Torsion axles are awesome, so Why not use torsion axles in tandem? It’s a frequent question, along with the corollary “Why shouldn’t I have torsion axles in triple?” They’re good questions, especially in light of all the misinformation about axles out there.
Then, of course, you see examples of manufacturers putting twin torsions under RV trailers and many more. If the manufacturers do it, then it must be OK — Right? Same for styles like axle-less suspension, Moryde axles, and others. So, Can I use them in tandem? Or triple?
These are the questions of the day. So, let’s dive in. We’ll also use the trailer above (triple torsion axle 5th-wheel) as a case study. But first, let’s ditch the propaganda and the salesman spew, and get down to the engineering.
Trailer Fundamentals
Let’s start with the basics — a simple, Single Axle Trailer. There are 3 points of contact supporting the trailer: 1.) The hitch. 2.) The left side wheel. 3.) The right side wheel. No matter where that trailer goes (within reason), those 3 points of contact are stable. Up hill, down hill, over uneven bumps, whatever. The 3 points of support are always there.
Next, consider a trailer with 2 axles in tandem — 4 wheels as in the image above. This gives 5 points of contact.
Now, let’s compare these trailers moving down the road and over a bump. Of course the car goes over first, but that’s not our focus. For the single axle trailer, the bump is really no different because the wheels just carry the weight over the bump. 3 points of contact, always. No big deal. See the image below.
For the tandem axle trailer it’s different because of the 5 points of contact. Since we’re talking about torsion axles, let’s look at a trailer with Torsion Axles In Tandem. When the front set of wheels are on the bump, the back set becomes unloaded.
Without load sharing, the axles can’t equally support the trailer weight.
Does it matter? It means one axle (and wheels) are overloaded. Extrapolate this to 3 axles, and the problem is worse. While this is simplified for discussion, hopefully it conveys the concept.
Action In Trailer Axle Suspension
There are many trailer suspension types — see Trailer Axles 101. Also, the article on axle-less suspension, and a comparison for leaf springs and torsion axles. Yet, for this discussion, the type of suspension doesn’t matter. It’s whether the axles work together to share the load on uneven surfaces — or not.
All suspensions have a compliant member between the wheel and the trailer frame (load). As load increases, the suspension deflects more. Think of squeezing a tennis ball. A small force gives small deflection. Larger forces cause greater deflection. Now, think about a trailer going over a bump. The simple trailer with one axle has no change in wheel loading on the bump. (Dynamics aside.) However, for tandem axles, if they are independent, then as first axle goes up, it’s suspension deflects more because the back axle is still on the flat ground. The back axle can’t carry as much in that situation. That means the wheels on the bump carry more. (Remember, this is for INDEPENDENT axles — whether they have springs, or torsion, or whatever.)
The goal is to have the axles share the load regardless of bumps or holes. Fortunately, there are good ways to accomplish it. The most common is an equalizing bar for leaf springs, yet it’s not the only way. Walking beam suspensions and linked airbags do this well too, and there are others.
Uneven Travel Surfaces
Road surfaces are uneven everywhere. Potholes, curbs, washboard, speed bumps, . . . the list goes on. The simple fact that we pull trailers on uneven surfaces, means tandem axles don’t stay perfect with each other. So for multiple axles, if we want our wheels to carry roughly even amounts, then the axles must interconnect somehow. As one goes up (or down), it must NOT carry more load (or less) than the other.
The classic example is pulling into a gas station. While we drive in, up a short ramp (to sidewalk level), we are also turning so one side of the trailer starts up the ramp before the other. In fact, (think about tandem axles), one wheel starts up first, and there is a complex motion of suspension as all transition to the ramp, and as the trailer pitch changes with the tow vehicle going up and over.
Another simple example is a pothole. One wheel goes into a hole while the other 3 are on the road surface. All of these situations require suspension movement — all while (hopefully) not overloading one wheel or axle.
If the axles are independent, then some wheels carry more than others in these situations. If, on the other hand, the axles share evenly, then all wheels carry the same load — even when the travel surfaces undulate. This is the goal.
Interconnecting Axle Motion
The simplest way to conquer uneven travel surfaces is to interconnect the axle motions. If one wheel or axle can move up (or down) without changing the load it carries, that’s the solution.
The most common interconnection technique is for leaf springs with an equalizer between the axles. Basically, as one wheel goes up (or down), the equalizing bar pivots to offset the height of both wheels. Obviously there are limits to how much, which limits how big a bump it can compensate. Yet, for most driving, this does the job for load sharing. Both wheels carry the same weight over bumps and through dips. (Here’s an example where it goes wrong.)
A walking beam suspension does something similar. The beam pivots to accommodate an uneven travel surface while keeping the load equal on both wheels.
Another successful interconnection is with air ride. As one wheel goes up, air from its bag passes to the other keeping pressure equal in both. While the volume in one bag increases and the other decreases, pressure stays the same in both. This keeps loading the same for all wheels as the axles traverse over uneven ground.
And, there are other good solutions like this low profile trailer suspension.
Torsion Axles In Tandem or Triple
It does not matter if we are talking about springs, or axle-less suspension, or torsion axles in tandem or triple. If the axles mount independent, the result is the same. Motion of one wheel does NOT affect motion of the others, so there is (at times) a miss balance of load.
Think about it. With standard leaf springs, if you place 2 axles in tandem WITHOUT an equalizer bar, then they act independent. That is exactly the scenario with torsions — or axle-less suspension, or Moryde axles. If the axles don’t interconnect, then they can’t equally share the load.
Again, I’ll stress, this does not matter what axle suspension technology you use.
Leaf springs are easy to interconnect because the equalizing bar is simple, and it’s been around a long time. Criticism for torsion axles in tandem or triple comes because there is not a good interconnection. When they are in tandem or triple, they are almost always independent, and — as in the illustration — that’s a problem. Unless a walking beam or some other mechanism is incorporated (like in the image above).
Illustration of Torsion Axles in Tandem
The enlarged view of the tandem axles illustration above highlights the issue with torsion axles in tandem. The transparent wheels in this view allow us to see the torsion arm angles. Note that the second axle has the arm angle way down, and the first axle is up.
In this case, the first axle is carrying all the weight, so it deflects a lot. If the axles are the same (which they certainly should be), and if combined they are rated at capacity of the trailer (typical), then in this condition, the first axle is overloaded by Twice. If the tires are rated for the axle, then they are also overloaded by Twice. This is how travel-stopping failures happen.
While this image shows the front axle on the bump, you can easily see as the trailer moves forward, the second axle will have a similar overload.
Is this example extreme? The bump is 3″ tall. Do you ever encounter a 3″ speed bump? Or a small curb? Or a pothole? No, this is not extreme. More to the point, a 1″ bump or a 2″ bump does some of the same thing. Physics don’t lie.
The Misnomer of Independent Trailer Suspension
People say they like the independence of torsion axles — or axle-less suspension, and others. We’ve been taught by the automotive world that “Independent Suspension” is good. Because, in our cars it is! So why not on trailers?
Let’s compare. A car has 4 wheels, one at each corner. A tandem axle trailer has 4 wheels, all bunched together. Oh, and a trailer has a hitch — attached to the tow vehicle setting it’s attitude. Cars don’t have a tongue and hitch forcing the vertical orientation.
Load sharing for a car with independent suspension is great because the 4 wheels are near the 4 corners of the vehicle. 100% of the load is shared by the 4, and the attitude (pitch and yaw) are controlled by the position of the 4 wheels. Nothing else. (In extreme situations, opposite corner wheels may carry extra, but those are extreme cases like 4-wheeling.)
Trailers have the axle(s) central and close. Most important, the attitude is set mostly by the hitch (height) on the tow vehicle. As illustrated above, axles must interconnect to act as one. Independent suspension for trailers is not helpful.
For another example, look at big trucks with tandem rear axles. They interconnect for the same reasons trailer axles should interconnect. The front wheels are independent, but the back tandem axles interconnect for load sharing.
A Real World Example Of Torsion Axles In Triple
A website visitor contacted me about problems with his brand new trailer. He sent a bunch of images and video. There are some structural integrity issues too, but the big problem is the triple torsion axles.
Here is a video of the empty trailer while the front jack is going up and down. Think about what is happening. As the front of the trailer rises, the angle changes, and because the axles are independent, it takes some weight off the front axle and puts more on the rearmost axle. It seems like such a little change, but you can see how the structure responds.
In this case, the effect is more visible because of some structural issues, but it highlights changes in load for the 3 axles. This customer reports that poor road surfaces make the trailer dangerous to pull. All because of issues exacerbated by independent triple torsion axles.
Why Do Many Torsion Axles In Tandem Seem To Work?
There are many examples of Torsion Axles in Tandem, and in Triple, that seem to work just fine. Dexter even has a line of axles that they sell for use in tandem. Why?
This image shows a new RV trailer underside with torsion axles in tandem. You can see that they do not link in any way — meaning they are independent. In flat towing, they’ll perform well, but as soon as one axle goes over a bump, or goes into a depression, these axles become unequally loaded.
To get around the possibility of overload in such situations, you can use axles and tires that are way overrated. For example, the trailer is 4800# with a max load of #6400. Using 3200# axles will absolutely overload one axle. Using 1600# capacity tires will almost certainly cause a tire failure if using the trailer anywhere near max capacity.
Instead, if the axles are 5000#, and the tires are 2800# capacity, then the system will probably work well enough. Even though the axles don’t load share, one is capable of almost all the weight, and the other will usually carry at least some of the load (even if it’s much less).
Is There Anything Wrong With That Solution?
That depends on what you mean. Over-design is one way to solve the problem. BUT — you don’t get the real advantages of a torsion axle. Even if the suspension is torsion, higher capacities are stiffer. So, an over-design solution gives a harsher ride. Why not just get a single 7000# torsion axle? From an engineering point of view, if the goal is a nicer ride, use the hardware that accomplishes the goal rather than mixing up something that will either counteract the goal, or create a world of potential problems?
The big issue is that systems are not over designed as indicated above. For the 6400# trailer example above, most applications will have 3500# axles. When lightly loaded, i.e. 4800#, the axles and tires just suck it up. On the other hand, with a full load (near 6400#), failures begin to happen.
One Explanation
Here is one idea someone gave. He said, on RV’s, people don’t tend to overload them. They just don’t put that much camping gear in them to approach the limits. Utility trailers, on the other hand, are often at (or over) the limit. That’s why it works (most of the time) for RV’s, and tire failures are common for utility trailers with torsion axles in tandem.
OK, maybe that explains some of it, but I’m not totally embracing it. And, it isn’t really a solution. I do appreciate, however, the observation and opinion.
Failure Statistics
I would really like to see a correlation for axle, hub and tire failures — First to see how age affects the failure rate, and Second to see how overloading affects it. I’d love to know if failure rates are higher for single axles or tandems or triples. Then, for multiple axles, see the failure rates divided into groups where the axles load share and where they don’t. Too bad such data is not available.
My gut says lack of maintenance is the biggest contributor to failures. (Things like insufficient grease, old tires, etc.) Second is probably overloading. Tire failures often happen AFTER the overload — meaning they don’t just pop as you go over the speed bump. They fail later with heat or something because of the internal damage from overload. Think about it next time you see a trailer along the highway with a wheel off. It happens all the time, so make sure you have a good spare tire.
One other point. I don’t see or hear a lot about other axle failures. Mostly it’s the tires, the hubs, or the bearings. Maybe, to fix an existing problem, the hubs, bearings and tires can be changed to beefier versions? Leave the axles as they are, and that may decrease the chances of failure. It’s just a guess, but it might be worth asking your parts supplier to see if your axle can do high strength end parts.
Trailer Manufacturers With Torsion Axles In Tandem?
Why do some manufacturers design with torsion axles in tandem or triple? Why do they set their customers up for failure? That is beyond me, for sure. In the video example above, with a brand new trailer, it’s obvious the company didn’t do their engineering and QC. I don’t know how else to say it. That’s an engineering failure.
My hunch is not enough engineering goes into many trailer designs. I don’t know each case, but you can see the result. I also don’t know what Dexter puts in their line of axles for use in tandem. Let us know if you have specific and real knowledge.
What I can say: I’ve consulted for a couple RV manufacturers, and they tend to do things by seat-of-the-pants more than by engineering. Their effort has more focus on the upper and the interior — the things customers interact with. The chassis is almost an after-thought, and the one company didn’t have an engineer. Lots of really smart guys that produce beautiful work, but no engineer. (Interestingly, they didn’t like my advise about load sharing for the axles. Oh well.)
In the meantime, if you really want rubber suspension for tandem axles, use one of the load sharing varieties like the Timbren Silent Ride or torsions on a walking beam. If you don’t like the cost, then do a significant over-design or use rubber linked leaf spring equalizers. Either way, don’t set yourself up for failure.
Arguments
While I have heard many opinions about why I’m wrong, none have given the physics to support their arguments. Anecdotal evidence is not enough because that goes both ways fast. The truth is, I’m open to being wrong, so if you can show otherwise, please share it. I’m sure many of our website readers would also love to know. We invite your thoughtful comments below. Thank you for taking part.
One other factor you didn’t mention is the sensitivity of tandem torsion axles to hitch height (and the axles being level).
My club has a custom 28′ trailer that the builder unfortunately put the axles in the center of, and some people had no problems towing it and for others it was a nightmare. Someone who works on trailers (for semis) suggested that it was related to the height of the hitch changing the effective length of the tongue (ie: when the trailer was tongue low, the front axle carried more weight and when tongue high, the rear axle carried more weight. This changed the load center of the trailer and made it unstable when tongue low or less unstable when the tongue high.)
We confirmed some of this by measuring the weight on the hitch at various hitch heights. With a 10,000# trailer, the tongue weight varied by 500# (800#-1300#) over a 6″ variation in hitch heights (3″ +/- from level), with it weighing around 1050# when level.
I’m sure I didn’t phrase that in correct engineer-speak, but you should understand the idea.
You communicated the situation, and that’s more important than the “engineer-speak”! You bring up a great point, and you’re exactly right. Thanks.
Yes you are correct as to hitch/tongue height causing towing problems.
No matter the axle type… you NEVER want to tow a trailer tongue high.
You want to be level (Always the best situation as to stability and tire heat/wear as well) or even a bit low in the front, if you have to go one way or the other.
Otherwise the trailer will always sway all over the road.
Never tow a trailer high in the front.
Definitely not low in the front, that takes load off the hitch and puts more load on the front axle on a tandem arrangement. That is a very dangerous combination especially while engine braking going downhill. Run your combination over a scale and shoot for 15% on the hitch
Love everything in this articular. We specialize in building a product to load and unload cars in the trailer. I also tow a lot of miles each year myself. I have seen and experienced the damage to many different manufactures trailers that use tandem and triple torsion axles. What are your thoughts about putting an air bag on each end of the of the walking beam? My thought is to help spread the load better across the beam rather than concentrate that load at the pivot.
Thanks for the endorsement. Yes, you can use air bags with the walking beam suspension, but make sure they are linked — as the front one compresses, it puts the air to the back one — so they both stay the same pressure. Yes, that would spread the load. Incidentally, that center pivot is high load, but it’s big & fat — designed to take it. Load is then spread with the underpinning beam between the trailer frame and the pivot.
Thanks for a great article. I was planning to replace the rough spring suspension on my horse trailer with rubber torsion (which is what nearly everyone recommends) But now I am leaning towards Timbren silent ride. The trailer weighs about 2000# and the maximum load it would ever have would be 2 1500# horses. Though much more likely 1 or 2 1000# horses. The number one priority is smooth ride for the animals. It seems that the Silent Ride would be the best of both worlds. Do you concur?
I like the look of the Timbren Tandem axle Silent Ride Walking Beam Suspension. I’ve never had my hands on one, but I like what I see. Speaking from experience with a different walking beam, that does a great job of smoothing the ride. Good luck with your project.
Firstly I want to thank you for this article. I’m currently in the market for a new race car trailer and see a lot of the tandem axle trailers with torsion axles, so as an ME myself I appreciate informed information like this that cuts through the marketing hype. Overbuilding is common, and a trend I’m noticing is “spread axles” where they move the axles farther apart than you would see on a leaf spring multi-axle trailer, apparently in an attempt to mitigate the single wheel loading problems you mention when traveling uneven roads. How effective is this?
If you can support a long trailer at more separated points, then the structure does not have to be as strong and stiff. So, if the axles load share properly, then “spread axles” are great, but that’s not what I see. As in the article, independent suspension does not make sense for a trailer. Why do people do it? This is my warped opinion: They are trying to make the trailers cheaper and lighter while ignoring that trailers do not travel on even flat ground all the time. I think there’s some amateur hour in the engineering at the source. FEA looks great if you input easy conditions, but as we know garbage in = garbage out.
Another and possibly the main reason horse and car trailer manufacturers use torsions is to maintain a low ride height for loading purposes. I dislike the spread axle design because it stresses tires, axles, and the frame when turning sharp more than a conventional spread
Dave: One big problem with spread axles would be tire wear on tight turning/backing maneuvers. Most people on occasion need to make sharp turns to get their trailer where they want it. The more distance between tires whether because of triple axles or spread axles, the more rubber you will leave on the pavement. I spend a few thousand dollars a year on tires and this is a significant contributor.
Question: I have a single torsion axle on my boat trailer. My boat is a 21′ fiberglass center console with a 150 hp Yamaha outboard. The weight means the tires continually look flat though they are properly inflated. The boat pulls exceptionally well as is but I was considering adding a second axle to re-distribute the weight. After reading your article I’m thinking leave well enough alone. A misguided attempt may actually make things worse. your thoughts would be appreciated.
Interesting question. I’d do a few things: 1) Weigh the boat and trailer fully loaded. There are vehicle scales at truck service centers, recycling centers, garbage dumps, etc. 2) Check the weight compared to the axle capacity and the tire load ratings. 3) I assume the trailer came with the boat, so the axle is probably good. Maybe it just needs higher capacity tires? I recommend a load rating that is 20% or so over the actual load. Go a little larger diameter if you have the room.
Thank you mechanic. It is amazing how many people do not pay attention to the sidewall weight rating when they buy tires. I’ve seen tire sales websites that do not even list it. My first commercial driving ticket was for under rated tires in comparison with the scale weight of the trailer.
Very interesting Article on Torsion Axles. I just replaced the torsion axles on my 1973 Argosy (Airstream) travel trailer with tandem axles. They lasted 47 years before losing almost all their travel and going lob-sided. Looking at your article, it appears what Airstream did to handle the overloading problem you mention was to totally over-engineer it. The trailer itself weighs 3400lbs- Axles were rated at 2600lbs apiece. 4 Bias-ply tires rated at 2900lbs each. When you consider probably over 400lbs would be in tongue weight- it would be nearly impossible to overload an axle. Even so, I bumped the new Lippert axles up to #2800lbs each. If they last as long as the original; I wont have to replace them until I am 106 years old:)
Great article and comments! I was looking at putting triple-axle air ride on my 5th-wheel. I notice Ridewell and others offer tandem air ride suspensions, but most offer suspensions without walking beams or other equalizers. My question, as long as the air bags are interconnected so air can flow between bags that should be enough axle equalization but I wanted to hear other thoughts. Thanks!
Air ride is cool, but I have to admit I need to learn more about other systems out there. Yes, 2 things are important: 1) interconnected air chambers that allow flow between them BUT only for each side. Don’t interconnect from left side to right side. 2) Make sure you have adequate travel for each axle. If one bottoms out, it messes up the load sharing. Good Luck.
Hello, I have a Keystone Fuzion 3 axel and a powerpoint I’m sending to Morryde 4000 kit. My rig has 12′ garage. Can i send you the powerpoint for your review. I hope your not a bears fan!
Great article, thank you for your educating input. This will help me choose our next RV in the future
Thank you for this article. Which type of axle do you think would be better on a 2 horse straight load trailer, torsion or spring? I usually only have 1 horse in it so the weight is mostly on one side. Thank you for any help you can provide.
I like the shock handling character of rubber for important loads like horses, but axles must share the load. Torsions don’t. Fortunately, there are products that augment the classic tandem leaf spring suspension. Look at center equalizers that are enhanced with rubber. I don’t have enough experience to recommend one, but do some YouTube searching to see what you like.
I found your wonderful site while looking for solutions for tandem torsion axle load balancing. Application is for a custom toy-hauler trailer (2 dirt bikes + living quarters) that will have a GVWR of up to 10,000 lbs. It occurred to me that a simple low-cost solution would be to start with a standard leaf spring + equalizing linkage type of suspension, but substitute a pair of torsion axles in place of the standard axles. This creates a hybrid suspension where the leaf spring equalizing linkage takes care of the load balancing and the torsion axles soften up the ride better than the leaf springs alone. Has this ever been done and if so, what was the result?
Yes, and there is a patent. See this page about tandem axles and scroll down to the bit on hybrid suspension. I don’t know if the patent is expired or not. Also this page about our torsion axle walking beam suspension. Unfortunately, we don’t have one for 10,000 lbs. You’re on the right track, keep thinking.
I have a flagstaff rv tandem torsion has terrible pulling characteristics due to the torsion new trailer under warranty have to deal with it.
One manufacturer of a trailer with a asphalt tank tandem torsion. Rents the trailer they are having problems breaking the frame. Wish I would have researched this before buying my RV
I should have known better I’m a mechanic
Equipment Superintendent for a contractor now thanks again