Trailer Build: Where Does The Axle Go?

Trailer axle position?  This is a good question.  Unfortunately, there is some popular, but misleading information around.  So, how do you know proper trailer axle position?  Here is the whole answer, from The Mechanic.

There once was a boy who came to his mother and asked “How do rockets get to the moon?”  She replied “Ask your dad.”  To this, the boy mumbled “I don’t want to know that much about it.”

If you are looking for a simple – maybe misguided or incomplete answer – this is not the right article.  If you really want to know about trailer axle position, then you have come to the right place.

Trailer Axle Position Goals

For this discussion, we will focus on trailers with a traditional tongue attached to the rear of the tow vehicle.  (Bumper Pull.)  5th Wheel (and Gooseneck) trailers are similar, but are not the focus.

These are The Goals that drive trailer axle position:

1. Stable, predictable towing.
2. Proper weight balance for both the tow vehicle and the trailer.

There is a lot in these goals including construction (straightness, flatness, perpendicularity, etc.), proper stiffness and more.  However, with respect to axle position, we follow some well proven guidelines.

1. Follow manufacturer specifications for the tow vehicle.  Note:  there are 2 different limits – trailer weight, and tongue weight.  Don’t exceed your tow vehicle limits.  Enough said.
2. In general, more weight on the tongue is better for stability.  Example:  Over-the-road trucks (ie. Semi or Lorry) trailers can have roughly half the trailer weight on the hitch.  It works because that vehicle is made for it.
3. For a 5th wheel (or gooseneck) trailer, weight at the tongue should be in the 20% – 30% range of total trailer weight.  (Perhaps we will dive into this in a future post.)
4. For rear connecting trailers (traditional, bumper pull), weight of the tongue should be in the 10% – 15% range of total trailer weight.  (10% minimum, 12% – 15% is great.)

As a side note, many vehicle manufacturers and hitch products assume 10% tongue weight, so they can inflate their tow numbers.  Just be aware of this trend and adjust to a little higher % for better towing.

Stable Predictable Towing

The guideline above for 12-15% of trailer weight on the tongue is time tested for dynamic towing stability.  If tongue weight is too low, the trailer will buck more over bumps, and wag around corners.

I won’t go into all the engineering, but the summary is inertia.  If there is not enough tongue weight, a change like a bump or turn or steering correction leaves the trailer mass pivoting, and it takes additional energy to get things back to stable.  As a driver, you feel this motion as bucking or a wagging of the trailer.  In bad cases, it is very unnerving.  In severe cases, a crash can result.

If there is significant tongue weight (above 10ish %) gravity serves to help settle things back to stable.  The more tongue weight, the faster stability is achieved, because the Center of Gravity is further from the axle.

Measuring Tongue Weight

One technique for setting axle position is by measuring tongue weight.  Most of us don’t have a scale that will go high enough, so take the trailer to a vehicle scale.  (Or use this DIY method for a good approximation.)

For an existing trailer, load it for travel.  (This is difficult for utility trailers because you never know what it will haul.)  Drive onto the scale where only the trailer wheels are on the scale.  Take a measurement.  For example, 2250#.  Unhook the trailer on the scale so it measures the full trailer weight (tongue and axle).  For example, 2600#.  Subtracting:  2600# – 2250# = 350# tongue weight.  For percentage, divide:  350/2600=13.5% which is great.  Adjust axle position if needed.

When building a trailer, one easy way is to set the axle in place under the trailer, but don’t mount it.  Clamp it in place, then measure as above.  Move the trailer axle position forward or back as needed, then verify loading.  This technique works well when you have a defined load for the trailer – like a boat, or ATV for instance.

On the other hand, for something like a Tiny House Foundation or a Utility Trailer, this method for trailer axle position does not help much because you don’t have the actual load when constructing the trailer.  See the Calculation Method below.

That said, using this measurement technique helps when locating bigger loads like a battery pack or water tank for a Tiny House.  You can set an axle position that suits the overall design, then place the bigger loads where they measure for proper tongue weight.

Calculating Tongue Weight And Trailer Axle Position

We can also calculate trailer axle position.  I will give an example below.  However, for existing trailers (if you want to check the axle position or maybe move the axles), please read this other article also about Calculating Axle Position.)

This calculation example uses a “balanced lever” approach for loading.  First, we sum forces in a vertical direction.  There are only 2 points that support the vertical forces (tongue, FT; and axle(s) FA).  Then, there are several “loads” (depending on your trailer):

• Trailer Weight – WF – Weight of the trailer including frame, sides and flooring can be measured or calculated pretty easily.  You must also know the position of the center of that weight, L4.  (Measure this by placing a board on edge under the trailer frame, then move it till the frame balances on the board.)  This is NOT the geometric center of the trailer.  It is the center of MASS.
• Evenly Distributed Loads – WD – Loads that can be assumed as even along the length of the trailer bed.  For a utility trailer this may be rocks, or firewood.  For a Tiny House, this is the walls and roof.  A good estimation is OK, but more accuracy gives a more accurate final answer.  The location of this load, L5, is the center of distribution.  If you don’t have an evenly distributed load, leave this out of the equation.
• Points of Specific Loads – WT – These are big loads at specific locations.  The image shows one as a toolbox in front of the bed, but these can be on or under the bed as well.  For a utility trailer it may be an ATV, or lawn tractor, or tank.  For a Tiny House, maybe water tanks, battery packs or kitchen cupboards.  You may have several loads, giving more variables like WT1, WT2, . . .  and L1a, L1b, . . .

Measurements are from the center of the hitch ball to each load (center of mass for each load).  If any of these are not present, just leave them out.  If you have more point loads, just add them in as illustrated.  Obviously, Tongue Length L2 has a big effect, so it is worth reading this article too about Choosing the Right Tongue Length.

Now we know the loading if the tongue has 12% of the load.  If that works for the tow vehicle, then we can move to the next step.

On the other hand, we may know that our tow vehicle can only handle 300# tongue weight.  By dividing the max tongue weight, 300#, by the total trailer load, 3000#, we get 300/3000 which is 10% tongue load.  That is on the margin, but it can work.  So,  FT = 300#  and we calculate  FA=3000#-300# =2700#.

Summing Moments (Load by Position)

Now we know the forces, the next step is to figure out where the axle goes so all the forces balance.  We do that by summing the moments – basically, summing the loads multiplied by their distance from the ball.  By setting the moments with forces up equal to the moments with forces down, we can solve for axle position.

For equation 2, please note that the distance from the tongue (ball) to the tongue load is Zero (so the multiplication of that Zero * FT = Zero).  We take advantage of that nice fact so we have only one variable to calculate – which is why FT is not in the equation.

From the example above, if we remove the toolbox, the calculated axle position changes to 100.9″
Or, using the example, if WT is 15% (instead of 12%), the axle position becomes 97.6″

When building your own trailer, run the calculations a few times with different load scenarios, then design for the maximums.  After evaluation, make a judgement call to choose the final trailer axle position.

That’s it.  Now you know how to calculate proper trailer axle position.

Trailer Axle Position for Multiple Axles

What is different for multiple axles?  Nothing.  Use the central position of the axle group for all the measurements and calculations.  Treat them as a single axle.  Note:  This simplification works for loading and trailer axle position on load sharing multiple axles, but not for stress or stiffness calculations.

What about torsion axles?  Use the location of the wheel center, NOT where the axle attaches to the trailer frame.  And that goes for any of the axle types and suspension types (like trailer air suspension, or an axle-less, etc.) where the wheel center is not at the same location the suspension attaches.

Proving Or Disproving The Common 60% Rule

On the internet there are several websites and YouTube videos saying to position the axle at 60% of the trailer bed.  What do you think?

In our example above, the trailer axle position is ~55% of bed length.  If we put it at 60%, the tongue weight becomes 501 Lbs, ~16.7%.  Or, if we take the toolbox off and went to 15% tongue weight, the desired trailer axle position is 65% of the bed.  These examples show that the 60% guess-of-thumb is not always best.

In some cases it makes the tongue load below 10% which borders on dangerous.  Other situations it can make the tongue load too high.

My Opinion?  Setting the axle at 60% is the lazy way.  In many cases it gives a reasonable answer, but why settle for a lazy guess when you can simply run the numbers and gain understanding.  Sometimes the answer is near 60%, but calculate and be confident.

All that being said, I recommend biasing the trailer axle position a little farther back for trailers where the load will change – like utility trailers.  It improves towing stability, maneuverability, and safety.  You can place the load appropriate for the conditions.  Just make sure you have the right materials and design safety factors for the loads you intend to carry.

And now you know how to calculate trailer axle position.  We invite you to read Trailer Axles 101 and some extra tips on mounting a leaf spring suspension.

Good Luck With Your Trailer Building Project!!

A Word About Center Of Mass

This seems to be a point of a little confusion, so here is a quick explanation on the difference between the Geometric Center and the Center of Mass.  Think about a Hammer.  The Geometric Center is approximately half-way down the handle.  The Center of Mass is where the hammer will balance – say on your finger.  That balance point is the Center of Mass, and that is much closer to the head.

The same is true of a trailer.  The main body of the trailer is much heavier than the tongue, yet the tongue also has weight.  So, the Center of Mass is not the Geometric center, nor is it the center of the trailer bed.  While it is nice to think about, the only real way to find the true Center of Mass is to balance the trailer on something like a pipe or 2×4 as mentioned above.

Perhaps most important with respect to trailer axle position, one of the key points for loading is the simple case of where do you put the load?  Obviously all the loading calculations above are pretty meaningless if you put things in the trailer in a different place.  So, if you are loading the trailer with “stuff”, position it so the tongue weight is appropriate.

Hopefully this explanation helps.  Have a wonderful day.