What Is Axle Overhang? How Much Is OK?

When you get into the details, trailer axles are not as simple as we often like to think.  Dimensions we most often discuss are:  Capacity (strength), Spring Centers (where they mount on the trailer frame), and Hub Face (distance between the wheels).  These all relate, and define the fit on your trailer.  So what is Trailer Axle Overhang?

For a good overview of axle basics, see our Trailer Axles 101 article.  We cover a bunch of the options and measurements for trailer axles – not only for fit on your trailer – also for ride comfort and stability.  In the end, it is the details that make them work properly, comfortably, and reliably.

So, our 3 big measurements for fit are:

1. Axle Capacity:  How much weight will it carry?  If the axle is too light or too heavy, it will not serve you well.
2. Spring Center Distance:  The distance between the centers of where the springs mount.  This is applicable in most cases to the centers of the beams where the axles mount.  (The term “Spring Centers” comes from Leaf Spring axles, but the mounting locations are applicable to all sorts of axles.)
3. Hub Face Distance:  Measurement from the face where one wheel mounts, to the face at the opposite end of the axle where the other wheel mounts.

Basics Relating to Trailer Axle Overhang

What is Axle Overhang?  It is the distance from the Mounting Point – Spring Center – on one side, to the Hub Face of the same side.  It is the distance for which the axle sticks out from the mount.  The easiest way to calculate it is:

Axle Overhang = ( Hub Face Distance – Spring Center Distance ) / 2

While the above description and equation is technically correct, “Overhang” is often discussed (and even published) as the Total Overhang for both sides.  That can be a little misleading.

For example –  IF:

Hub Face Distance = 75 inches
Spring Center Distance = 60 inches

Then:

Subtracting, we get:  75 – 60 = 15  which is the Total Overhang.
Actual Overhang (see image above), is half of the Total on each side.

Actual Overhang (each side):  15 / 2  = 7.5 inches
But, the published Overhang value is often 15 inches

Sometimes in published data it will be labeled “Overhang Per Side”, and sometimes “Overhang Overall” which does help clear things up.  I will not try to justify the tradition, just be aware that it can appear either way.

Why Is Axle Overhang Important?

There are 3 parts of Why.  First, the axle must stick out far enough so the wheels and tires have space to rotate.  We definitely need wheels, and they need space to move without rubbing the trailer.

Second, trailer axle overhang is part of what defines the capacity.  Axles have a design load limit they will carry.  At max capacity, strength of the axle beam will carry the load without exceeding stress limits.  However, if there is too much overhang, the max load will put too much strain on the beam and components.

You can think of it as holding a weight with your hand.  If you hold it close to your body, it is much easier (less stress) than if you hold it out at arms length.

To compensate, the manufacturers recalculate a lower max capacity if the trailer axle overhang becomes more than the published max overhang.  That means, capacity of the axle decreases.

Third, for trailer function.  With some trailers we want a short overhang – like when we are trying to maximize the trailer width within the legal limit.  Our 6′ 10″ wide trailers are a good example.

For other trailers we want a long overhang – like when we want oversize tires.  An example is our Off-Roading Jeep Trailer.  Big, wide tires increase ground clearance, and help with floatation in sand.  These big tires require a longer than normal trailer axle overhang.

In Summary, the axle must sick out (overhang) enough for wheel space and clearance, but not so much that axle parts become over-stressed.  Various trailer functions may also dictate how much overhang is good.

Axle Bearings

Ultimately, the reason for an axle is to allow the wheels to roll and carry your stuff.  That requires bearings of some sort.  At the center of the axle hub there are bearings.  Typically these are tapered roller bearings set opposed so they carry a lot of load straight up, and a little bit of load side to side.  The straight up capacity carries your stuff.  The side to side capacity carries your stuff around corners.

How do axle bearings relate to overhang?  When the axle has very heavy loads, it bows slightly.  That puts the spindle (internal part where the bearings mount) at a slight angle.  That means the bearings are not running together on a perfect horizontal.  Under normal operating conditions, this is fine.  However, if the overhang gets large, the added bow creates unwanted forces in the bearings.  If the condition persists, the bearings will have a shorter life.

With bearings and axle bending in mind, axle manufacturers specify an “overhang” distance that is acceptable.

As A Side Note, axles with Camber are not just for holding the crown of the road better.  They also help with the bowing condition.  When the trailer is fully loaded, the bearings are better positioned to hold the load.  (While the best reason for camber is tracking, bearing load is another nice benefit.)

Driving Stability

While this topic might be argued from many angles, the concept of narrowing the spring centers usually means a narrower trailer deck.  But, if the axle is wide, people can mistakenly feel like they have more stability in the corners than they do.  Applying trailer axle overhang in the extreme means the load becomes “tippy” even while the axle is stable.

Stability is not a big reason for avoiding excess overhang, but to be complete in our discussion, it is something to consider.  Obviously, the overhang will exceed the manufacturers specification long before you get into a super tippy situation.  Just keep this in mind because it comes in degrees depending on many factors, not just axle overhang.

Ratings With Axle Overhang

What happens if the trailer axle overhang gets to large?  Manufacturers derate the axles (lower capacity) as the spring seats move farther from the hub.  Each manufacturer has their secret calculations, but they all do it.  I spoke at length with Dexter one time, and they showed me a secret program to calculate derating due to increased overhang.  Super interesting as they ran some numbers.  Unfortunately, I do not know what equations they use.  They would not tell me.

Of course, we can calculate our own, and it might be close, but it would mislead because it is not their calculations.

Keep in mind that the capacity for every axle assumes half the weight is on each wheel.  If the trailer is poorly loaded such that significantly greater load is on one side, you might be overloading the one side – even if your total is less than the axle rating.

If you really need to know the derated amount, you can request information from the manufacturer.

As a matter of GROSS Oversimplification, you can use a rule of thumb of losing 10% axle capacity for every inch, per side, of added axle overhang (beyond the max specification).  So, if the axle is 3500 lbs capacity with 9″ overhang (max per side), then with 11″ overhang, the capacity drops 20% to 2800 lbs.

Again, this is a gross oversimplification, and on the safe side.  Taken to extreme, the axle will have a negative capacity, which we know is not correct.

The actual derating is non-linear, and it depends on many details like the actual axle beam.  These things make it hard to create a good rule of thumb, so if you really want to know, talk with the axle manufacturer (or their representative).

Potential Failure Modes

Wheel bearings are often the weak link in axle loading, and it gets worse if the overhang is large.  It is unlikely an axle will just bend (unless at extreme overload like if it hits something).  Also, bearings probably will not just shatter if you overload them, but when overloaded (or not properly lubricated), they will certainly fail sooner.

Bearing failures can be minutes, hours, days, weeks, or whatever, because bearings do not like overloading.  That can take many forms, but the most common is smoke, then wobbling, and if allowed long enough, the wheel falls off.

When we introduce a lot of trailer axle overhang, the potential for bearing failure increases.  Also, the potential for an axle tube failure.

Axle tubes are admittedly quite strong.  The one hiccup to remember – impact, like a pothole or bump – will apply a much higher load.  With bending, it only takes once going over the limit to create damage.  It might knock the wheel out of alignment, or it might bend the axle.  Either way, it is undesirable, so stay within the axle load limits – even if those limits derate because of extra overhang.

How Can I Get More Overhang?

If you have a trailer, or want to build a trailer with a lot more overhang, it is certainly possible.

I had a trailer for many years with about 20″ of axle overhang.  Yes, each side.  The trailer was narrow for carrying a motorcycle streamliner, but the trailer also served as the workbench for the machine so I did not have to work on it on the ground.  I needed to get all around it, including between the wheels and the deck.  And, I had a jib crane mounted just in front of the one wheel so I could lift the bike off the trailer and set it on the ground for racing.  The extra overhang was key to all of that.  (Plus it added a bit of stability when traveling.)

How do you safely have extra axle overhang?  We leverage the manufacturers derating factors.  For instance, we can use a 5200# axle, then put the spring centers in board (giving the larger overhang), but with the derating, treat it like a 3000# axle.  That is the simplest way.

If you want to strengthen an existing axle to carry the full load even with a larger overhang, build a truss under the axle, welded to the axle, going over the spring mounts.  Yet, unless you know all the engineering, it is hard to predict the improvement.  While that is feasible, this approach is a hack, so be careful with it.

What If I Want Less?

The short answer is you need enough space for the wheels and everything that goes with it.  Space for tire movement and flex.  Things like all the crap that sticks to the wheels – mud, muck, etc..  Also different tire sizes (not just the skinniest ones).

Secondly, if the overhang becomes too short, even if there is space for the wheels, the axle construction might get in the way.  Usually there are welds where the axle tube connects to the spindle assembly.  You do not want to add stress to the axle right at the weld by attaching spring seats there, or by clamping a U-Bolt right on it.

Yes, You can fudge the axle overhang some for your own trailer, but generally, I do not recommend making it smaller than the manufacturers published data.

Things are never as simple as they first appear.  Why would we think it is so simple to haul around thousands of pounds?  I marvel sometimes at the crazy tasks we ask of our trailers.  Yeah, they are made for it, but really, it is some bits of steel, some grease, and a little rubber.  Awesome.  But, there are limits to everything.

Practical Application w/ Trailer Axle Overhang

In the end, there is theory, then the practical application.

I let the manufacturers do the theory part.  They do a good job of it, and they tell us the appropriate range for trailer axle overhang.  That is application of the theory.

In practice you can often get away with pushing the boundaries, but doing so comes with risk.  If you think it looks right and if the trailer will not be carrying a full load, then it might work.  Or, maybe you can find a cool way to improve capacity.

Anyway, to me, DIY includes thinking about all the possibilities – which I do often – even if professionally I can’t recommend it.

What I do recommend is staying within the manufacturers limits.  They know the theory, and I personally like the practical side of avoiding failures.

Good luck with all your projects !!!