There is no opinion stronger than one totally unsupported by evidence, experience or education. That describes the widely held opinion that tandem axle trailers are naturally superior to single axle trailers. I’ve got an opinion too but, by contrast, it’s based on well over a million miles of firsthand experience, not to mention the secondhand experience we get from our customers.
Specifically, we’ve used six or seven drivers to deliver Ironhorse trailers all over the U.S. So far those drivers have racked up well over a million miles. Roughly half of those were in a dually towing a tandem axle transport trailer with zero, one, two and three Ironhorse trailers on board. The rest of the miles were rolled up using pickups and SUV’s to ground tow one or two single torsion axle Ironhorse trailers at a time.
So why do so many believe tandem axle trailers are naturally superior? Typically it seems to be that they are fixated on a single imaginary advantage. Specifically, what we hear most often is the opinion that: 1) if you have a flat or blowout on one side, the other tire will keep you from running off the road, 2) a tandem axle trailer is much more stable, 3) if the trailer comes off the ball, a tandem axle trailer won’t go crazy like a single axle trailer will, and 4) you can carry more load with a tandem axle trailer than with a single axle trailer.
Flats and Blowouts
So what happened when our drivers had blowouts? As you might expect, it depended on whether they were towing tandem or single axle trailers. When a tandem axle trailer had a blowout nothing much happened, unless someone flagged the driver down. BUT that’s not typically what happened. Instead the remaining good tire on that side carried the load only a few more miles before it failed too. Just what you would expect since the remaining tire was only strong enough to carry half the load.
If somebody flagged the driver down before the second tire blew, it was easier to change the flat—just roll the remaining tire up on something thick enough to get the flat off the ground. That is much easier than jacking up a single axle trailer under similar conditions. BUT if nobody flagged the driver down and he wound up with two flat tires, not only did he have to jack up the double axle trailer, he had to change and we had to replace two tires.
What about the running off the road bit? To make a long story short, it didn’t happen. Instead both our tandem axle transporters and single axle trailers continued to pull straight and true. In either case, until one flat tire on the single axle trailers and two flat tires on the tandem axle trailers came completely apart, the driver would first notice that his tow vehicle seemed to be a little bit sluggish. Next he might notice some minor but rapidly increasing vibration which would eventually become strong enough to get his attention.
In one case, a customer was towing his single axle WideBody with a Class A motorhome when a truck driver tried to flag him down. Although there were two Harley Street Glides and one mini golf cart in the trailer, when he looked at his rear view camera display, he kept going because he couldn’t anything wrong. But the truck driver didn’t give up and, if anything, redoubled his efforts. Ultimately, our customer stopped only to find a tire completely gone except for some of the radial wire circles around the wheel. When he looked a little closer, about 3/16” was worn off the outer edges of the aluminum wheel!
How about the difference between the stability of single axle trailers and tandem axle trailers? Our experience suggests there isn’t any. On the other hand, single axle trailers with very short distances between the coupler and the axle are much less stable than tandem axles trailers with a much greater distance between the coupler and a point midway between the two axles. Likewise, single axle trailers are generally harder to load properly—too far forward and you get bumper sag, too far backward and your trailer tends to sway from side to side. Tandem axle trailers, while still not idiot proof, are nevertheless less sensitive to load imbalances.
What about the relative stability of the two types of trailers under extreme conditions? Specifically what happens to each when they “come off the ball”? It doesn’t seem to matter whether it’s caused by human or mechanical error. The result in either event is a severe case of the safety chain mamba. According to a couple of our drivers who experienced this sphincter tightening experience, it doesn’t seem to matter whether one is towing a single or tandem axle trailer. It’s terrifying either way.
Load Carrying Capacity
It is well known that in the enclosed cargo trailer market, low price, not cash, is king. Clearly trailer builders can’t sell for less unless they build for less and you can bet that the largest low price builders have figured out how to do that. Not only do they use a lot of automation, each component of their trailers is designed to meet specific cost and performance targets. No less and no more—excess component performance capability raises costs and makes a specific model less competitive.
Now suppose one of these trailer builders has a hot selling single axle model with a 3500 lb. maximum weight with moderate sales while the sales arm of the company insists an identical trailer with a 5000 lb. maximum weight would sell even better. That leaves the builder in a quandary. One option is to switch to a 5000 lb. single axle and beef up the chassis to handle the extra stress. A second option is to slap two 2500 lb. axles on the original chassis and hope that spreading the load over a longer portion of the chassis will be enough to keep it from failing under hard use.
The third, and least attractive, option for the builder is to redesign the chassis to accommodate tandem 2500 lb. axles–not the least cost path nor the path of least resistance. That’s something to remember the next time you are tempted by a relatively small but nevertheless flashy looking tandem axle motorcycle trailer.
While some of the widely held biases against single axle trailers are obviously incorrect, clearly there must be some limit to how long a single axle trailer can be. Not really, as long as the trailer chassis frame rails are strong enough to support the entire load concentrated over the few inches where the axle is attached. Long fifth wheel trailers with single width axles with dual tires on each side are especially common in areas with stiff per axle tolls. In this case, of course, the single axle is mounted so far to the rear and the front is so firmly supported by the rear suspension of the trucks towing them that there is little danger of loading such a trailer too far to the rear.
Why else do knowledgeable buyers often prefer single axle to dual axle trailers? Maneuverability is the main issue. Add a rolling jack to the front of a single axle “personal” trailer and its owner can move it around at will. A fact that greatly simplifies hitching and unhitching and putting it away and retrieving it. Put dual axles on the same trailer and you have to get the ball on your tow vehicle directly under the coupler before you can hitch the trailer—no matter how many tries that may take.
Turn a loaded tandem axle trailer too sharply and it pivots on one of the tires on the inside of the turn. Sadly, it can and often does actually drag one or more of the other tires off the rim. And even if it doesn’t, the cumulative effect of overstressing the sidewalls dramatically shortens tire life. That’s why trailers used to deliver goods to retail outlets rarely have tandem axles. Because of the sharp turns such trailers have to make and the tire problems that result, single axles make a lot more sense. This is such a common problem that back along the way the tire industry developed “Special Trailer” tires with extra strong sidewalls. (Although ST tires are widely used on single and dual axle trailers, it’s not absolutely clear that they are even necessary on single axle trailers.)