Single Trailer Axle or Tandem Axles
(for the Same Capacity)

Question from a customer:  “Do you have any articles that speak about single axle vs double axle advantages?”  Specifically, with the same load carrying capacity.  Something like a 14 foot trailer with a single axle @ 7,000 lbs – or – 2 axles at @ 3,500 lbs each.

That is a good question, and worth writing about.  Certainly from a simple load perspective, both the single trailer axle and the tandem axle trailer will do the job.  They hold the same amount of weight, and assuming the axle position is correct, the tongue load and dynamics will be nearly the same.

From another customer, we received a related note about a video of a similar topic.  I will not share the video because the info shared is misleading, and well, to me, a bit disturbing.  Where are the fact checkers for YouTube?  Oh well.

So, in this article we will take a look, a little deeper, at some of the small things that can make a difference in choosing axle(s).

Differences With A Single Trailer Axle vs. Tandem

The most notable differences when considering a single trailer axle, or tandem axles:

  1. Single trailer axle trailers are easier to maneuver in tight situations.  The tandem and triple axle tires scrub sideways, and resist twisting when the trailer turns tight.
  2. Multiple axle trailers spread the load a little more onto the trailer frame.  (3 mount points rather than 2 – for leaf springs.)  – Except for axle sets such as a walking beam suspension (like the Timbren Silent Ride).
  3. Multiple axles can make the ride a little smoother with load sharing, (and a lot more reasons).
  4. A single trailer axle offers more options in technology – like torsion axles for smoother ride.
    (It is a toss-up as to whether a tandem axle with leaf springs is smoother than a torsion axle single.  It depends on road conditions.  On washboard I think the tandem is smoother.  On one-off big bumps or potholes, the torsion is probably less jolting.  Tech like Axle-Less trailer suspension act much like torsion axles.)
  5. A single heavy duty axle is lighter weight that a pair of lower capacity axles.  Tires too, even though they are bigger and heavier, 4 smaller ones weigh more than 2 larger ones.
  6. A single trailer axle will have less rolling resistance, and less wind resistance.  (Though these are both relatively small effects.)
  7. Single axles are less expensive to build, easier to maintain, fewer parts, have inherent alignment from side to side, etc..
  8. For the same load capacity, the single trailer axle will require larger, heavier tires.
  9. Multiple axles gives more footprint on the ground.  On soft ground, like sand, that can be a slight advantage.

Now To The Myths

I have heard a bunch of very questionable things people say about the differences between tandem axle and single axle trailers.  It is worth repeating them here and looking at why we can categorize them as myths.

1.  Stability?

Some people say tandem axles are more stable, but as they say it, they are confounding effects.  Be really careful when listening to YouTube or sales people try to explain this stuff.

Tandem Axle TrailerFor the same length trailer, with axles centered at the same location, the same wind conditions, road conditions, and same load distribution, there will not be much difference.  This myth gains some value at the extremes of traction and high wind, which explains a little of why it persists.  However, it is really hard to test such a thing, especially with a long trailer, because tandem trailers are usually heavier, so of course they appear to have better traction.

There is nothing fundamental that makes a tandem trailer more stable.  It is much more about proper loading and the other stability factors.

For these reasons, I do not find the arguments very convincing.  I will say, however, it is much harder to bounce with multiple axles, so maybe that is something they feel.  Again, this myth is compounding effects.

2.  Reverse?  (With Single Trailer Axle vs. Tandem)

Some people say backing a single trailer axle is easier.  Some say backing a tandem axle trailer is easier.  Again, watch for the BS as they make up reasons for “Why”.  There are differences, of course, but there is also a lot of BS out there.

Single Trailer AxleOne of the biggest effects for backing is wheelbase.  With a greater distance between the rear wheels of the tow vehicle and the wheels of the trailer, then the easier it is to consistently back it into a specific spot.   Since most single axle trailers are shorter, Yes, you can argue that they are a little more difficult in reverse.  But that has nothing to do with how many axles.

I call it a myth, because the assumption includes mixed effects that are not directly related to the number of axles.

Maybe in tight reverse turns, wheel scrub for tandems can arguably enter the equation and make the tandem axle trailer slight more difficult?  But that is a tiny effect unless you are pushing the trailer around by hand.

3.  Tires ??

Some people claim that multiple axles help if you ever have a blowout or flat tire.  What do you think?

Also, tandems have twice as many tires, so there is an argument that more tires give more possibilities for a flat or bearing problems.  (As I see it, regardless of how many axles, if you get a flat, then you are stopped, changing a tire.)

Some people claim tires will last longer on a single trailer axle.  (Probably due to the inherent alignment.)  While this is often the case, it is not a function of one or many axles.  Issues with tire wear are related far more to proper axle placement and alignment.  For multiple axle trailers, the axles must align, or tire wear will be a problem.

Tire problems are much more a function of neglect.  If you keep your tires and axles well maintained, then tire problems are rare.

The one area where tires do wear more with multiple axles is during tight maneuvering.  The tires scrub side to side when the trailer turns tight.

Trailer Tire Failure

This is neglect, not because of the number of axles.

4.  Your Trailer Should Be Long To Carry Heavy Things

Where does this come from?  I will guess from some bad experiences (see below).  Yet, I am not even sure how to answer it because it really is not true.

Yes, there are some stability differences with heavy loads for short trailers, but that is more of a dynamics thing than a number of axles thing.  Most long trailers have multiple axles, so maybe that is where the confusion starts.  Again, it is important to consider what effects we are discussing, otherwise we confound the facts.

Here at Mechanical Elements, we have some short, single axle, fairly high load trailer plans.  For instance, this one is 6′ x 10′ with 7000 lbs. capacity!  And another which is similar, but 12′ long – also with a single trailer axle.  Build one of these at 6000 lbs or 7000 lbs capacity.  The choice is in the plans.  They are both excellent trailers, not long, yet carry heavy things.
6x10 High Capacity, Single Trailer Axle Plans

Summary – Tandem Axles vs. Single Trailer Axle

None of the above are singularly convincing arguments for me.  The choice for number of axles should be about strength, and need.

Adding up all of the above, we see the longer the trailer, the more advantage there is for spreading the load and for the larger footprint.  A longer trailer is easier to back up – assuming you have space to maneuver.

In the end, I do not think it is something to stress over.  If your design wants a single axle, go for it.  Just make sure the frame is sufficiently strong to handle it – especially if you use a torsion axle.  (But frame strength is important no matter what axle(s) you use.)

Please Note:  You can make either choice worse by poor implementation.  For instance, poor load distribution in a trailer will make it unstable, no matter how many axles you have.  Same for axle alignment.  If a single trailer axle does not align to the frame properly, or if the hitch is not on center with the frame, the tires will wear out faster, and the trailer is harder to control.

For a tandem axle trailer, if the axles align perfectly and load distribution is good, the tires will not wear as fast, and the trailer will be a pleasure to tow.  Yes, much of this discussion depends on the implementation.

Your Experience Matters

So much of what people think about trailers comes from their experience.  If they have had a tandem trailer with poor axle alignment, they will likely have a poor opinion of tandem axles.  Likewise a person who has towed a really well made trailer.

When new information is presented, always consider the source, and compare it to the logical side of what you already know.  Ask questions (like the customers who stimulated this article).

The key for me, when I don’t know the facts, I always question statements of “Why”.   Observations are often correct, but causation assumptions . . . . well those are another story.

To Video Creators

Yikes !!  As an engineer, I cringe when customers send links to videos asking if the information is true.  For the record, your observations are likely spot on.  Trust yourself.  On the other hand, trying to explain “Why” can go off into the weeds quickly.  A bunch of good info, mixed with bits of BS – will make a video pretty dangerous.  The dangerous part – the video sounds convincing, even as it misleads the audience.

My recommendation to video creators, PLEASE DO NOT make up reasons or try to explain things you don’t have the technical background to understand.  Thank you.

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