I see a lot of comments on websites and hear references once in a while about the wonders of independent trailer suspension. What do you think? Does it deliver on the hype? Or is it a sales hoax? We will look at some video and think about the engineering to find out. What are the real benefits, and what are the fallacies?
Call this the “Myth Busters” approach to looking at independent trailer suspension. We will do some physics, then let you decide if it is actually worth it.
Unraveling The Jargon
Here are two related terms we often use / misuse / interchange. The First is “Trailer Suspension“. The Second is “Axle“.
Trailer Suspension is the flexible connection between the trailer frame and the axle. It may have springs or air bags or rubber globs, or a combination of things. It may have arms, or levers, or be more direct. Either way, it is there to carry the trailer weight and absorb some of the road irregularities.
So, what is an Axle? Trailer wheels contact the ground, and rotate on something we call a spindle. (Basically the bearing assembly or “Hub” that allows the wheel to spin.) Technically, that spindle is the “axle” for the wheel because it defines the center about which the wheel spins. However, there are other ways we use this word too.
(Yes, that means the Axle-Less Suspension is false advertising. But, don’t get hung up on that. Hey, hub-less wheels have a better claim to axle-less, but even they have a secondary axle.)
Traditionally, spindles on both sides of the trailer connect by a bar — which we often refer to as the “Axle” because spindles attach at each end. Sometimes the spindles and beam are in line like on a “straight axle”. And sometimes the connection is anything but straight, like a drop axle, or a torsion axle, or even an axle-less suspension. It is easy to confuse the many broad uses of the word “Axle” with trailers.
So, the tire rolls on the road, and it spins about an Axle. The Axle connects to other hardware and eventually to the trailer (usually the frame) via a flexible connection we call Suspension. Whew.
Read more about axle jargon in the article, Trailer Axles 101.
Dependent? or Independent?
Now we know “Trailer Suspension”, and “Axle”, what makes it “Independent” or the opposite . . . “Dependent ? ?” ?
These words have to do with how portions of the suspension work together. If one wheel moves up or down (like going over a bump), does the motion also make another wheel or wheels move? Of course we know that if you put more weight on one wheel it takes some weight off the other(s), which makes them move, so it is not truly independent. However, that is not really what we are talking about.
I see on YouTube and in many web articles praises for “Independent Trailer Suspension”. That is especially true when looking at suspension with trailing arms that look kind of like this. (Image from Curt.)
We will skip the detail since we have covered various axles and configurations before.
Independent trailer suspension basically means each wheel can move up and down “unlinked”, or “independent” of other wheels. This definition works with one “axle” (as in the image) or with many. Common examples are like the images above, as well as Torsion axles, Axle-less suspensions, and others. Even leaf springs that do not link are “independent” trailer suspension.
If that is “Independent” . . . is the antithesis “Dependent” ??
We don’t really have a common word for the opposite, like “Dependent”. It means that as one wheel moves (up or down) there is a similar, usually opposite action by the adjacent wheel(s). They “Link” to each other so they can “share the load”. To understand this, we need to consider 2 directions. Fore and aft, then side to side.
The Opposite of “Independent”
The first direction is across the trailer. If the wheels directly across the trailer from one another connect by a solid beam, they are NOT truly “independent”. If one wheel goes up over a bump, it has some effect on the other wheel on the other side of the trailer. In the case of a traditional straight axle, it is not a strong effect, but it does have an input.
For a single axle, this connection effect will change with various “trailer suspension” techniques. Think first about a typical straight axle with leaf springs, then think about trailing arms (like the front suspension on an older VW Bug). Also, coil springs (or air bags). The response is different for each type, and perhaps that has more to do with the “suspension type” than the “dependence”. We will talk more about single axles below.
Now we look in the other direction, fore and aft. In the case of multiple axles, we don’t call them “dependent”, but we do refer to them as “Load Sharing”, or “Linked”. Examples are equalized springs, walking beams, some air-ride systems, and a lot of less known others (like this low profile suspension arrangement).
These systems are not so much “Dependent”, that is a wrong use of the word. More like “sharing”. They link so the load on each wheel stays the same, even as one goes up on a bump. This is the real differentiation as it applies to trailers. Independent trailer suspension does not attempt to “share” the load, therefore it is easy to overload one (or more) wheels. Linking suspensions will share the load — for each wheel, during articulation — within the limits of suspension motion.
So what do we call axles without suspension? Is rigid mount independent?
Single Axle Trailer with Independent Suspension
Here are some examples starting with single axle trailers, because they are the easy to visualize.
If you remember back to your Mathematics and Geometry classes, we learn that any 3 non-coincident points create a plane. In trailer terms, that means the orientation of a trailer is always set by the 3 points — 2 wheels, and the hitch. In any static condition, the relationship of those 3 points will always be the same for the trailer, as if the suspension did not exist.
The suspension comes into play ONLY with dynamics. What do I mean? If the trailer moves super slow, the suspension will not change. Sure, as you put a load on it, the suspension will compress some because the dynamic load is changing. Once things are steady, even if you drive over big bumps super slow, the suspension will not change. (Except when the trailer tips and there is a weight shift). The orientation of the trailer may change — like tilting fore or aft, or side to side — but the 3 points of contact will always define the trailer position.
But, trailers are not static. We hit bumps, and dips in the road. We drive around corners. These are the dynamics. If the dynamics didn’t happen, we would not need suspension on a single axle trailer.
In fact, since the 2 wheels define the plane for the trailer position, independent suspension for 2 wheels offers only a very tiny benefit. As above, slow makes it irrelevant, so the benefit is only at higher speed. Want to see that in action? Here is a YouTube video showing it in an off-road application.
I like this video. Check it out, because I think they did a great job of showing a close comparison. https://www.youtube.com/watch?v=FysLZt04iR0
Starting at 3:24 see the slow motion views. Please note: It looks like the suspension is going up and down, but it looks that way because the camera is mounted to the trailer. In fact, the trailer is tipping back and forth causing dynamic weight transfer which is what the suspension is reacting to. You can see this starting around 7:33, and especially at 9:23. It is the same for both suspension types.
To me, the independent trailer suspension appears to be a little “softer”, but it is hard to tell. Second, the independent suspension has an advantage in the very middle for high protruding obstacles, but only the short open section offers the advantage. The disadvantage of “softer” springs is the susceptibility for tip-over since the springs compress more allowing more dynamic weight shift.
We also see shock absorbers seem to dampen some of the chatter. They are common with this style of independent trailer suspension, but can also be put on others — like the springs in the video.
It is a great video with great shots, and good comparison views. Thank you for doing it.
To me this video pretty clearly supports the physics. If there are dynamic advantages, it is other things, like softer springs and more travel, not the independence.
Tandem Axles with Independent Suspension
With tandem or triple axles there are more than 3 points of contact. That means over bumps and undulations the wheels with respect to the hitch will not always create a plane.
We made a lot of images to explain the situation with multiple axles having independent suspension. The whole explanation is worth reading in this article “Why Shouldn’t I Use Torsion Axles In Tandem Or Triple?” This image below is the essence . . . but read the article for the full explanation.
An example in real life is the image at the top of this page. The trailer is nearly empty, but one wheel is off the ground because the suspension does not share.
So why are we talking about it again? We have had questions about videos like this one on YouTube, and others.
Do a video search and watch some. In some, they really talk up the benefits, but misrepresent and simply miss some very important facts. (IDK, Perhaps they are stuck in a fantasy bubble and only think about the ideas that support what they want to think.)
For instance, in the video link here, at 3:18 he says “the right tire doesn’t even know it”. This is not really true. As one wheel goes up or down, the other axle carries more or less. It absolutely does know. Watch carefully at 3:24 and you will see that a change on one wheel absolutely effects all. (And it is a small bump.) It is independent, but it is misleading to make that claim, because it contributes to overload and failures.
(Please forgive the couple doing the video. They do not understand and are only showing what they experience.)
If you do not believe me, watch this video showing overload in a real-life road condition.
The greater up and down travel can be true, but it is not unique to independent trailer suspension. Adding more travel and “softer” springs (or longer springs) will help the ride of ANY trailer.
I think it is cool they add the riser spacer beam as it does what we talk about in this article about mounting axles and this one about torsion axles. What they did not tell you – it is required because the suspension now has more travel, so the piece keeps it from hitting in the wheel wells. I do not like the excuse about ride height (though it might be true for many trailers). Here it is misleading to give that as a reason.
Other Interesting Points
Also, the comment about adding a custom crossmember 16:12 — supporting once again the wimpy nature of so many RV and other mass produced trailers like in this little frame bending disaster. I think the added crossmember is a good idea.
As we have pointed out, axle alignment is really important. While most of us do not have this equipment, the alignment they show at 20:10 is pretty sweet.
“What is that noise?” 32:50 Yes, that is a benefit of the suspension type, but again, it is not a function of independent. In fact, I will guess if they had proper bushings in their leaf springs, they would not have had the noise. (Yes, many manufacturers skip it to save a few bucks.) Either way, with multiple axles, you will get tire scrub when turning. Now they get more, without noticing it.)
What About The Touted Benefits?
Staying with this video, looking at 24:48 and the discussion about the “new” better ride. Look at the axles and notice how much more they move than previous. Yes, the effectively softer springs and more travel change the ride. Notice that the “bounce” with the axles is pretty even for all of them. Of course, when the trailer is on flat, even ground, yes, they will each carry an appropriate distribution of the load. But there are many times when you are not on flat, even ground. Again, watch this video showing overload in a small dip.
Please understand. I totally agree that the ride will change with the longer travel and softer “springs”. The rubber has a natural dampling as well which is nice. No, my objection is attributing it to independent trailer suspension. Soft springs, longer travel, and rubber will make a big difference in ride — anytime — and you don’t have to risk overload and failures to achieve it.
I love the longer travel of this MORryde. That in itself will help so overloads are not as severe, but they still exist. As you know from other articles, I like rubber in suspension — for single axle apps, and when applied in a way that multiple axles share the load. Also, here is the full Engineering Case Study if you want it.
But I Was Taught Independent Suspension Is Better
Sure independent suspension is great for cars because the 4 wheels are at the 4 corners of the vehicle. The separation is important. And, the key point, cars do not have a tongue.
On trailers, the wheels are all in the same area, close together. That is a big difference. Yet, more important, the trailer pitch is set by a connection (the tongue) to the tow vehicle. No matter what suspension, it is not truly independent because of the tongue. If the tow vehicle goes up, so does the tongue, and that changes the relationship of the axles to the ground. So, they are not truly independent after all.
One more important point . . . . Suspensions like the MORryde, the Axle-Less, or Torsion axles all handle some overload pretty well. At first, that makes the independent trailer suspension look attractive, but that is not the whole story. Bearings and tires DO NOT handle overload well. Which is one reason we see bearing and tire issues often. Trailers are notoriously overloaded, and the problem is worse with independent trailer suspension.
So, all the above just to say — this is why we recommend axles which share the load. There are many styles, like the Timbren Silent Ride, or our Walking Beam, besides the standard leaf springs with equalizer links. Also, some suspensions that look independent can link to share the load — like linked air bags. (Frequently on over-the-road trucks.) So, please avoid this trap. Use the practical side of physics to avoid the lies.
Trailers That Will Benefit
Are there any trailers that can benefit from independent suspension? Well, yes. There is some minor dynamic benefit for a single axle like torsion axle trailers. Because the unsprung suspension mass is so low, in fast dynamic situations, there is a minor benefit.
The other case is for trailers like the one in this fun article. We do not see many of this style because they are almost impossible to back up. That said, much like a car, there is some benefit for independent suspension with one like that.
This is a look now at independent trailer suspension. For a single axle trailer there is not much functional advantage, but there is also nothing wrong with it. While there is little to gain from being independent, there are several benefits that often combine (and confuse). Things like more suspension travel (longer/softer springs), and sometimes rubber isolation. These have some value, so if it works for you, go for it. — Just avoid the confusion about the source of benefits.
On the other hand, we also see the cool factor of independent for Tandem or Triple axles is actually a detriment. Independent suspension for multi-axle trailers will cause overload at times. Of course, manufacturers will not talk about it, but it is a very real problem. Overload is the main reason we do not recommend independent trailer suspension for multi-axle trailers.
For 2 or more axles, use a method that shares the load. Sure, the typical leaf springs with an equalizer is one, but there are attractive options which are more advanced . Check out Linked air bags or the Silent Ride if you are looking for something with a softer ride, or more travel.
Good luck with your suspension choices.