Choosing Tandem Axles (and Triples)
It all rides on the trailer axles, so get them right. Here are some common technologies (and misnomers) for tandem axles and other multi-axle trailers.
The most common are tandem axles configurations. Triple axles are less common, but you do see them once in a while. Going up from there is far less common, so we’ll keep this discussion to Tandems and Triples.
Why use Tandem Axles? (or Triples?)
The concept is easy . . . ya gotta carry more load, so put more axles under the trailer. Easy enough, but Why not just buy a beefier single axle?
There are 4 big reasons for multiple axles, plus one more little one:
- Tires. To carry more weight you need beefier tires, and higher capacity (road rated) tires are usually bigger diameter. That affects bed height, and a host of other details.
- Tires. With a single axle, regardless of how beefy it is, if you damage one tire, you have a major issue. With 2 or 3 tires per side, you can usually stop before serious damage occurs.
- Load distribution. With more axles, the load is spread over a much broader portion of the frame. That helps with strength.
- Ride. With multiple axles, load to the ground is distributed over more points, so when a tire encounters a bump or pot hole, the load sharing linkage mitigates the contribution to jarring the trailer. This gives a smoother ride.
- Some say it gives Better Tracking, which it can if everything is set up correctly because multiple axles give a much larger footprint. If the axles are off, just a little, it will do the opposite and make the trailer less stable.
What Technology Should I Use For Multiple Trailer Axles?
There are tons of places to purchase axles, and the hardware is pretty standard and common. So, just buy the axles and weld them on. Right? Mostly. Let’s look at some of the technologies.
Leaf Spring Style:
These are the most common, by far, and also the cheapest — but those are not the only advantages. They are the most popular for lots of good reasons. Choose between Slipper Style and eye-eye style. (Slipper shown here, eye-eye style shown below in the image for Hybrid.) As a rule of thumb, think Slipper Technology for heavy loads — 6000# Axles and up. Think Eye-Eye type for lighter applications 4000# axles and below. Yes, there is a lot of wiggle room, and it’s just a rule of thumb, so consider the application.
I will refer to this as “Twin Axles” rather than “Tandem Axles” because the 2 do not interact with each other. They do not equalize, nor do they really load share unless the road is flat and level. This configuration is a disaster waiting to happen IMO. If you go up a bump (or down), one axle will completely take all the load and, depending on your capacity, maybe overload. True, torsion axles handle overload conditions better than springs, but tires don’t, and frames don’t. (See Hybrid below.) Also see our previous post comparing torsion axles with leaf springs.
Hybrid Using Rubber:
One of the desires for using twin torsion axles (as shown above), is the vibration damping that comes with rubber. By simply using rubber in the equalizer with leaf springs, much of that effect is achieved. See the image below. This is just one example, and there are many different hybrid types, but the rubber provides vibration damping AND the equalizing. This allows the axles to share the load when the road is not flat and level while offering vibration damping. This is a much better choice.
Hybrid Using Mechanisms:
There are several ways you can use rubber for suspension yet still achieve load sharing. As an example, here is a patent drawing showing a simple method to connect a pair of torsion axles for equalization with a mechanism. And there are more methods too. Just be careful not to violate someones intellectual property rights by copying.
Another approach entirely is that of a center pivot to accommodate equalization. This is the best equalization, and I can see why they are so proud of it. Click the image to watch a YouTube video of it in action. Very cool, but this one from Timbren is very expensive — like almost 10 times as much as leaf springs. Also it does not spread the load over a broad area of the trailer frame, so attention must be given to strength with this kind of a trailer suspension much more than with leaf spring tandem axles.The second image shows just the mechanism.The concept of the center pivot is not new, over the road trucks have been using them for years, but with leaf springs rather than the mechanism shown here. It is not, however, common with smaller trailers.
There are, of course, many more options and variations of the above options, but we’ll suffice with this group as they are the most common and most easily available. Some others require a lot of fabrication. It’s fun and interesting research if you want to know more.
Considerations In Choosing A Trailer Suspension
A final choice for trailer axles should balance many factors and, of course, your situation. Consider how the trailer will be used, what environment (wet, dry, on-road, off road), cost, etc.. Here are a couple keys to success.
One key with multiple axles is making sure they will share the load. There really is no point to having multiple axles if they don’t work together to share the trailer load. This is referred to as “Load Share” or “Equalization”. Since the road is not always flat, and the trailer not always level, the axles need to accommodate going over the uneven ground while still sharing the load.
Imagine for a minute a trailer with tandem axles, without springs or suspension. As the trailer rolls over a speed bump, the first wheel rises, then because there is no equalization, the back wheel may lift off the ground. This is an example where the axles don’t share the load. On top of the bump, the one axle (and it’s poor tire) takes all the weight.
Think again about a trailer with tandem axles, but this time, think about it with an equalizing suspension. (The little gold up-side-down Y shaped thing between the trailer springs in the image above is the “Equalizer”.) It moves like a “Teeter-Totter” to adjust the relative vertical position of the wheels as they go over bumps or into dips. Having that ability is, in this engineering opinion, absolutely essential. Whether the choice is springs or torsion, they need to share the load. To illustrate, please see our post Springs Vs. Torsion Axles. There we show how Torsion Axles naturally cause higher stress, so imagine that doubled! It’s not a good idea — especially when you can accomplish almost the same damping with a hybrid system as shown above.
Trailer Axles And Stability
A second key to success is making the axles stable under the trailer. For discussion purposes let’s look at leaf springs — which includes both slipper and double eye as well as rubber isolated hybrids.
The biggest part of stability is connecting the axles closely to the frame. When you think about the classic single axle leaf spring situation, in reality the forward portion of the leaf spring acts as a trailing arm connected to the frame. A trailing arm is a very stable way to mount an axle — and it’s one of the reasons people like the feel of torsion axles.
When 2 leaf spring axles mount in the classic way (like in the image below), the back axle is not a trailing arm, but a leading arm. That is not as stable. Yet, because they connect with the linkage in the center, and because motion in one moves the other, it becomes stable. It’s a quasi stable condition that is proven to work. As a side note, this is why you sometimes see trailer brakes on just the front tandem axle, and not the back one.
A reason to use slipper style springs is the pinning of the back axle springs to a more supportive front piece. Looking at the photo above showing slipper springs, it is easy to see that the back springs attach to the center equalizer, allowing them to “float” at the back. This is also quasi stable because movement in the equalizer actually moves the back axle fore and aft just a tiny bit. This is not bad or dangerous in any way, but it’s worth understanding, and it makes rear axle brakes work better (more stable). That’s one reason for the 6000# rule of thumb.
Triple Trailer Axles
For really heavy or really long trailers, triple axle configurations are common. In context of the discussion above, making the middle axle both equalized and front anchored becomes a bit of a challenge. Slippers solve that problem for the center axle just like the mechanism shown for the tandem axles in the photo of leaf springs above. Double eye spring configurations solve the same task with another type of equalizer. Compare the equalizer rocker for the tandem axles and triple axles in this drawing below. Note the front equalizer as compared to the back one for the triple.
Please note the discussion above is about axle stability under the trailer. Overall trailer stability requires axle stability, but there is more too, including proper axle position under the trailer. For more information, please read our article on Synthx.com about trailer stability.
Trailer Axles Recommendations
If you have tons of money and lots of space under your trailer (and an extra strong frame), the Timbren Silent Ride may make the perfect tandem axles. If you’re like most of us, cost is important, so the next best choice is a leaf spring suspension with a rubber isolater in the equalizer as shown above. There are many kinds for tandems and for triples. Finally, the standard leaf springs on tandem axles (or triples) are a well proven, solid (and the least expensive) choice. Use eye-eye for lighter applications, and slipper style for the heavy duty ones.
Again, I would never use twin torsion axles without some accommodation for load sharing. Before dismissing this “un” recommendation, read our other post about the engineering side of Springs Vs. Torsion Axles and realize it is compounded with heavier trailers and more axles.
Watch for more about trailer axles in general and tandem axles in specific — in coming posts — like this one looking at spring mounting. There is so much more to discuss than can be done on just one page. Another good source is the set of articles titled “What Makes A Good Trailer?” from Synthesis.