Where Is The Weight In Your Trailer?

What’s the big deal with Weight in my Trailer?  Isn’t Weight just Weight?  Yes, I know about tongue weight, but the trailer is made to carry weight.  So, if I’m below the specified weight ratings, what does it matter?  If you’ve ever felt this way, you’re right – mostly.

Of course, the mass we call weight is just that, and trailers are made to carry it.  However, the location of the weight with respect to other parts of the trailer also matters.  It matters in the design, and also in the practical setup for a comfortable, stable towing experience.  Most of all, it matters with loading the trailer for a stable ride.

Starting With Design

One of the most important decisions when designing or building a trailer – Where Does The Axle Go?  We set the axle position so that the tongue weight is appropriate.  That is a HUGE contributor to trailer stability.

We know if the mass on the trailer is too far forward, the tongue weight will be too high.  High tongue weight puts more weight on the hitch, and extra stress in the trailer tongue.  – Both of these conditions are OK if both the tow vehicle and the trailer frame can handle it.

On the other hand, if weight in the trailer is too far back, the trailer will be very unstable.  Things like sway and fish-tailing are frequent descriptors of this kind of instability.  Here’s a video of a trailer having some of this instability.  Both trailer weight and where it is on the trailer definitely come into play with this kind of instability, for sure.

With most calculations for Axle Position and Tongue Weight, the distribution of weight does not come into play.  We look at the total weight and the total center of gravity (a little in front of the axles).  For example, follow the calculations in both of these articles:  “Trailer Build:  Where Does The Axle Go”  and  “Calculating Axle Position“.  These show 2 methods for setting the axle position based on where you start – from scratch, or from an existing trailer.  Yet, these are Static calculations, so they don’t consider dispersion of the load.

The static calculations are certainly good, but today, in this article, we will go one step farther in understanding where and how placement of the weight in the trailer matters.

Location Of Trailer Load

In practice, it’s not just the weight of the trailer, or even where the center of mass is for the trailer.  When towing, the dynamics also include distribution of weight around the trailer.  Here are 3 images illustrating what I mean.

Image 1:  Evenly Distributed Weight on the Trailer.

Even Trailer Weight Distribution

When doing calculations, we often think about the trailer weight as evenly distributed over the bed.  In practice, that is not usually true.

Image 2:  Weight Front and Back on the Trailer.

Ends Loading

This 2nd image has the same amount of weight as in Image 1, so both the tongue weight and axle weights are the same.  Calculation results for tongue weight and axle position are exactly the same.  However, as you can see, the distribution of trailer weight is totally different.  Does that matter?

Image 3:  Weight at Center-ish of Trailer.

Center Weight

Our 3rd example has weight focused near the trailer deck center.  Like the 2 above, the tongue weight, axle weight, and calculation results for axle position are the same.  Again, this is just a change in where the load is physically located.  The center of mass for all 3 is the same (horizontally).

Now the question.  Does any of this matter?  And, Why?  or Why not?

Trailer Weight For Towing Experience

I was always taught to put the heavy things toward the front of the trailer, then lighter things toward the rear.  While that is not bad advice, a better approach is available.

Look at a top view of a trailer like these 2 images.  If you try to “twist” the trailer as with the arrows, which one is easier to move?  More important, once it starts to twist, for which trailer is it easier to stop the motion?

Rotational Inertia
Top-ish View of two trailer loading conditions.  Left with End Weights, and Right with a Central Weight.
Red arrows show the minor rotational motion as it relates to dynamic stability.

This is the physics concept of rotational moment of inertia.  We intuitively know if the weights are at the ends of the trailer deck, it is harder to twist back and forth.  So if we have a load which is more central, it has far less rotational moment of inertia.

When we’re driving, there are forces that make the trailer want to twist like this — wind, road undulations, and driving inputs.  We can’t get rid of the forces that cause these minor rotational movements, so we need to make it easy for the trailer to “settle back” after inputs happen.  More rotational inertia (like the blue weights) makes the trailer harder to “settle” after such inputs.

What does this mean?

Here are several important implications for distribution of weight on the trailer.  (Please note, we are not talking about Weight Distribution Hitches, those are loosely related, but not part of this article.)

  1. Focusing weight toward the center of the trailer decreases the rotational inertia about the vertical axis, and the trailer becomes more stable.  While moving the load to the trailer center is not always possible, we certainly don’t want to put the heavy bits near the ends.
  2. Heavy bits near the front are less intrusive for stability than at the rear.  The hitch at the tow vehicle acts as a secondary pivot, which gives more control for weights at the front.  However, we don’t want to overdo it for tongue weight.  See more below.
  3. Even distribution of load usually works just fine.  Don’t get to worried about the distribution of weight, but do be conscious of it.
  4. Lower center of mass is also better.  The images above simplify the truth of dynamics, because there are forces, like wind and cornering, that want to tip the trailer, which is another order of complication.  As we think about dynamics, height is worth mentioning.  While this is a lower priority effect, it is important just the same.  More care in corners, and slower speeds are recommended for trailers with a high center of mass.
  5. Finally, keeping the weights generally centered left to right.  Yes, it stands to reason, but we mention it here for completeness.

How much does any of this matter?

Let’s shift gears a little and look more at a few details.  Don’t worry, we’ll come back to the practical applications.

What Is Trailer Weight?

It’s easy to make the argument that axles, wheels and tires are unsprung weight and therefore not included in calculations with trailer weight.

Another one that’s similar . . . Is tongue weight really part of the Trailer Weight when we’re figuring out how much load the axles carry?  After all, if the tongue has 10% of the trailer weight, then we could actually have an 11,000 lb trailer on 10,000 lbs axles.  Right?

These are both easy arguments to make when thinking about a trailer.  While I don’t have a definitive answer about when it is right or wrong to consider unsprung and total weight or to know exactly what an axle will carry, I guarantee you if you push the limits you will have problems.

I always recommend using the trailer weight total, as measured on a scale.  So, the total trailer weight includes the axles, wheels, and the tongue weight.  If 10% of the trailer weight is on the tongue, and a hundred or so pounds are are in the wheels (unsprung weight), that gives you a small margin for safety at the axles.  Why push it?  Why tempt disaster?

Then there is a third bit that enters in.  What is actually the best tongue weight percentage?

Calculations for Tongue Weight

All of the above arguments hinge on subjective and empirical observations about tongue weight and stability.  The 10% to 15% tongue weight numbers we talk about come from observations that tend to work almost all the time.  It is a “Rule of Thumb”, as a recommendation.  If you want to get into super nitty gritty detail, there is always plenty of room to calculate possibilities.

On the other hand, mother nature doesn’t follow exactly our calculations anyway, because we don’t consider all the various possibilities of conditions.  That is why the rules of thumb exist.  If you think about it, the difference between 10% tongue weight and 15% tongue weight is actually quite large.  If you target 12% tongue weight and leave a little hand waving space, you’ll be right most of the time.

I guess what I’m saying:  Don’t get too wrapped up in the exact numbers because the true numbers change with many factors (wind, wind direction, pitch, yaw, bumps, other passing vehicles, road crown, loading unevenness, dynamics, etc.).  There is all too much in the variations to even think about.

The truth is, regardless of all the changing variables, the trailer needs to be stable.  So, we get there by observing the millions of trailers that have gone before.  Some have stability issues, some cause horrific accidents.  Yet, some always pull perfect and stable.  We know what generally works and what doesn’t, so we create rules of thumb as guides.  Yes, every trailer is different, so we need to take that into consideration, especially in the design process.  That’s where experience comes in.

Side Note on Tongue Weight

I had one European reader tell me I was promoting tongue weights that are much too high.  He claimed it is unnecessary.  He said small tow vehicles don’t need as much tongue weight.  I mention this to dispel a myth.  What he’s saying is opposite the physics.

All the science says stability is best when the tow vehicle significantly outweighs the trailer.  Then, you can get away with a little lower tongue weight – into the 7% or 8% range.

Another trick that works – if the load on the trailer is concentrated near the axle then a really long tongue allows a lower tongue weight.  So if the tongue weight would be 10% with a normal tongue, if you make the tongue really long, the actual tongue weight is lower because you are using a longer lever.  (Please note there are other considerations too, for a long tongue, both positive and negative.)

Finally, you can get away with very low tongue weights if you drive really slow.  At speeds below 20 Kph, the dynamic energy is just not enough to create instability.  Driving super slow is not an option in most roadway situations, but for completeness, it is worth mentioning.  The opposite is also true:  Higher speed, puts more energy in the system, so it is more susceptible to trouble.

A trailer that seems stable at moderate speed can become a huge problem at higher speeds.  We see this in serious trailer sway accidents.  Someone pulling the RV just fine for many miles, accelerates to pass, then the wind disturbance of passing excites the trailer uncontrollably, and a horrible accident happens.  Distribution of weight in the trailer is almost always a big factor.

Where To Put Weight In The Trailer

Theory is good and all, but how about a practical approach?  OK, here are Practical Recommendations with a list in order of importance.

  1. Make sure all weights are within the tow vehicle specifications.  That’s GVAR (axles), tongue weight, trailer weight, and total system.  This includes all the people and things in the tow vehicle also.  If you’re not sure, measure them.  (Believe me, the hassle of getting weights is a whole lot less than picking up pieces after an accident.)
  2. Adhere to the recommended weights for the tongue and axles.  Total weight of the trailer less than GTWR, tongue weight in the 12% range (between 10% – 15%).
  3. As much as practical, focus weight toward the center of the trailer – see the images above.  For many trailers, like an RV, it’s not really practical to do completely, but don’t put extras at the ends of the trailer.  For instance, don’t hang a generator or a storage box on the back.
  4. Keep center of mass as low as practical.  Again, this is not always easy or even possible, so do what you can, and don’t make it obnoxious.  Like, don’t put a generator on the roof of a tall RV.  If you have a high center of mass, slow down.
  5. Please note the above recommendations are more important as the trailer gets longer, if it has a lot of side surface area, as it gets taller, and as it gets heavier.  (In that order of importance.)
  6. If the trailer weighs significantly LESS than the tow vehicle, these weight issues are less important.
  7. Finally, if the trailer EVER acts squirrely, stop and figure it out.  Redistribute the weight if needed.  Check tires for low pressure.  Check springs for breakage.  If that is not practical, slow down.  Speed increases system energy, and if it starts to get unstable, it is much more difficult to bring it back into control.

Have a Wonderful Trip

I believe every accident from instability has some warning signs, often long before the crash.  It’s a matter of seeing, or feeling them, then acknowledging what it is.  An ostrich with his head in the sand is much more likely to be eaten than the ostrich running with his head high and his eyes wide open.  For you own safety, please don’t ignore even the earliest hints of instability.

For more on this topic, check out the Video Review on Trailer Sway.

Good luck in your travels.

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