Video – Why Do Some Trailers Tow Stable?

The way a trailer tows makes a big difference in the ownership experience.  So, Why do some trailers tow stable, while others do not?  Even a small trailer that does misbehaves by swaying is rather intimidating on the highway, and a lot of work to tow.  Bigger trailers that waggle are downright dangerous.  (Of course, that does depend on the severity of the waggling.)  Many people call this “Trailer Sway”, or “Fishtailing”.

On a recent trip, I caught one such small trailer waggling away behind the tow vehicle rolling down the highway.  My wife was a great sport and took this video for you.  It’s a little shaky, but, with the help of YouTube stabilization, not too bad.

Learning From Another Example

Look at the trailer and how it tracks behind the tow vehicle.  For the most part it follows well, but when there is an small interruption — like going around this curve, or a bump, a minor steering correction by the driver, etc., the trailer fishtails just a little.

This vehicle caught my attention when the driver changed lanes and the trailer did a pronounced waggle.  Fortunately for them (unfortunately for the video), the driver did not do that again while we watched.  Nevertheless, you can still see some tail wagging in the video as they round this bend.

For reference, this waggling from side to side is called Yaw — one of the important vehicle attitudes.  Important, but not desirable (except in a purposeful full drift!)

This waggle is actually not that bad.  However, I’ll bet the driver feels some weird steering inputs and probably has to fight it just a little from time to time as they drive.  That said, a little can be a lot if it makes the driving more stressful.

Making Trailers Tow Stable

What makes some trailers tow stable, while others waggle down the road?  We wrote a full article about Trailer Stability, (with related trailer design articles) still posted on our parent site Synthx.com.  It’s a good place to start reading.

For the one in the video, it’s hard to say exactly the cause of the semi-stable behavior just by looking.  However, I will guess it is poor loading.  Probably the center of mass is too far back leaving insufficient tongue weight.  Also, (not in the video) the trailer tongue is pretty short, which can amplify loading (and stability) issues.  See the post on setting the right axle position with discussion on loading for more on making trailers tow stable.

Of course, there are likely some other contributing factors.  Look at the weight distribution side to side.  Again, this is not a big deal by itself, but the right side motorcycle is bigger, and probably heavier than the left.  (Note the motorcycle rear tire size.  Typically a bigger tire means more HP, and therefore likely more weight, though these are guesses.)  With all that, I’ll bet if they stop and redistribute the trailer contents for a more centered (side to side) and slightly forward center of mass, then this trailer will be much more stable and enjoyable to tow.  While it would be fun to show them on the spot, I’m sure the intrusion would not be welcome.  Oh well.

I will give them props for securing the load well.  As another driver on the road, I really appreciate it when folks take the time to strap down and secure their loads.  Stuff flying from trailers (or vehicles for that matter) is no fun.  Thank You.

Other Contributors In Making Trailers Tow Stable

Other typical contributors (not shown in the video) include wheels, tires, axles, and alignment.  Those are in a section by themselves below.

Aerodynamics can also play a role especially in highway cross winds.  Large profile trailers can get a push by a wind gust, or even by air flow around a larger vehicle on the highway.  Wheelbase also plays a part.  Longer is usually better — if you get the other contributors right.

Finally, the tow vehicle is important — especially when the trailer size exceeds the tow vehicle size.  The bigger the waggle, the more it takes to control it, so make sure your vehicle is good for it.  Too often, the little waggle will  flip the whole system — that’s now where we want to be.  The closer the rear wheels are to the hitch point, the better also.

The key in all of this — It’s never just one thing.  It involves the whole system of car and trailer.  And the smartest way around it is to stack the deck in your favor for stability on the road.

There is so much more information on this topic of trailer sway and stability.  For starters, I highly recommend the article Trailer Sway – Video Review and Discussion.

When It All Goes Wrong

Stability is more than just a buzz word.  As mentioned above, minor instabilities are mostly just an annoyance that make towing less pleasant.  On the other hand, when we think of it as teetering the balance toward disaster, it has a different light.

The example in this article about safety chains is a stark reminder that all the details matter.  It’s never just one thing, it’s the whole of circumstances and situation.  That’s highlighted even more in this article from several news stories about dangling from a bridge by just one trailer chain.  Though I don’t have proof, I suspect weight stability is one of many contributors in both these accidents.  It’s really no joke.  Perhaps both could have been avoided a with setup so the trailers tow stable.

Don’t Forget Wheels And Tires

While it does not appear to be an issue with this trailer, one other factor to help trailers tow stable is the wheels and tires.  Without going into much detail, here are a couple thoughts.

First, tire pressure makes a difference.  One side low and one side high will make the trailer do weird things when you hit a bump or swerve.  So, make sure the tire pressure is right, and the same on both sides of the trailer.  Tire pressure is also very involved in trailers that bounce.  That’s a different kind of stability.

Second, if a trailer wheel is not balanced, or if one gets damaged, like bent, the trailer will wander — especially at speed.  Have the wheels balanced and inspected once in a while.

Finally, for multi-axle trailers — tandem and triple — make sure the axles are parallel with each other, and perpendicular to the direction of travel.  Straight and square as they like to say.  Misaligned axles will not only cause weird dynamics and the trailers may not tow very stable.  They will also wear out tires fast.

For more about tires, read this about choosing trailer tires, and this about spares.  Enjoy.

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