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’s 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’re 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’ve 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.  5th Wheel (and Gooseneck) trailers are mentioned, but are not the focus.

To determine trailer axle position, we must first understand what drives the decision.  The Goals:

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

There is a lot that goes into 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 tow vehicle limits.  Enough said.
2. In general, more weight on the tongue is better for stability.  Example:  Over-the-road trucks (ie., a Semi or Lorry) trailers have roughly half the trailer weight on the hitch.  It works because the tow vehicle is built 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’ll dive into this in a future post.)
4. For rear connecting trailers (traditional), weight of the tongue should be in the 10% – 15% range of total trailer weight.  (10% minimum, 12% is OK, and 15% is great.)

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.  What you, as a driver, feel is a 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 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

Of course, the easiest way to know tongue weight is to measure it.  Most of us don’t have a scale that will go high enough, so take the trailer to a vehicle scale.

For an existing trailer, load it for travel.  (This is difficult for utility trailers because you never know what will be hauled.)  Drive onto the scale with your vehicle enough that just the trailer wheels are on the scale.  Take the 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.

When building a trailer, one of the easiest ways to measure is to set the axle in place under the trailer, but don’t permanently mount it.  Clamp it in place, then measure as above.  Move the 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, for instance.

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

That said, using the measurement technique can help when setting 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 out to give proper tongue weight.

Calculating Tongue Weight And Trailer Axle Position

We can also calculate tongue weight and trailer axle position.  I’ll give an example, then if you need more detail, please contact us, or do some searching on the internet.

We use a balanced lever approach to calculate loading.  First, we sum forces in a vertical direction.  There are only 2 points that support the vertical loads (tongue and axle(s)).

• Trailer Weight — WF — Weight of the trailer including frame, sides and flooring can be measured or calculated pretty easily.  You will also need to know the center of that weight.  (Measure this by placing a board on edge under the trailer frame, then move it till the frame balances on the board.)
• Evenly Distributed Load — 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.
• Points of Specific Load — WT — These are big loads at specific locations.  For a utility trailer this may be an ATV, or lawn tractor, or tank, or toolbox.  For a Tiny House, this may be water tanks or battery packs or kitchen cupboards.

Measurements are to the load centers.  If any of these are not present, just leave them out.  If you have more point loads, just add them in as illustrated.

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, let’s say 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 and that yields 10% tongue load.  That is on the margin, but can work.

Summing Moments (Load by Position)

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

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.  Design for the maximums.  After evaluation, make a judgement call for 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 frame.

Proving Or Disproving The Common 60% Rule

On the internet there are several websites and YouTube videos saying to place 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 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.

And now you have the trailer axle position figured out, here are some tips on mounting a leaf spring suspension.

Good Luck With All Your Trailer Building Projects!!

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