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  Home  >  Technical Articles  >  Trailer Design  >  Page 3:  Trailer Stability
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1. - Introduction.

2. - Trailer Strength.

3. - Stability.

4. - Versatility.
 


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Choosing the Right Utility Trailer.

Trailer Towing Tips.
 
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Page 3:  Trailer Stability
Why are some trailers stable while others wander all over the road?

 

There are dozens of factors related to towing stability.  No one factor is an absolute for making a trailer stable, or not, but they all have an affect.  Here are a few of the most common things to look for:
Poor Load Distribution
An extreme example of poor load distribution.

Load distribution is the most common cause of stability concerns -- and the easiest to correct.

First, the trailer load should always be reasonably distributed from side to side.  Don't put a lot more weight on one side or the other.  Center the load on the trailer as much as possible.

Secondly, load the trailer front to back with at least 10% of the trailer weight on the hitch.  With typical trailers, more weight on the hitch is better for stability.  15% is usually a good number.  To illustrate, 5th wheel trailers have much more weight on the hitch, and they tend to be pretty stable.  The extreme example is over-the-road trucks.  They carry up to 50% of the load at the front.

Third, (hopefully this is common sense) secure the load properly.  If the load shifts, the load distribution has changed, probably in a manner that is not helpful.  Always make sure the load is secure.

Load distribution extends beyond just how a load is put in a trailer.  When a trailer is hitched to a tow vehicle, they become a system and the dynamics of the system are what makes stability (or not).

For the tow vehicle, "Motor Boating" is a big stability concern.  A large weight on the hitch -- at the rear of the vehicle -- can make the back of the tow vehicle sag, and the front rise.  When this condition is significant, it will adversely affect stability, driveability and control because weight is lifted off the steering wheels.  To correct this problem, load distributing hitches can be used to level the trailer with respect to the tow vehicle.  A sample is shown below.  These hitches can make a big difference -- especially when trailer size and/or weight is near or exceeds that of the tow vehicle.

Note:  Before using a load distributing hitch, make sure the tow vehicle, hitch and coupler can handle it.

Load Distributing Hitch
A load distributing hitch example from reese-hitches.com

Usually if a Damper (like a big shock absorber between the trailer and the tow vehicle) is needed there are other, perhaps fundamental, issues that should be addressed first.  A trailer properly configured and properly loaded should not need a damper -- though in a pinch, they can help.  (Note:  One is shown in the photo above.)

Trailer length and length with respect to width is another stability factor.  Longer trailers typically tow better -- or more accurately, a greater separation between the rear most wheels of the tow vehicle and the fore most wheels of the trailer.  A long tongue (as discussed in the Strength section) can facilitate such separation.  Evaluate trailer length in the context of width.  Look at the width of the trailer with respect to the tow vehicle and with respect to length (or more accurately, with respect to separation of the axles).  In general, the wider the trailer, the more it will benefit from length.

Another length factor is the distance between the rear wheels of the tow vehicle and the hitch point.  When this distance is shorter, the trailer can't "wag" the tow vehicle as much, nor do the "bumps" of the tow vehicle affect the trailer as much.  Fifth wheel trailers are a good example.  Finally, a hitch at the back of a really long overhang (like the back of a school bus) will have a negative effect on stability.  Extend-a-hitch bars that push the hitch point back with respect to the rear most wheels of the tow vehicle are a similar bad example.

A couple of Axle and Wheel related items:
First, use a trailer axle - an axle made for a trailer.  The wheels on a proper trailer axle have just a little camber to them -- which greatly enhances tracking.

Next, Axle alignment with respect to the pull direction is very important.  The axle(s) should be perpendicular to the direction of pull -- and perhaps more important, if more than one axle is used, they must be parallel.  (This condition is easy to see from behind the trailer on the highway -- because the trailer follows a little to the side of the tow vehicle.)

>> This condition is not only less stable, it also wears out tires prematurely and wastes fuel. <<

Finally, Wheels and tires should be in good condition.  I can't tell you how many wobbly (bent) rims I have seen on trailers going down the highway.  The wobble may come from a bent rim, or from a wheel that is not attached properly.  Either way, it causes problems with stability -- and safety.

Tires should be properly inflated.  Inflation pressure will vary depending on conditions, tire type, load, etc..  When the trailer is to be pulled a long distance empty, or without significant load, tire pressure should be reduced -- perhaps as low as 10 or 15 psi.  This allows the tires to soak up more of the road inconsistencies, and the tires become a damper to keep the trailer from bouncing around.  When the trailer is loaded, the inflation pressure should be increased per the tire manufacturers recommendations.

Though not a specific stability concern (until there is a blow-out) Dry Rot on Trailer Tires is one of the most common causes of trailer tire failures - and when a tire blows, it's a big stability AND SAFETY concern!!  It's a horrible way to interrupt your trip, so carefully check the tires before traveling.

Tandem Axles with Brakes
Example of Tandem Axles with Brakes

Trailer brakes, and calibrating the brakes to the tow vehicle is worth a long discussion (which we won't give here).  Just note that the brakes and application of the brakes need to be appropriate for the load and the trailer size.  This is less of an issue with electric brakes than with surge type, but it applies to both.  Numerous issues with setting and calibrating surge brakes have caused several states to consider banning their use.  Check local regulations before buying a trailer with surge type brakes.

One idea (please be careful in how you might apply this) is to connect and use the trailer axle brakes when the trailer is loaded, then disconnect the brakes when the trailer is empty.  This can work well for a trailer that (empty) is light compared to the tow vehicle.  This will not work if the trailer is large or significant with respect to the tow vehicle.  (Note: this idea addresses concerns when brake calibration for the loaded trailer causes the trailer brakes to "skid" when the trailer is towed empty.)

Dynamic loads can have a significant effect on stability.  For instance, if a large, heavy pipe is carried untied, it will roll from side to side or front to back with motion of the trailer.  This constantly changing load makes the trailer move.  If the dynamic load is small compared to the weight of the trailer, it may not be so important, but if the moving load is a large, the effect can be quite important.  Live loads like animals can create this concern.  In general, the load should be secured as much as possible to accommodate stability.

Center of gravity is another important consideration.  In general, a lower center of gravity helps stability.  I once saw a trailer that tipped over around a corner because the CG was too high for the conditions.


The topic of Aerodynamic effects, on trailers and specifically on trailer stability deserves a page by itself.  There are hundreds of studies on the subject, mostly on large, over-the-road trucks, but they are applicable to trailers of all sorts.  It's a fascinating subject, but I won't delve too deep here.

When discussing aerodynamics on trailers, the answer is almost always ... "Minimize the effect of wind on the trailer, and it will help with all sorts of concerns - including stability, fuel economy, driveability, etc.."

Aerodynamics are more than just wind drag when towing the trailer down the road.  Effects from side winds or other passing vehicles can contribute to stability issues.  Typically high profile and larger trailers like campers suffer from this more, but be aware of aerodynamic loading on utility trailers too.  Things like high profile objects sticking up, tarps that flap relentlessly in the wind, even sharp corners on the trailer contribute (some in a minor way) to aerodynamic stability.

Aerodynamic Trailer
Some trailers are built specifically aerodynamic like this one from U-Haul.

There are all sorts of ways to improve aerodynamics.  Some trailers incorporate a "V" nose; others use full bulbous fronts; some use trailing spoilers; yet others have full fish tails.  All the above will help, but not all are necessary.  The largest "bang for the buck" is keeping the trailer directly behind and in the slip stream of the tow vehicle.  If the trailer has a larger profile than the tow vehicle, the biggest "bang for the buck" is the rounded corners.  It does not have to be dramatic like the U-Haul trailer pictured above in order to have an effect.  A little goes a long way like the picture below.

Rounded Trailer Front
Just rounding the front (leading) edges of the trailer box will result in significant aerodynamic improvement including fuel savings and better stability.


Along with aerodynamics, trailer attitude has an effect.  Trailer attitude is the relationship of the trailer with respect to the ground and tow vehicle -- basically, the trailer should be somewhat level (not with the butt dragging the ground or the tongue extra low).

One final item is the strength or rigidity of the trailer itself.  If the trailer is flimsy it will twist and bend with the dynamics -- especially if the load is also dynamic.  Just one more reason to choose a sufficiently strong trailer as discussed on Page 2:  Trailer Strength.

 
Concluding Thoughts ...

There are numerous things that affect safe towing and stability for a trailer - all need consideration, but a few are most important.  Items of construction include wheel and axle alignment, axle placement, brakes, calibration, and overall strength.
 
Effects to consider with each use include (first and foremost) load distribution and load securing.  Other things (also very important) are trailer attitude (with respect to the ground and the tow vehicle), tire condition, and tire inflation pressure.  By considering, adjusting and making sure these things are all correct for your situation, the trailer can tow in a stable, predictable and enjoyable manner.

 
ContinueNext Up:  Versatility   -  A look at trailer options.
 

 
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