How To Mount A Trailer Axle Straight

Of all the important things to do when building a trailer, getting the axle(s) right is near the top of the list.  It’s the position on the trailer, yes, and proper alignment as well.  In this article we look at “How to Mount a Trailer Axle(s) Straight and True.”

Run Like A DogHave 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 – presumably to avoid hitting their feet.

When a dog runs, it’s hips will offset (to the side) slightly from their shoulders.

Well, if you don’t mount a trailer axle straight, your trailer will do that too.  Basically, the trailer frame will tow skew 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.

I’ve seen this effect while driving down the road a number of times.  I’ll be following a trailer, site down the road looking at the tow vehicle.  I see the vehicle straight with the direction of travel, but the trailer pulling behind at an angle, skewing to the side.  Here’s an exaggerated perspective — yes, it’s exaggerated for the illustration.

This is not a good thing for stability, or for fuel economy, or tire wear, or anything I can imagine. . . . .  Let’s get it straight.

Axle Not Straight
Exaggerated effect of pulling a trailer when the trailer axle does not mount straight. The trailer will not pull straight behind.

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.  Let’s limit this article to good axle parts.  Secondly, this could be a book if we look at all the variations of misalignment (like this one), so let’s limit the discussion to the plan view (looking down from the sky).

With a single axle, misalignment usually means the dog jog issue.  (Assuming good parts.)  Basically, the axle is not perpendicular to the trailer frame.

For multiple axles, that can mean the dog jog (where the axles are 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.)  None of these conditions are good.

Tandem Axles Skew
Exaggerated example of tandem axles not parallel.  These seem so obvious, yet minor variations of this are not that uncommon, and they cause trouble in towing.

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 so that the wheels themselves are not parallel.  Here’s a much more detailed article I recommend reading about mounting an Axle-less suspension.

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 let’s elaborate on what measurements to make.

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

Trailer Axle Placement

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.

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.  Let’s 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 just the front spring brackets positioning, but we do recommend actually 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.
    mount a trailer axle
  • Step 5 — Tack weld the two front brackets in place, then double check all the measurements.
  • Step 6 — Carefully measure back for the additional spring brackets making them the same on both sides of the trailer.  Tack them in place.
  • Step 7 — Double check all the measurements.  When satisfied, remove the axle(s) from the brackets, and weld the spring mount brackets in place.

This method also works 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.

Assumptions

In every process, there are caveats, and that is certainly true to mount 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 be mounted central on the frame.  We measure from a central point on the tongue, so it must be right 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 onto the springs.
  4. Accuracy in measurement.  The more accurate it is, the better the result.
Running Like A Dog
Don’t make your trailer run like a dog 🙂 (Image from tenor.)

It’s worth pointing out when Dimension ‘A’ won’t equal ‘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 or tongue mounting is not square or true.  If that is your case, bias your work to making ‘A’ = ‘A’ while keeping the brackets aligned on the frame beams.

What If . . .

If something is not perfect, 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 ‘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 are, so work to make them all match.  For me, when things don’t match, I figure out why, then make an assessment, before moving on.

If you are careful and take the time all through the trailer building process, it will come out straight and true.  Also, mounting the trailer axle, then the end result will be awesome.  Good luck as you mount your trailer axle straight.

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