Shock Absorbers For Trailers?

For good ride quality in your car, Shock Absorbers are super important.  So why don’t they use more shock absorbers for trailers?  Do they make a difference with trailer ride?  These are good questions that need a some background, so we will dive in and talk through it.

Books are written about suspension dynamics and about things that make a good suspension, so we cannot possibly go into all the details.  However, we will take a broad overview and offer some perspective.  Please forgive us if we gloss over some things in the process.

What Is A Shock Absorber?

In simple terms, a shock absorber is a passive element of a suspension system that resists motion.  “Passive” meaning that it does not create action, it only reacts to action.  As the suspension goes up and down, they “absorb” a little energy so the suspension movement cannot start “bouncing”.  The amount of resistance depends on speed of the movement.  Fast motion gets more resistance than slower motion.

Shock Absorbers Or Combined
Typical Shock Absorber (Left), and Combined Shock Absorber with Load Carrying Element (Right).

Again, in simple terms, shocks have oil in them (hydraulics) and as the suspension moves, a piston pushes volumes of oil through small holes.  The oil goes back and forth through a series of holes as the suspension goes up and down.  (Super simplified explanation.)  The resistance is usually different for the up stroke and the down stroke because the oil goes through different sizes of holes, up and down.

Shock absorbers do not carry load.  They resist motion.  They slow things down.  This action helps to keep the tires always on the ground, and smooth the ride.

Understanding shock absorbers can get confusing when they combine with other functions.  For instance, when they also include load carrying element like a spring (for struts in a FWD car), or an air leveling function (to lift the rear of a saggy vehicle).  While these are sometimes mislabeled as shock absorbers, they are actually a combined function of both shock absorber and load carrier.  For convenience in this article I will call these multi-function parts “Struts”, though that is not always technically correct.

How Trailer Suspension Works

Suspension systems have a load carrying element.  This is the portion that supports the weight, and usually it flexes or gives as the tires go over bumps.

For a leaf spring trailer suspension, it is the leaf springs.  With a Torsion Axle, it is the rubber that compresses to hold the load.  In an Axle-Less suspension, it is the rubber blocks.

Whatever the type, the load carrying element compresses more as we apply more load.  Fully loaded versus empty, for instance.  When we drive over a bump, there are extra forces (momentarily), and the suspension compresses more.  When the bump is over, the suspension rebounds back out to the load needed.

Bumps and such usually happen really fast when you are driving, so the suspension must react, then rebound fast.  This gets complicated because the rebound can be too much, then it compresses again, then that is too much, so it rebounds again, and it can all become a “bouncy” cycle.

How Can Shock Absorbers Help A Trailer?

Shock absorbers act to resist the suspension motion.  So, instead of things getting bouncy, the shock absorbers will slow things down just enough that it kills the bouncy effect.  That is simplified, so if you want more information, try this Wikipedia Article about Shocks.

There are 2 things that can make a trailer “bouncy”.  The first we spoke about in our Trailer Tires article.  The second is spring bounce as described above.  If you have spring bounce, then yes, shock absorbers can help with ride quality by quelling the bounce.  Shock absorbers do NOT soften the ride like a soft bed to sleep in, instead, they damp the oscillations so the reaction to bumps are not as severe.  That is a really important distinction.

Sometimes we talk about a softer ride with Air Suspension or with Torsion Axles.  That is more like the soft bed.  As shock absorber is different because it does not carry the load – it reacts to suspension movement.

But, before running off to buy shock absorbers, we need to learn a little more about a normal trailer suspension.  Like why trailers typically do not have shocks.

Why Don’t Trailers Usually Have Shock Absorbers?

We can look at the 2 generalized suspension types – flexural and compressive.  Flexural is like springs, where the load carrying member flexes.  Compressive is like rubber a suspension where the rubber deforms or squishes.

For flexural suspension, the load is supported and released easily.  Especially for coil springs, they will take and give the force and movement without much loss.  In these cases, like coil springs, shock absorbers can really help with suspension control.  (Most cars have coil springs, so they need shocks to control the ride.)

On the other hand, leaf springs do not need so much help because the inertia of the system and the internal friction between the leafs tend to damp the oscillation.  This is especially true for tandem and triple suspension, because there is so much going on it is hard to set up a resonance.

For compressive suspension, the rubber has a natural hysteresis.  Hysteresis is a technical word for resistance to movement – which is the same as a shock absorber.  It damps the travel naturally so things don’t happen as fast (not as fast as a spring), so they do not need a shock absorber.

Please note, different types of rubber have different Hysteresis properties.  In rubber trailer suspension, hysteresis is part of the design.

Another important Compressive Suspension type is Air.  Air compresses, but does not have a natural hysteresis.  So, air ride suspensions, like the coil spring suspensions, can really benefit from shock absorbers.

One Other Big Reason . . .

. . . for Why trailers mostly don’t have shocks . . . People do not ride in trailers (at least they are not supposed to), so super ride quality is not needed.  (The dirt, or rocks, or trash, or lumber you carry in the trailer don’t care about ride quality.)  An RV trailer is another matter, but still perfect ride quality is not as important as in a car where people feel it.

What About Tandem And Triple Axles?

Some questions I receive, ask about adding shock absorbers to torsion axles so they can function in tandem or triple.  Unfortunately, the shocks  will not affect the problems with Tandem or Triple Torsion Axles.  Torsion axles (just like springs) compress more with more load.  If the ground is uneven, or it it becomes uneven (like rolling over a bump) then the axles carry a different amount.  The more compressed one is carrying more load, often overloading.  Shocks do not change that.

As a side note reminder:  With a mechanism like the equalizer between sets of leaf springs, the suspension is self compensating by changing the “height” of the spring mounting so the wheels can be a different heights yet carry the same load.  The same is true of the Walking Beam styles of suspension.

Air ride does this different so achieving the load balance is different.  With Air, if you plumb it correctly, a nice load balance is easy to achieve.  The down side of air suspension is standard components are not readily available, and the systems are expensive.

Air can be very quick responding, like the coil spring, so shock absorbers are a good idea.  For tandem and triple axles with air, some system damping can happen with the right orifices in the connecting lines.  However, getting the orifices right can be a big chore.  Use needle valves if you are attempting to do this.  Even at that, with the large air bags, a good shock absorber is often needed to keep oscillations under control.

An Engineering Perspective

First, I am not an automotive suspension engineer.  I have designed areas of engines and transmissions, but only 2 suspension systems (plus a bunch of bicycle suspension), so take this next thought with that in mind.

If I really wanted to slow things down for a leaf spring tandem or triple suspension, I would build new equalizers, and attach the shock absorber to the equalizer.  Often the equalizers do a lot of rapid motion, so slowing equalizer action seems like a good way of damping suspension movement.  Also the forces there are lower, so the shock absorber would not need to be as large.

Could we damp both axles with one shock absorber (one on each side)?  I think so, but I have not tried it.

Substitutes For Shock Absorbers

For most people that ask, the questions about using shock absorbers for trailers is mostly about making the trailer ride better.  For that, we concur.  There is always room to improve trailer ride quality.

Trailer suspension is actually pretty archaic compared to automobile suspension.  But, I do not think that matters.  It does well for the job we ask of it.

While we can certainly use something far more complicated and expensive, there is really not a compelling reason for it.  That said, we can make improvements even without all the sophistication and complexity of modern automotive suspension.

For leaf spring suspension, the easiest approach is adding rubber.  For tandem and triple axle trailers, that can be as simple as adding a rubberized equalizer.  Many styles are available.

Shock Absorber Substitute

These are 2 from Lippert.  I have not used them, so don’t take this as a referral, but it is something to explore.  Dexter and others also make similar products.  Actually, the Dexter E-Z Flex Equalizer is one of the simplest, and I like the construction.

Spring Helpers Instead of Shock Absorbers

Trailer Spring HelperAnother class of products to help trailer ride are spring helpers.  They come in a wide variety of options from rubber blobs (in various shapes), to bolt-on helper leaf springs.  While these all do things a little different, they have some common functions.

The image here is Sumo Springs, and it is advertised to reduce trailer suspension vibration.  In many ways it acts like shock absorbers to help improve the ride.

It is important to note that these also carry some of the trailer load (just like adding a spring).  See the comments about struts, both above and below.

I think these products have a place, especially in maintaining ride height, but please recognize that they also limit suspension travel, and thus effectively stiffen the ride.  They can help with reducing vibration, and they do it by taking some of the load off the springs.  So, you will get less vibration, but also less suspension travel, and a stiffer net suspension effect.

One more thing I will point out.  For multiple axle trailers, like tandem and triple, items like this (and other spring helpers), will decrease load sharing for the axles – which is bad.  When the road is not level and flat, one axle will carry more of the total, which means you can overload one axle.  Just think carefully about the products you add to be sure they are not creating new problems even as they are fixing something else.

Products Sold With SCAM Claims

Misleading Equalizer InfoThere are a bunch of other damping and softening products on the market to improve the ride of your trailer.  Some are definitely better than others, so look closely before spending your money.

I have seen a bunch of products that appear to be good, but somehow the manufacturer gets carried away in the marketing making scam claims.  That is too bad, because to me, if the claims are unrealistic, it diminishes my desire for the product.

Please watch out for scam advertising, because there are a lot of false claims floating around.  See this image and my notes off to the side for example.  It does add a little suspension travel, but minimal.  If it did add 3″ travel, it would also have to give your trailer a 3″ lift, (or your axle would hit the frame).  And, if it did add 3″ travel, it would all be softer than the springs, would make your trailer very wobbly.  (As in rocking side to side, which would make going around corners quite scary.)

So, I am not saying this is a bad product, but I am saying think about claims like this that are not true.

Your search may start with looking for shock absorbers, but that is not usually the best place to end – at least for trailers.

One Point To Note

All of these suspension enhancements to improve ride also increase travel just a little.  A little, not 3″ more as in the above Scam statement.  Make sure your suspension mounting brackets are tall enough to allow a little more travel.  You do not want to end up like this trailer problem.

Normally you should have 3.5″ or so from the wheels to the fenders, AND from the axles to the trailer frame.  You will want more than that if you add these soft rubbery suspension pieces.  The amount more depends on the actual movement of the rubbery parts.  We do not want parts to hit.

 


Side Note About Struts

Yes, adding a load carrying member (like struts or overload bumpers or helper springs) will change a suspension.  Maybe not the way some people think.

We don’t normally suggest adding extra load carrying “helper” elements to a trailer suspension  It is like increasing suspension capacity without changing the wheel bearing load capacity.  For most trailers these items will limit suspension travel (less travel), making it a rougher ride.  This, because it requires more load to compress.  Yes, these items can help increase trailer ride height, or to help it “squat” less.  Things like the Sumo Springs above can help with vibrations, like shock absorbers, too, but there is a trade-off.

In light of the article above, if we want to soften the ride, we really want more compliance, not less.  Usually that means more travel and more sensitivity in compliance, in the right ways, not less.  (Things like longer leaf springs.)

Greater compliance helps to avoid wheel lift and load imbalance, which is great.  It also helps to achieve a smoother ride.  So, consider carefully the full effect of things to add, not just a few advertising points.  Struts and other capacity increases definitely fit this category.  Sometimes shock absorbers too.

Good luck with improving the ride of your trailer.

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