How To Mount A Trailer Axle Straight

Of all the important things to do when building a trailer, getting axle(s) right is near the top of the list.  It is the position on the trailer, yes, and proper axle alignment as well.  When you mount a trailer axle correct, it makes a big difference for overall safety, tire wear, and towing stability. When the axle position and axle alignment all all correct, the trailer is so much more pleasant to tow.

In this article we look at How to Mount a Trailer Axle(s) Straight and True.  Use the steps in the guide below to build your trailer right.

Does Your Trailer Run Like A Dog?

Have you ever followed a trailer that seemed to roll down the highway like a running dog?  This is not an insult to dogs, they just have a strange way of running – with their hips offset to the side – presumably to avoid hitting their feet.

When a dog runs, watch the hips.  They will offset (to the side) slightly from their shoulders.  If you watch, you will see some trailers do this too.  (In fact, some vehicles do this, but that is another story.)

If the trailer axle (or axles) does not mount to the frame straight, the trailer will run like a dog.  Basically, the trailer frame will tow askew to the tow vehicle.  The axle will align with direction of pull, even while the frame is at a bit of an angle.  See the exaggerated example in the image below, and note the angle of the axle.

You can see this effect on the road.  When following a trailer, sight down the road looking at the tow vehicle.  You can see the vehicle straight with the direction of travel, but sometimes the trailer pulling behind is at a slight angle, skewing to the side.

The image below is an exaggerated perspective.  While it is an exaggeration for the illustration, the concepts are important.  This is not a good thing for stability, or for fuel economy, or tire wear, or anything I can imagine. . . . .  Really, it is time to re-mount the trailer axle to make it straight.

Enumerating the Problem

There are a host of issues that come up in this discussion that we are not going to address here.  Things like a bent axle, or a damaged trailer mount are discussions for another time.  We will limit this article to good axle parts.  Second, this could be a book if we look at all the variations of possible misalignment (like this one), so we will limit the discussion to the plan view (looking down from the sky).

Finally, we will limit our discussion to avoid situations where the axle spindles do not align.  This condition is unfortunately all too common with rigid axle stubs, and independent sides like the Axle-Less suspension.

With a single axle, misalignment usually means the dog running issue.  (Assuming good axle parts, and tight axle mounting.)  Basically, it means the axle is not perpendicular (true) to the trailer frame.

For multiple axles, it can mean the same (where axles are parallel, but not perpendicular to the frame).  Or, it can mean the trailer is trying to run circles while you are pulling it straight.  (This happens when the axles are not parallel with each other, like in the image below.)

Then there are the “axle-less” trailer axles which pose another level of difficulty in alignment.  With axle-less, it is very easy to mount the trailer axle in ways the wheels themselves are not parallel.  While this is a more involved discussion, it is worth the note here.  If this is of interest, I recommend reading the article about mounting an Axle-less suspension for a lot more detail.

Mount a Single Trailer Axle Straight

In the process of building, there comes a time when you need to mount the trailer axle.  So, you set it up, set all the brackets on the trailer frame, make the measurements and weld them in place.  Yes?  Pretty much right, but we will elaborate on what measurements to make.

Trailer plans will usually show where the axle goes with a drawing something like this.

Maybe the plans will also show a top or bottom view so you get more perspective.  Anyway, we always think about making it all straight and true (like the pictures), but how do we actually get it that way?  After all, there are more ways to do it wrong than to get it right.

Squaring Up With Diagonal Measurements

One really easy way to help mount a trailer axle straight is by measuring in multiple directions.  Sure, left and right, and front to back.  Then, also measure diagonal.

If the trailer frame is square (meaning the sides are parallel and the corners are perpendicular), then the method shown in the image below will work.  The image shows front spring brackets position, but we do recommend setting them in place with the full axle assembled.

• Step 1 — Assemble the axle(s) with the springs and all the brackets attached.  Make sure everything is the same on the Left side as on the Right side.
• Step 2 — Read this article on Mounting Trailer Spring Brackets.  There is more to it than simply welding them on.
• Step 3 — Set the full axle(s) assembly on the trailer frame (usually with the trailer frame up-side-down).
• Step 4 — Focus on the position of the Front Spring Brackets, left and right.  Fiddle with the positioning until the dimensions B = B and A = A.
  
• Step 5 — Tack weld the two front brackets in place, then double check all the measurements.
• Step 6 — Perform the same type of measurements to the axle hubs.  They are really the most important, but we do spring mounts first for a known basis for the axles.  (See notes below about Final Position.)
• Step 7 — Carefully measure back for the additional spring brackets and axles making them the same on both sides of the trailer.  Tack them in place also.
• Step 8 — Double check all the measurements.  When satisfied, remove the axle(s) from the brackets, and weld the spring mount brackets in place.

Final Position To Mount The Trailer Axle

The steps above also work for setting a Torsion Axle.  While the key is making the dimensions B = B and A = A, it does not matter to what point you are measuring, as long as it is to the same point on both sides of the trailer.

Repeat the above dimensions for the trailer axle hubs as you mount the trailer axle.  We apply the same type of measurement to the axle hub instead of the spring mounts as illustrated above.)  See Step 6.  It adds one more level of complexity because we must center the axle both side to side, and on the springs.  If you buy an axle with the spring seats already welded on, they SHOULD BE straight, true, and centered, but measure to be sure.

1. If the spring hanger brackets are positioned correct, then we have a good place to measure spring position.  Check that the springs mount to the axles fore and aft the same both sides.  Most have some type of alignment at the spring seat, like a pin, which should make it easy.  If they are not the same for both springs, make some adjustments.
2. The axles must center side to side on the trailer.  Again, if the spring brackets on the trailer frame, and the spring seats on the axle are set correct, then the axles should align easily, but check anyway.
3. When the measurements for A and B above (applied to the axle hub instead of the spring mounts) are the same, the axle alignment will be good.  Repeat for all the axles (if you have multiple axles).

Assumptions

In every process, there are caveats, and that is certainly true for mounting a trailer axle straight.  Here are a few.  Get these right FIRST, then mount the trailer axle.

1. The trailer frame must be straight and square for this method to work right.
2. The tongue must mount central on the trailer frame.  We measure from a central point at the end of the tongue, at the frame center.  The tongue must be correct (center), or it is very difficult to get dimensions A = A.
3. Axle parts must be the same on both sides.  Things like brackets, spring lengths, and where the axle connects to the springs.  Symmetry really helps the trailer axle to mount straight and true.
4. Accuracy in measurement.  The more accurate the measurements, the better the final result.

It is worth pointing out that when Dimension A will not equal the other A, while B = B or vise-versa, it indicates that one of the items in our assumptions list is not correct.  Usually something about the frame is not square, or the tongue not really in the center.  If that is your case, bias the effort to making A = A, while keeping the brackets aligned on the frame beams.

What If . . .

If something is not perfect when you mount a trailer axle, then think about this hierarchy for the positioning.  Having the brackets sit side to side properly on the frame rails is 1st most important.  Matching the two ‘A’ dimensions are 2nd most important.  Matching the ‘B’ dimensions are 3rd most important.

Please note, this does not mean A or B are not important.  They both are, so work to make them all match.  For me, when things do not match, I spend some effort to figure out why.  Then, I make an assessment, and fix it, before moving on.

Building Notes:

If you are building a trailer from our plans, the instructions document talks a lot about how to build the frame so it is straight and true.  This includes the setup procedures as well as the welding techniques – and when to weld.

If you are careful and take the time all through the trailer building process, it will come out straight and true.  Measure a lot as you build.  Use good welding technique to avoid “weld pull” out of true.  Take care in mounting the trailer axle, then the end result will be awesome.

One of my old favorite quotes:  “Quality is FREE if you do it right the first time.”

Mount Multiple Trailer Axles

We do need to take a moment and quickly mention some added complications with multiple axles.  Though we have shown them above, Tandem axles, and triple axles add levels of complication.

When you mount a single trailer axle, you measure to the front set of spring hanger brackets, and to the hubs of the axle for alignment.  When mounting multiple axles, do the same for each set of axle mounting brackets, and each axle.  Also measure parallelism – first axle to second, and first axle to back axle.

For a triple axle situation, the middle axle does “float” a little.  As the equalizer rocks up and down, the middle axle will move fore and aft just a little, so it can be hard to assure it is parallel.  On the road, there are very few times when the equalizers (left side and right side) are not in a similar position.  Usually only in transition states like going over a bump, so middle axle float is not usually a problem.

As illustrated above, the goal is to get the axles all aligned parallel, centered, and true to the trailer frame.  This does take some time, but it is well worth the effort.

Good luck as you mount your trailer axle straight.