Should trailers have suspension?

Hiyas, I'm starting to think seriously now about my trailer for a circuit of Oz in 2017. I'm looking at either buying ready-made or a custom build and placing a cheap cargo pod on type. There are various options for bike trailers for carrying bikes (just a frame and runners), and I'm wondering about modifying one of those. However, they typically don't have suspension.

As a result, I'm wondering about the effectiveness of suspension on a lightweight trailer. It seems that the smallest suspension load that can be bought is around the 250kg mark - probably more than double what I'd expect to be towing. I'm thinking that the suspension wouldn't actually be loaded much and would just bounce around behind the bike anyway. So, for those of you that have trailers, do you find you get much movement/travel in the suspension or does it actually bounce?

Cheers, Rob.

What happens if you hard mount an axle on your trailer – without a suspension?  The question has come up a few times, and it’s worth talking about.  There are some distinct advantages of eliminating the suspension, but there are also some pitfalls.

On this website we discuss a lot about various axle types, which we usually distinguish by the suspension system.  For instance, leaf spring axles, torsion axles, even axle-less.  We have a whole article on axle basics – Axles 101.  But, what about a trailer axle without suspension?  (We call it Rigid Mount.)

How To Mount A Trailer Axle w/o Suspension

While there are various ways to approach an axle without suspension on a trailer, there are 2 most common.

• The first is attaching a full length axle.  It can be welding the axle direct to the frame, or by making brackets (which weld to the axle), then bolt to the frame.  (I prefer bolting so the axle can easily remove for repair or replacement.)  A full length axle assures the spindles are straight and true (one to the other), because the axle manufacturer builds them that way.  No worries about wheels alignment.  It is also good for accurate camber (if you choose it) because that is also set at the factory.
• A second way is welding spindles to the trailer frame.  (Often called “Stub Axles”.)  You can purchase separate spindles in many varieties similar to the ones shown here.  (One of these has a brake flange, the other does not.)  These can weld right to the trailer frame.  The catch?  You must find a way to assure they align and are not only straight with each other, but also with the trailer frame.
  

Rigid Mount Advantages?

There are several nice advantages for eliminating the suspension from a trailer axle.

1. Simplicity.  Without a suspension there are fewer parts, so less to maintain.
2. No Motion.  Normally we allow space for axle and suspension movement.  By eliminating suspension, the wheels don’t move up or down with respect to the trailer frame, so fenders and other parts can mount close without worry.  We see this sometimes with special show trailers.
3. Low Deck.  Suspension takes space.  If the space requirement is not there, then the trailer deck can be lower.  While there are low suspension options, they all require more space than rigid axle mounting.
4. Really Cheap.  Without suspension, rigid axles are pretty cheap.  Less cost for parts, so less cost to build the trailer.  (Please note, however, the extended cost in the list of disadvantages below.)
5. Axle-Less.  Separate spindles offer freedom in trailer construction.  You can create an axle-less trailer with a lot of ground clearance.  Or, build in an open area between the wheels where the deck can drop to the ground.  (Some motorcycle trailers are built this way for easy loading.  See below.)

Let us know in the comments if you have other advantages we’ve missed.

What are the Disadvantages?

Admittedly, some of the advantages are quite attractive for the right situation.  But, what are the risks of pulling a trailer without springs?  Unfortunately, there are some disadvantages, like these:

1. Rough Ride.  Yes, without suspension the trailer feels every bump.  This goes right to the trailer frame, jolting the load and jostling the tow vehicle.
2. Bouncy Ride.  One job of the suspension is isolating action from the road.  Tires are part of helping with bumps and normally work with the suspension.  However, without suspension, tires are like basketballs which bounce when the trailer does not have a heavy load.
3. Prone to Failure.  Rigid mount axles put more stress on the spindle.  With nothing to help shock loads, the axles, bearings and frame take the brunt, and can fail.  Failure can be bending (especially if there is a bracket), but more likely a spindle break, or bearing destruction.  Unfortunately, a spindle break will let the wheel leave the trailer to roll off and hit who knows what.
4. Cost of Repairs.  While building rigid is cheap, it can get super expensive if you break something.  – Especially if far from home.  (Not only expensive to repair, but expensive liability if a wheel goes rolling.)  While this is true of every trailer, breaking things is more likely for a trailer axle without suspension.
5. Load Reduction.  Trailer axles rate for a particular load with suspension.  Dexter says the load capacity is one half for a trailer axle without suspension (rigid mount).  So, a 6000 lb axle is good to carry 6000 lbs with suspension, but only 3000 lbs rigid mount.
6. No Load Share.  Most applications of rigid mount are single axles.  That makes sense.  However, in tandem or triple, the axles won’t share at all, so any attitude other than perfectly level makes one axle carry most or all of the load.  So, the rating of the trailer – no matter how many axles – is half the rating of one axle.  See the example below.
7. Sensitive to Attitude.  When multiple axles are rigid mount, the trailer becomes very sensitive to position attitude.  If the tongue is up slightly, the rear axle takes the load.  If the tongue is down, the front axle takes it.  This also make trailer stability suffer because the effective load balance changes drastically as the trailer goes over a bump.
8. Alignment is critical.  Separate spindles must align or the wheels will fight each other – as in this previous axles article, and the article about axle-less suspension.

Unfortunately, these disadvantages compound, meaning they add up.  That makes them very important.

One More . . .

While not technically just for a trailer axle without suspension, we often see this as a disadvantage with rigid mounting.

9. Overhang.  It is tempting to use a rigid mount axle to extend width.  Like the motorcycle trailer photo, top of the page.  I understand the temptation because extra width can benefit many things.  However, axles are made for connections to springs (or other mounting) within a short range from the wheel (hub face).  If the axle extends beyond the manufacturer’s specified distance (overhang), then they de-rate capacity.  For instance, if the max overhang is 19″ (that’s 9.5″ on each side), then extending beyond that, means the axle will carry less.  —  I believe the failure mode is bearings, not usually bending or breaking the axle.  As a high force is applied, the axle bows a little, and that puts extra stress on the bearings.  (I mention it here because it is more common to see it on a trailer axle mounted without suspension.)

Again, if you have something to add, we want to know.  Please leave a comment.  We want to hear your insights.

An Example:

Consider a trailer with 2 rigid mount, 3500 lb. axles.  Without load share, there are many times when only one axle will carry the entire load.  Also, with rigid mounting, the axle rating is half.  So, this trailer with 2 axles has a true load rating of 1,700 lbs . . . NOT  7,000 lbs.  We get half of one axle for load rating instead of the combined capacity of two.  If someone chooses to ignore this, then disadvantage #3 above, “Prone to Failure”, will very likely come to visit.

In this example, the rough ride becomes rougher as the loading effectively shifts from one axle to the other as the road undulates and bumps.  This happens because attitude of the trailer shifts a bit, and loading to the tongue shifts with it.  At some points the trailer may travel quite stable, then suddenly it can become unstable just because the surface of the road changes.

These effects become more pronounced as speeds increase.

Our Take:  Axle Without Suspension

There is a time and place for every possibility.  While I do not suggest a trailer axle without suspension for most highway applications, there are many good ways it is useful.

Hey, we often see small trailers (to pull with an ATV) or farm trailers without suspension.  Many, if not most, bicycle trailers go without suspension.  On the heavy end, there are many mining trailer applications and cargo waggon trailers without axle suspension.  (The image is a link to these two.)

These work great for what they do – low speed, with balloon tires (which act as suspension).

While there are many other applications, we will highlight two, below, in more detail.  These come from customer questions.

Here are 2 other good examples.

Motorcycle Trailer Example

Several people have asked about low show bikes.  Others have asked about low race cars and the difficulty in getting them on a trailer.

If you need a low trailer for hauling something that does not load easily, perhaps this can work.  We’ll explain with a motorcycle example.  Recognizing that the motorcycle has its own suspension, using rigid spindles can achieve the low trailer.  Remember, the axle capacity is half, so you could put 2000 lb spindles (one 1000 lb spindle on each side), then call the trailer good for 1,000 lbs.  Such a trailer may not ride well on the highway due to bouncing and jostling, but it might work well for local trips.

Have you seen the fun trailers that lower the deck platform to the ground with a hand crank?  That makes for really easy loading of a motorcycle.  Mounting separate trailer axle spindles without suspension is one way to accomplish it.

While it can work, there are some hiccups.  Stability and ride quality.  You might not want to subject your motorcycle to all the extra bouncing.  Another issue with any low deck is the possibility of dragging the end when the road undulates – like in and out of a driveway.

With a light load and a single axle it is probably not so much of a concern.  It will accelerate tire wear, which, for that kind of a trailer perhaps it doesn’t matter much.  To enhance things, use wider trailer tires, bigger tires, and low air pressure to accomplish some semblance of suspension.

Off-Road Trailer Example

This may seem a little counterintuitive because of the rough ground, but for the right situation it works.  I will point to the YouTube personality at Matt’s Off Road Recovery.  He has a small trailer they use for UTV recovery.  Because of the harsh action this trailer sees, they have had a lot of trouble with suspension.  (I personally think it was using the wrong application of the suspension – which I did offer to assist them with – but that’s another story.)  To solve the issues, they went to rigid mount axles – tandem.

Of course tandem axles in rigid mount is normally a big No-No, but in their case it works.  They want the greater floatation of 4 wheels on the sand.  (Note that the soft sand helps share the load.)  The double axle also helps them crawl over rocks because it effectively increases both the approach and departure angles for the trailer.  And, the UTV’s they carry have their own suspension (or they are crashed so it does not matter).

Rigid mounting allows a fairly tight clearance to the deck-over to keep the deck low.  (For a low CG.)  So, for a situation like this, if each axle is capable of twice the entire load, then it works.  (Twice because of the reduced capacity for rigid mounting, and each axle twice because the trailer is often on only one axle.)  Actually, in their case, I’d go even higher than twice because of the severe shock loading the trailer sees sometimes.

Summary

So that’s it for trailer axles without suspension.  Some of you have asked the questions, and this is the answer – from the Mechanic.  The rigid mount has some applications, but generally I’d discourage it.

I’m pretty sure there are people around that have rigid mounting and love it.  They probably drive all around and have no problem.  If that’s you, be happy and know you are the exception.  Most people with rigid mount axles end up replacing broken parts.  Most people find pulling a trailer without suspension is very annoying because it will jolt and bounce so much.

It’s all good, use the concepts here as you see fit.  There is a time and place for every possibility.

Now, from the rigid to the cushy . . . try this article about Air Suspension.  Good luck in all your travels!