What happened? I can see that I did a number of things wrong. One of them was NOT tongue weight - I had plenty. There are other factors that are contributors to fishtailing and I want to help you all understand them and avoid having the same adventure as I did.

Central to my thinking is the concept of a “resonant system”. This is an engineering term describing a system that has two (or more) ways to store energy. The most common is a spring-mass system, and we are all pretty familiar with it: the mass of our pickups is carried on springs. Some of us have experienced driving a vehicle with poor shock absorbers. If you go over a dip or bump that sharply “excites” the system, you can have a long series of resonant vibrations. The word excite refers to energy being quickly added to the system and causing a resonant (vibrating) response. In this example, there is usually enough friction in the system to calm things down, but some some systems, the oscillation can build up. This case is referred to as an unstable system.

If you look from above on a vehicle-trailer system, there are small forces (e.g.: wind) that try to move things around laterally. The suspensions and the tires all have some give, and act as springs - if you put energy in by pushing on them they will store the energy in their elasticity and return the energy by pushing back. There is also mass - a lot of it in the system. Mass stores energy by moving - if you push on a mass to get it moving, it can push on something else until it stops. Once you have this condition, you can get resonance and oscillation. In a trailer, we call it fishtailing.

 

In my case, the highway with its alternating patches provided the exciting energy. I was unfortunate that the series of pulses were just what my “system” was waiting for. Once the tow vehicle had rotated one way and the trailer, the other, energy was stored in the tires and suspension. This energy the pushed back, transferring into moving the mass of the vehicles. This system did not have damping for horizontal motion (think: shock absorbers), so when it was moving back to the straight position, there was enough energy to carry it on past to the other side. Each time the system was at its side position, the road movement and air motion over the body sides, helped push it back, adding a little more energy each time, and increasing the amount of fishtail. (Note that friction anti-sway control systems do their work by adding friction into the system. A hydraulic damping system, a.k.a.: shock absorber, should also work.)

I made this situation worse by having a heavy trailer with a short tongue - A VERY BAD COMBINATION. The trailer literally pushed the rear end of my tow vehicle around. A long-tongue trailer could not have had such a drastic effect. I still have the trailer and I will be fabbing a much longer tongue for it before I use it fully loaded again. (I also plan on adding a vacuum system to my trailer tongue that will allow me to manually actuate my surge brake master cylinder.) I also now have a F-350 with dual rear wheels, so there is a lot more lateral stiffness. I even have plans to build a fifth-wheel trailer which will move the hitch point in front of the rear axle, removing all problems that come from overhang.

I have more thoughts, but want this to be a conversation. I am open for questions, comments, and even disagreements.

 

There is a post here that mentions a 5er getting squirly. I just purchased a 32' Toy Hauler with a Lil Rocker hitch and an electonic brake controller with a manual over-pressure button. How much danger am I in from fish-tailing fun? Oh yea, 2003 F250 SD SB V10 FX4 XLT with tow package(sway bar and electronics).

 

Just make sure you don't overload the rear of that toyhauler and you shouldn't have any problems. I was the one that posted about the squirrely 5er, to this day I still don't know why that happened, it made 2 swings and straightened out on it own, never did it again.

 

Jeff,

Could your one-time event have come from something hanging up during an earlier turn and then popping free while you were going down the road?

I join RagdollRacer in being interested in fifth-wheel stability, so I am starting a new thread where I will be mostly listening:

https://www.ford-trucks.com/forums/6...ml#post4626018

 

Archie, we were on the interstate running probably 65-70 MPH and had been on the road for 1-1/2 -2 hours when it happened. I pulled over and checked tires, brakes, wheel bearings, anything I thought could have caused the sway, I never found a thing. I pulled this same trailer with the same truck many more miles and never had it happen again. I have no idea what happened, there was no wind, roads were dry, no trucks within a mile, just a freak occurance that never happened again.

 

When we started camping again in '03 we had an '02 Expedition and after a lot of research my wife chose a 27 ft Surveyor261t hybrid. This combination was a real good match for the Expedition. I towed it all the way to Florida, 1200 miles and back once and to Smokies, 800 miles twice. I actually forgot it was back there.

Then I got tired of poping out that one end bunk and putting it back if it rained so decided I had to have hard side TT and Queen walk around bed. As we always take at least two grandchildren we chose a Jayfeather LGT 29Y for them to have their own beds.

The 29Y is 30 ft and 9" in length and about 2000 lbs heavier and right away I could feel the difference. Well we made two more long trips to Smokies but took our time traveling it was okay but not the same as that shorter, lighter TT. Well last trip I swerved just a little to miss a large pothole and it started to fishtail not so much as I thought to be a close call but it did scare the devil out of me. That was our last camping trip for summer of '06. I decided that was the last trip towing with the Expedition so just picked up a used 2000 F350 dually, didn't get to tow yet and it is overkill but I'm thinkin it has got to be a better experience with that 172" wheelbase. I did not even think about tongue weight just thought it was the wheel base being to short on the Expedition. I won't ever reccomend a 30 fter being pulled by that short of a wheel base.

 

Thanks, LA West, for a great addition to this thread. Your F-350 DRW will give you stability three ways: longer wheelbase (as noted), greater lateral stiffness of the extra tire sidewalls, heavier weight on those drivers. (Don't be afraid to carry weight in your bed.) Glad you had a more gentle "wake-up call" than I did. I am even more glad to hear you answered it.

I did not start this thread to brag about how stupid I was, but to spread the word about how sneaky fishtailing can be. Once it starts, it can go unstable in an instant. On FTE (in an old thread) I read one post that stated that if the driver is highly skilled, then fishtailing is not a problem. In my "adventure" the fishtailing took over and I found it to be impossible to "drive out of it".

Tongue weight is very important, but another critical item is the combination of the tow-vehicle's wheel-base, the tow-vehicle's overhang (rear axle to ball), and the trailer's coupler-to-axle distance. The longer the first & last are and the shorter the middle one is, the more stability. This stability is nice when the trailer is lighter than the tow vehicle, but it is critical if the trailer is heavier. With a fifth-wheel, the middle quantity goes zero or negative and one has very good stability.

P.S.: In my case, I did not swerve to avoid the road obstacle, but the road obstacle started the whole process. If the rig is capable of going unstable it will, sooner or later.

 

I made the mistake of loading a jeep wagoneer on a flatbed backwards. There was limited room, and it didn't run so we couldn't get it swung around. It was okay until about 45mph and then it started to sway. I ended up sticking to 35mph and staying on backroads until I reached my destination. The only other vehicle I loaded backwards was a 91 honda civic, and it was so short that I backed it up to the front of the trailer and the engine was right over the trailer axles with the rest of the car on the tongue.

I keep learning more about towing as I go. I had a standard cab long box 2wd as my first tow vehicle, then I got a boat and needed more power so I got a shortbox 4x4 with a BB. Plenty of power, no stability. I ended up trading the boat for a 20' flatbed. I then decided I needed a longer truck and now I have the F-250 in my sig.

I do have question though. I purchased a new 18' 10k flatbed last year. It has the 4 wheel brakes and surge brakes. I have no sway control devices or anything and have never had a problem towing with it. I plan on getting a TT next year in the 20' range. Is it more likely I will need some kind of SCD? They seem to be more prevalent with TT than flatbed trailers.

 

It is a strange thing that the word "sway" has more than one meaning. For clarity, I suggest using the airplane definitions:

pitch = rotation about the left-right axis
roll = rotation about the fore-aft axis
yaw = rotation about the up-down axis

In vehicles, an anti-sway bar (sometimes called a sway bar) controls roll.
In trailers, sometimes people mean yaw and sometimes they mean roll.

This is not totally crazy, because there are situations where these two motions are not always independent. The best example I have seen is small utility trailers with welders or compressors that are tilted up to be pulled by large trucks. Some of them get into a dance that works through all three axes. (This does not bother the truck much because of the big difference in weights.)

Many of the sway control devices add friction into the system in the yaw direction. They work by soaking up some of the energy so that sway does not build up. I have not had any experience with them but there seem to be believers and non-believers. I would like to hear from both sides, but please give your reasons for being pro or con. (It is interesting to note that most of the factory information for these devices states that they should not be used under "severe conditions", which seems to limit their effectiveness. Personally I think that these disclaimers are the result of the lawyers, rather than the engineers.) (Another thought is that wet or cold conditions may affect their friction material.)

I know I already said it, but the big down-side of surge brakes is that they cannot be applied manually to help straighten up a rig that has started to sway. I personally would only use surge brakes when the tow vehicle out-weighs the trailer.

Another answer is one of the (EXPENSIVE) systems that try to make a bumper hitch system act like a fifth-wheel. I got one of these for a friend and would be interested to see what experience people have had with them. These include the Pullrite which uses an extended tongue with a pivot point just behind the differential and the Arrow which uses a linkage in the tongue to give a similar effect. If there is sufficient interest, a new thread could be forked off to discuss them in detail - their price puts the out of consideration for most everyone but those who have had a "fishtailing adventure" that they do not want to repeat.

 

Sycostang67 - with proper tongue weight, you may be ok without the SCD for that short a trailer. I've pulled that size with no trouble with just a ball. It's probably a lot to do with the sail area catching the wind that makes people favor the SCD for campers.

acheda - I'm not real familiar with planes, so maybe I'm thinking of it differently - but it seems to me that a SCD is controling what you would call pitch. SCD is often coupled with a weight distributing hitch, I wonder if you are thinking of that section of a combination hitch ? When you set the weight controling part of it, that certainly would have an effect on the yaw, as it transfers how the weight is carried. The SCD part is off to usually one but sometimes both sides as an out rigger and has a friction part to limit wagging - not the up and down movement. Quite different from a vehicle's antisway bar which would control roll.

 

I agree that an SCD is sometimes part of a weight-distributing hitch (something of which I have many miles of experience). The weight distributing hitch does pitch the tow vehicle higher in the rear and the trailer higher in the front when the springs are loaded, so I would view that as being related to pitch. The SCD (from my limited experience) lies in the horizontal plane and adds friction when the trailer tongue and vehicle hitch move relative to each other, pivoting around the vertical axis through the ball. This would be what I would call yaw. I think where we are parting ways goes back to the definitions - yaw is a motion in the horizontal plane, around a vertical axis. Other than that, I think we are in agreement.

P.S.: The little ball joints on the SCD keep it from having any effect on pitch, but the friction inherent in the spring joints of a weight-transferring hitch do add to yaw stability. I could really tell the difference on one boat trailer I had - it was all over the place without the bars in place. With the bars in place it was very stable.

P.P.S.:

pitch = rotation about the left-right axis
roll = rotation about the fore-aft axis
yaw = rotation about the up-down axis

pitch = rotation in a vertical plane cutting the vehicle front-rear
roll = rotation in a vertical plane cutting the vehicle left-right
yaw = rotation in the horizontal plane

Example: When you turn a corner, a vehicle is moving in a yaw rotation. The motion is in the horizontal plane, but around a vertical axis. (In this case the axis passes through the plane at a point outside the vehicle - the center of the arc of the turn.)

 

Thanks - we're both talking about the same thing then. I'm just used to thinking of trailer sway as back and forth movement behind me - found the "vertical axis" plane thing a little confusing at first.

 

Just a few questions

If the trailer starts to fishtail should you try applying just the trailer brakes to straighten it up?

I know you want it exact but is a little too much tongue weight better then not enough?

 

I cannot promise you that one piece of advice could cover all possible events, but the "conventional wisdom" is that if the trailer brakes are applied moderately and the tow vehicle is still pulling, the resulting force will tend to straighten out the vehicles and pull them into line without any acceleration. Then if you ease out of the throttle without releasing the trailer brakes, you should be able to slow down safely.

Fishtailing is speed dependent. In my case, with surge brakes, all I could do was to ease into the throttle. This did help, but it resulted in an acceleration, so my speed was increased. This also helped, but there was still some motion. (The fishtail motion, in my case and those who had simiar stories, was much slower than you might expect.) I could not keep accelerating, so I slowly eased out of the throttle (never touched my brakes). When I went through the 60 mph speed, the magnitude of the motion increased and it did not seem to matter what I did with the steering wheel. Ultimately, the trailer pulled the back of the car clear around sideways to the direction of motion.

Within reason, I would always go for too much tongue weight rather than too little. Tongue weight to the point where the front wheels do not have any traction is obviously beyond reason, but sitting low in the rear with plenty of weight on your drivers is much better than zero or negative tongue weight where the drivers do not have enough traction to control the back end. I just read a pickup owner's manual yesterday and it called for 10-12% tongue weight. That is a pretty small target to hit without a scale.

My advice is to rig any sort of scale to find out tongue weight if you have any doubt. With heavy trailers, it is impossible to guess the tongue weight. The cost of an accident is much more than the scale's cost. Once you have the correct tongue weight for a particular tow vehicle & trailer, you can use a tape measure to turn your tow vehicle into a scale. Just measure the bumper height with trailer on and off the ball. This difference is what you want next time you use the trailer. If you do not get the correct droop in your bumper, then shift some of your load until you do. If you have a weight-distributing hitch, to all this before attaching the equalizing bars.