly you shouldn’t expect any issues from towing it nose up.
Well that’s just not right. The spectrum range of tongue height goes from 100% truck load (dangling trailer) to 0% truck load (hitch set below ground).
An example of what the original poster was asking about: I had the misfortune to be on the interstate with someone with a hitch so high the front axle of a tandem trailer was no longer consistently making contact with the road. If you want to tell me that it repeatedly having that axle go in and out of bearing a load instead of being a load didn’t change the forces discussed here, I weep for any institutions that certified your education.
All of this is within reason. If the trailer is really really nose up you may have to load a little further forwards to maintain 10-15% tongue weight, but that’s not a big deal.
Now if the tandems are coming off the ground, that trailer had to have been like 45° in the air. The tandems have a leveling link built in that will keep roughly equal weight on both axles within a rather wide range of angles.
It is worth noting that if the trailer axle was going in and out of contact with to road, it was fluctuating between having 0lb load and maybe 100lb load since it was barely touching down.
I don’t think I’ve ever seen a trailer being towed nose up enough to be a problem. Much more commonly is people driving around with 16″ drop hitches displaying a gleaming ball that’s never touched a trailer.
It depends on the trailer as well. A 2 or 3 axle trailer will totally change loading. If the hitch is high the rear trailer axle load goes up and the tongue weight does as well. If the hitch is low, the front axle is loaded (possibly over loaded) and the effective tongue weight goes down. Always best to tow level.
Tongue weight goes down the more you angle the trailer up, or increases the more you angle it down.
I’ve done it with 8 individual scales, one under each tow vehicle tire and each trailer tire, and with 5 scales one under each trailer tire and one under the trailer tongue. Every time I’ve done it, tongue weight decreases as the front of the trailer angles down.
The statics equations say the same. An easy way to think of it is the tongue high scenario. Remove the front trailer axle, but keep the rear in the same location. Now the weight that was on the front axle gets split between the rear axle and trailer tongue (because of the distance to the tongue vs distance to the rear axle, the rear axle will take much more of the load than the tongue will take).
I’d expect it to have at least some noticeable effect on the dynamic weight distribution. If the trailer’s nose-high, the tongue weight will drop under braking. Probably OK if the trailer brakes are adjusted right, but not good if there’s anything wrong with them (or the trailer tires are a lot less sticky than the tow vehicle’s)
Worst sway I’ve experienced was my first tow using new to me trailer to move a car. I had picked up a dakota truck and been on back roads for a while only getting up to about 40 mph. Once we hit the freeway and got up to 65, it got real scary, real sketchy, real quick. I could keep it under control up to about 55, so we slow rolled to the next exit and got off. the transmission was in the bed, as well as the bumpers and few other heavy things. We relocated what we could toward the front of the trailer some, ended up putting the transmission in the tongue storage to get the weight in the right places to tow safe. Freaking terrifying!
Great write up.
I’ve pulled a few trailers over the years and learned, thankfully without any incident, how important tongue weight it. While pulling my little Subaru and motorcycle while moving I had the car too far forward. While taking slower back roads, no problem but as soon as I hit a 55+ it got squirrelly real quick.
Also an item that should have been mentioned is weight distribution
hitches, especially ones with sway control. Pulling our 28′ camper with my Tacoma and weight distribution hitch with friction bars, it makes a massive difference in control. You’ll get pushed around some, the truck and trailer will stay in line really well.
I’ve tried to understand weight distribution hitches at least three separate times and each time I think I understand, I end up forgetting and have to look it up again. It’s just outside of being easily comprehensible and an Autopian primer would be welcome.
Yeah, they certainly don’t always make sense, especially the really exotic types like B&W, ProPride, or Andersons. Those really take a minute to figure out.
Also having to explain to someone that the hitch helps move weight back to the front wheels while ending up leveling the tow vehicle. This is not the same as leveling the vehicle with air bags, you still will have less weight on the front wheels.
Fastway trailer has some good videos on the topic.
Excellent summary! I understood the individual physics concepts but could not have combined them into such a concise explanation.
Well done.
I’m surprised you didn’t get into pendulum oscillations and K constants. Math is fun.
Neat article, Huibert. Thanks!
Is an empty cargo trailer prone to more or less sway? Does over or under tire pressure on the trailer impact sway?
If you want to see the havoc wind can have on a trailer, drive I-80 in Wyoming.
Empty trailers are designed to have 10-15% of their weight on the hitch so sway should not be a problem.
Tire pressure will certainly effect sway since an underinflated trailer tire will allow more slip angle in response to a side force so it will sway more easily.
Kind of a mixed bag, I’d say.
In windy conditions, empty could be worse because you have the same lateral force from wind but lower normal force between trailer tires and road, meaning less traction. But it also means if the trailer does start wobbling, the tow vehicle is less likely to be pulled around with it.
Been driving fire trucks and pulling 5th wheel trailers all over the country for 30 years. The amount of folks pulling trailers with no idea WTF they are doing is amazing. Tow limits mean nothing when it comes to what you can pull safely. It comes down to the vehicle payload because of what the tongue weight will be. You normally run out of vehicle payload way before you get near tow limit.
But instead the salesman gets them to buy a 8000lbs trailer because their vehicle can tow 8500. The tongue is going to be like 1000 and no way your SUV is going to handle that. Then add the 1000lb tongue, the driver, passengers, dog, cooler, etc; and that is way way way over vehicle payload and probably the rear axle. The throw in that the trailer is loaded incorrectly and it’s a total shit show pulling that thing.
There a lot of math and physics taking place pulling a trailer and the climb exponentially at speed. In this day and age where nobody wants it to be click and done, there’s not many of us actually doing it own homework when it comes to towing trailers safely.
I think there’s a great business opportunity for someone like you who’s extremely knowledgeable about towing. I bought a pop-up trailer that “only” weighs 2k lbs (against my SUV’s rated capacity of 3.5k lbs), but I’ve always suspected that after I load up the family, the 50-lb dog, the Yeti full of Mirror Pond etc, I’m probably exceeding either GVWR, rear axle limits, or both. But GOOD LUCK finding someone to evaluate all that for you. The dealer who installed the hitch…the shop that ran the power for the brake controller…even the local trailer dealer(!) were all competing to see who could be the least knowledgeable and least helpful.
When I put cross bars on the camper to carry bikes I opted for “just ahead of the axle” based on a bunch of blog posts and comments and questionable vids on sites like e-trailer and popup portal.
One time I stopped at an empty highway weigh station and asked the dude if I could put my fully loaded rig on his scale and he looked at me like I had three heads before shooing me off. Even for someone inclined to do it right, because of all the unique variables, it’s amazing the amount of pure guesswork involved in setting up your tow rig correctly.
Weird that the guy at the weigh station didn’t want to weigh you. If you’re really curious, it costs $14 to run your rig across the CAT scales at your local truck stop.
If all your axle weights are within reason and you have at least 10% tongue weight you’re fine. This doesn’t need to be as complicated as some people make it.
Half-ton trucks are so bad about this. I think it’s possible to buy an F150 with a 14,000lb tow rating and 1200lb payload. But of course the brochure says the payload’s 3200lbs.
Does anyone have any experience with wireless or inertia-based brake controllers? Scamp campers have a factory option to include Autobrake, which seems appealing for vehicles like mine that don’t come with a spot for brake controllers.
I have a Curt Echo wireless controller, and I think it’s a pretty decent item. I frequently tow various vehicles with various trailers, and wanted something that I could just take around with me and use on anything. It’s bluetooth, so, you need to be careful about connection. But. If it looses connection, it just sticks with the last input settings, so even if it looses connection, you still have trailer brakes – you just can’t manually apply brakes. And that’s only been an issue on some pickup trucks, busses, and UHauls. I wish they could make the design a little more compact, but, overall, I think it does nicely considering the available parameters.
Second on the Curt Echo. My tow vehicle also doesn’t have a built in controller and I find that this one works well. My gripes are the same as yours: relying on bluetooth tech for basic safety is a little sketch, and the module is so heavy i strap it to the vehicle because i’m afraid it might bounce out.
I have a curt echo for testing media vehicles. In my personal vehicle I always have a fixed trailer brake controller. Physical controllers are easier to control and modulate, especially higher end ones.
I had a Tekonsha Prodigy P2 on my Jeep and it worked fine. The Jeep didn’t come with a built-in brake controller plug, but it was fairly simple to tap into the trailer wiring harness. I had my local RV dealer do it because I had no experience with that sort of thing, but it was definitely DIY-able. The one problem I had was with the mounting location for the controller itself – I’m tall and they put it right where my knee needed to go. Fortunately I was able to relocate it without too much trouble.
How does a weight distribution hitch change the mechanics of this?
A WDH doesn’t change the tongue weight of the trailer on the hitch, but it does add a twisting force that pushes the front of the tow vehicle down, meaning there is more weight on the front axle (and less weight on the rear axle) than if the WDH was not used. By itself this doesn’t affect the trailer sway directly, but it does make the tow vehicle easier to control. Many WDH’s also have integral anti-sway bars, which add a lot of friction when the trailer moves side to side. This would add another red arrow to Huibert’s overhead diagrams, where the arrow would be parallel to the neck of the trailer but ~1’ offset. The arrow would point backward if it’s on the same side as the trailer is swaying, or forward if it’s on the opposite side as the trailer. Either way, it counters the direction of the sway moment.
I have rarely towed with trailer brakes and always just backed off the throttle when it starts to move. Whenever I have an issue I remember a Jackie Stewart driving course when he kept going smooooth ya gots to go smoooooth
This is one of the hundred reasons I will never own or tow a big camper. I have done it and will not do it again. There are just way too many people out there towing way too big of trailers and they are far outside of their knowledge and skill and are a danger to those on the highway with them.
You won’t tow a trailer because other people don’t know enough?
Because I know my limitations and capabilities and do not want to risk it.
The road to hell is paved with overconfidence. Tired of overconfident men endangering the rest of us. I will not contribute to that.
You know you are not capable of towing a large trailer? If you’re set up correctly it’s not that hard.
I agree that there are too many people who are overconfident, dangerously so. It’s just that I also don’t like people who fear monger and overstate the danger of something that can be done very safely with some common sense.
25 years in I’m still suffering from PTSD, thanks to EM6000 Theory of Vibrations.
Every time some smart-ass teenager asks why they need to learn their natural logs, exponents, and imaginary numbers, this is why.
They need to learn natural logs, exponents, and imaginary numbers so that they can understand that you need 10-15% on the tongue?
To really understand it, yes. All of the above plus so, so much more.
Maybe you do? I’ve always managed to load a very roughly eyeballed 10-15% on the tongue without busting out the graphing calculator……
Hubert also managed to write an explainer article about it without using any real math, definitely without logarithms or imaginary numbers.
25 years ago I could have worked it out I guess. I’ve been out of this way too long though.
Now I know just enough to realize what I do not know.
I had read a paper about 10 years ago that talked about this subject, with relationship to speed. The conclusion was that we negative tongue weight, the math was unstable below some very slow speed (25 mph?). A zero tongue weight was good to 55-60mph and 15% was good to in excess of 140mph. Don’t quote me on the upper and lower limits, it was a decade ago.
However, this does provide the reasoning behind the different towing regimes in North America and Europe. Europe tends to use a zero or 5% tongue weight trailer and they are limited to 50 or 55 mph while towing. North America has no real speed limit on vehicles while towing a trailer (except for some local/state speed control issues).
That is one of the reasons why if you find a car sold in both markets, it is allowed to tow in Europe and not in North America.
Apparently Washington State limits all vehicles towing trailers to 65mph, even on the many 80mph interstates in Washington.
Here in California the limit is 55 mph. Not that many people actually keep to that!
You should see the crazies in Utah doing 80-90mph towing large campers or toy haulers, it’s pretty scary.
We own a camper and I can’t count the number of crazies who have blown past me with their lifted trucks going 80 mph with a trailer.
80-90 is sadly just the average down I15! If you can’t do 100 on 40 inch tires with 35 ft trailer, I mean are you even trying? At least that seems to be how these people think. I’m starting to think my best bet is to go fast and furious, build a car that will go right under those trucks.
80mph Interstates in WA?? Where have you seen that I’ve driven 5, 90, and 82 from end to end in the last few months and I’ve not seen that anywhere. 70 is the highest I’m aware of. Now ID and MT have lots of places where the limit is currently 80 mph.
were it this one?
https://opposite-lock.com/topic/83534/tow-me-down-the-rehash
Also, in Europe, most trailers above a certain weight have to have their own braking system. Usually this is an extending/contracting element between the vehicle and the trailer that automatically applies the trailer brakes when the trailer starts to over-run the car. Trailers in Europe don’t put much additional strain on the tow car brakes.
In the US all trailer above a certain weight(2000lb unloaded in most states) have to have their own brakes. The sliding tongue thing is known as “surge brakes” and is generally understood to be mostly inferior to the more common electric trailer brakes.
Not just a European thing.
Yeah surge brakes suck, electric all the way since you can adjust them to lead the vehicle with a good controller and apply them independently to control sway. About the only place I commonly see surge brakes in the US are on boat trailers presumably because it is common to dunk the axles in the water, and on rental trailers.
I’ve never had issues with surge brakes on rental trailers; you don’t have to set them and they just work. But I’ve heard lots of people say they prefer electric brakes.
I guess it could be that you’ve been lucky and/or I’ve been unlucky. In theory if everything is properly adjusted and well maintained it should work well. I’ve experienced them living up to their name and surging, where it over brakes which causes it to release/let up on the brakes and then re-over apply and repeat.
Probably just got lucky with newer and well maintained rental trailers.
I have wondered why tongue weight would affect trailer sway, and I appreciate your explanation.
But this is a remarkably unsatisfying explanation to me. It doesn’t seem right at all.
So from the beginning, you describe a vehicle driving in a straight line with a trailer behind it, and a force suddenly causes the trailer to “move to the left”. This causes the trailer to rotate around the hitch ball. This does not affect the trajectory of the tow vehicle at all, the tow vehicle continues driving straight.
I don’t think that’s right at all. For a trailer to rotate about the hitch ball, the tires would have to slide across the ground. But we’re not talking about loss of traction here, and a small force like a passing semi most definitely is not capable of overcoming the traction of the trailer tires.
You also show no change in the trajectory or position of the tow vehicle. But in the videos you linked, and other videos I’ve seen showing trailer sway, one thing is very consistent: the tow vehicle is swaying as much as the trailer is. But you later specifically describe the tow vehicle driving in a straight line while the trailer follows a curved path, even though this is clearly not what is happening.
Looking at the videos, what seems to be happening is: A lateral force acts on the back of the trailer, behind the axle. This causes the trailer to rotate about the centerline of the axle(not the hitch ball), causing the tongue to push the opposite direction. This sideways force on the tongue, because of the distance between the tongue and axle of the tow vehicle, creates a torque on the tow vehicle that causes the tow vehicle to pivot about its rear axle. Both car and trailer pivot and neither loses traction. This type of sway makes more sense to me, but I don’t understand why weight distribution would affect this sway.
My description does explain why gooseneck/fifth wheel hitches mostly eliminate sway: there is no distance between the tongue and the rear axle of the vehicle, and so any lateral forces from the trailer have no leverage on the vehicle.
There are many mechanisms that could cause a trailer to get pushed to one side of the other and it doesn’t need to include sliding in the tires. Tires can move laterally through their slip angle without any sliding at all. That’s what happens in a turn on every car. When subjected to a side force, a tire will not roll in the direction it is facing but instead will roll at an angle. You can read more about this topic here: Tesla Model S And Ford GT Engineer Explains How To Design Oversteer Or Understeer Into A Car’s Suspension – The Autopian
Thanks for the response!
The way I understand it, the difference between the heading and travel direction is roughly proportional to the lateral force on the tire, meaning there will not be a whole lot of slip angle unless there is a whole lot of lateral force on the trailer, which there definitely isn’t in the case of a small trigger like a passing semi or a bump in the road.
I also don’t understand why the trailer would ever want to pivot around the hitch ball. The trailer naturally wants to pivot around the axle and it will do so unless there is a huge opposing force coming from the hitch ball.
You also haven’t addressed the tow vehicle swaying. I just don’t see how any explanation of trailer sway could be complete without considering the vehicle sway also, especially when it is very clear that vehicle weight and other factors affect trailer sway quite a bit, and when videos consistently show the vehicle swaying just as much as the trailer.
The reason the trailer will pivot around the hitch is because that is one of its constraints. It is true that the tow vehicle will also sway but that is in response to the trailer sway. It is a result, not a forcing function. I also glossed over it because it greatly complicates the discussion and adds nothing to the trailer weight distribution issue which was the focus of my post.
As far as triggering sway goes, there are many videos online showing trailer sway being started by passing a truck so it most definitely is enough to get the motion started. We own a camper and I can tell you from experience that the wind coming off the front of a semi-truck can shove the trailer sideways significantly.
The trailer would pivot around the hitch because it is a rigid constraint. But the tires are also a rigid constraint, and obviously if both were actually rigid constraints the trailer would never sway.
https://www.theautopian.com/why-automakers-overpressurize-tires-and-how-that-can-affect-your-handling-ask-an-engineer/
In this article you go into slip angles more in depth and actually share some numbers. You give 2.5° degrees of slip as an example of hard cornering(I know I’m oversimplifying). I’m going to assume that 5° of slip angle is beyond the traction limit for most leaf sprung solid axle trailers on skinny all season bias ply tires.
So I just don’t see how a trailer could sway even as much as 5° to either side without major traction loss or the tow vehicle swaying quite a bit. So it seems to me like the tow vehicle swaying would have to be a fundamental part of trailer sway.
I also don’t see how a significant amount of trailer slip could be started by the small forces of a semi passing or the like, but I know trailer sway can be triggered by such small forces. So obviously something else is contributing.
I think the hitch is a rather compliant constraint that the trailer can push around if forces are sufficiently high. This would explain why heavy vehicles towing light trailers are relatively resistant to sway, and why light vehicles towing heavy trailers are more prone to sway and can experience sway even when loaded correctly.
I also don’t see how the towing vehicle could be swerving several feet left and right and NOT be a major factor in the trailers behavior.
I’m not trying to give you a hard time, I appreciate your responses to my questions. I’m just trying to understand this as completely as I can, because this has been largely a mystery to me for years now.
Once trailer sway gets going, the movement of the tow vehicle will certainly have an impact. When I had my own incident, the truck was moving a ton and I had to countersteer a heck of a lot. I agree with you that a lighter tow vehicle with a heavy trailer will more likely sway but that’s not what starts it. Once the trailer starts moving, even a little, the dynamics that I described take over and grow quickly. All you need to do is search YouTube to find lots of examples of trailers swaying out of control. There is certainly sliding happening in the tires but only once it has gotten really bad.
Also, 5 deg of slip at the tires is a lot. Here’s a frame of reference. Think about the amount of steering you put in when changing lanes on the highway. You turn the wheel maybe 1-2 degrees and you’re changing lanes. Take that number and divide it by a number between 15 and 17 (which is the total ratio of the steering system). That’s the number of degrees the front tires are actually steering. You can see what a small number this will be but it is enough for you to change lanes. Compare that to 5 deg, or even 2.5 deg and you can see how much that would steer the trailer.
If it was only the trailer swaying back and forth with the tow vehicle staying straight it wouldn’t be as much of a problem as it is since the towing vehicle would stay in control.
Thanks! Keeping the center of mass ahead of the center of pressure is the corresponding prerequisite for stability in rocketry, too.
How about a discussion of flat towing, a special case in which the tongue load is essentially zero and the “trailer” wheels are more or less at the corners and, for the front wheels, passively steered?
That was a great write-up Huibert.
I’m wondering if we have any long haul truckers in the comments, and if so, do they teach this kind of information to them since it is something that directly impacts their driving?
Semi drivers do not have to worry about this specifically, fifth wheel semi trailers are much less sensitive to loading, much more resistant to sway, and when they do sway, they do it via a totally different mechanism.
Source: not a CDL driver but I know several and have plenty of experience driving heavy trucks.
If you look at the location of the wheels on a tractor-trailer rig the wheels are so far back on the trailer it would be nearly impossible to have the center of mass behind the tires. So I’m not sure it really impacts their driving. They carry much more trailer weight on their rear drive axles (tongue weight if they had a tongue) and are designed to do so.
They do have to make sure that their weight is distributed to ensure that an individual axle is not overloaded. There are axle limits in most states. They can adjust the distribution by sliding the rear tire carrier on the trailer fore and aft to change the balance.
They also have spike breaks to help straighten the trailer out if needed.
That’s actually why it’s called a semi trailer: a regular light trailer to tow behind your car has the axle roughly in the center of the trailer and the trailer carries 85% or more of its own weight. A semi trailer has the axle way at the back and carries more like 50% of its own weight, the other half resting on the truck. The trailer carries half of its own weight, giving you the prefix “semi-“.
That’s cool. I tend to refer to them as tractors and trailers. But a lot of people call them “semis”. I never knew why.
Yeah that was kind of a mind blown moment for me. I was always confused why a truck would be called a semi truck: that sure looks like an entire truck to me. But really it’s a truck and semi trailer.
I did a stint of about a year as an OTR operator for a company out of Joplin, MO. One thing we had to on the lookout for was improperly loaded trailers. You need to catch this while loading is taking place so you can correct it. Believe me it is entirely possible to pull into a yard pulling an empty, only to have some warehouse operator drop a 10K pound pallet on the tail of your trailer. If you had your trailer duals slip all the way forward to accomodate a previous load, you better believe you have a huge “pendulum” at the back end of your rig.
I was amazed at the actual skill involved in loading trailers. There are actually special loading patterns that can be used to load a trailer properly depending on the load. I used to unload dry vans full of sugar totes. At 2,500lbs a pallet you could see the damage from poor loading as soon as you opened the door.