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Published Oct 2024
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1 February, 2023 By: Brendan Batty
Whether it’s a caravan, boat or box trailer, we explain the pros and cons of single vs dual-axle trailers and what to look out for when considering which will serve you best.
While caravans aren’t the only trailers to work with multiple axles, they are the most visible and what’s applied to them can more generally be applied to any single or dual-axle trailer.
So we'll focus on caravans and their axle configurations to more broadly explain load carrying and stability as it relates to single and dual-axle trailers.
Why do trailers have one or two axles?
A cursory survey of Australia’s largest manufacturers of caravans highlights two things. Once caravans reach about 2000kg loaded weight, a second axle is usually added. That’s in line with regulations that suggest (but don’t specifically require) that a trailer over two tonnes needs more than two wheels with brakes. Of course, trailers can be heavier on single axles. AL-KO’s single-axle Enduro suspension system is rated to 2500kg GTM. If you need a refresher on trailer weights and masses, see our article on caravan towing weights.
Similarly, for caravans, a second axle is usually added once they reach about 17 or 18 feet in length. A good example of that is Jayco. All its caravans 16 feet or shorter have one axle. All its caravans 17 feet or longer have two axles, regardless of weight. Of course, that varies slightly across manufacturers (and European-built caravans tend to stick with a single axle until about 19 feet), but we can consider it a general trend.
Neville Hauffe of Simplicity Axles, one of the longest-serving axle builders for the Australian trailer industry, says it comes down to the personal choice of the designer. “Obviously if you’ve got four 12in brakes instead of two 12in brakes, the four aren’t working as hard. It’s not just the brakes either - but each tyre, stub axle, bearing, and even chassis points are carrying less load.
Secondly, once you start stretching a van much over 17 feet, they start to get unwieldy because the mass of the trailer is so far away from where it pivots on the wheels.”
According to Hauffe, while there are benefits of the extra axle for longer, heavier trailers, he doesn’t believe the number of axles is the main contributor to how well or poorly one handles. “Poorly designed caravans will tow poorly, no matter how many axles you put under them,” he says. More critical to handling is how a trailer’s weight is balanced, where the bulk of that weight is found, and how that all sits on the towing vehicle.
In summary, axles are added to trailers to increase the load-carrying ability and improve braking performance once that capacity increases. Particularly, but not exclusively, in caravans, axles are added to help improve their stability when their weight is spread over longer distances.
Trailer costs and maintenance
It’s reasonable to assume that adding more axles, brakes, tyres and suspension components to a trailer will increase its cost. And it’s not always just as simple as doubling it. Online parts store DIYCaravans.com.au lists single-axle CruiseMaster XT Freestyle Air suspension at close to $2700. The dual axle version is nearly $7000. Even on simpler, more traditional leaf-sprung systems, there’s a level of complexity added, which increases the cost per axle.
Once you’ve got your trailer behind your vehicle, the maintenance costs of a dual-axle trailer or caravan will be higher. Four sets of bearings, rather than two sets, will need to be checked or replaced at each service interval. When tyres wear out, you will need to buy four, not two. You get the picture. As a dual axle trailer is more likely to be heavier, it may also put more stress on your towing vehicle, leading to higher maintenance costs.
Are dual-axle trailers safer than single-axle trailers?
The tyre-blowout scenario is highlighted in many arguments about the safety benefit of dual axle trailers. It suggests that if you get a blowout on a single axle trailer, there is nothing left to support one side of it, and therefore it will result in catastrophe (although the arguers often forget that the trailer will still have a wheel rim and suspension). On the other hand, tandem trailers have a second wheel on each side to support the weight, so losing a tyre isn’t as much of a drama. The truth is, neither type of trailer is unsafe if it’s well maintained and loaded correctly.
Dual-axle trailers have a safety advantage when they can effectively spread the loads carried by the trailer over larger contact points on the road and chassis and apply more effective braking because there are more wheels to brake. The load-bearing componentry of dual-axle trailers is generally less stressed than in single-axle trailers.
For longer trailers, and particularly caravans, tandem axles help overcome stability issues caused by heavy loads which may be at the extreme front or very back of them.
Dual-axle vs single-axle suspension
On single-axle trailers, suspension choice is much more flexible. With most of the trailer’s load on just one axle (the rest of the load is on the tow ball), you can be reasonably confident that over any terrain, the axle, tyres, and bearings can handle that load. Whether that suspension is a leaf pack, torsion, coil springs or airbags, the load on the axle should be reasonably consistent.
Dual, or even triple-axle trailers, are different, though, and suspension systems can be separated into two categories – load sharing and not load sharing. According to the Vehicle Standards Bulletin about light trailers, “load sharing suspensions must be designed so that each axle in a group carries the same load irrespective of the relative movement of the wheels.” In layman’s terms, the load on the front axle is always the same as what’s on the back axle.
Most leaf-sprung dual-axle trailers have a load-sharing system, often referred to as rocker, or rocker roller suspension, after the trade name of the AL-KO version. Simplicity Axles also has an independent leaf, coil or air-sprung version, but most other independent trailer suspensions are not load-sharing.
Without load sharing across axles, trailers rolling over large bumps or undulations (i.e., ruts, large speed humps or even big potholes), the entire load of the trailer could rest on just one axle, even if only for a moment. Poorly loaded trailers may also have different loads on different axles, even if a trailer seems level.
As load-sharing systems maintain an equal load on each axle, braking performance is also more reliable, as there is less chance of a wheelset being unloaded during heavy braking. To compensate for the possibility of more load on a single axle, non-load-sharing tandem systems must have a rated capacity 20 per cent higher than the axle load at ATM.
Do dual-axle trailers affect tow-vehicle performance?
As dual-axle trailers and caravans tend to be heavier than single-axle trailers, they will contribute to higher fuel use. But how and where weight is distributed and loaded onto a trailer has the most significant effect on how tow vehicles handle them. For the best towing performance, the trailer’s centre of mass should be as close as possible to the axles so that around 10 per cent of the trailer’s weight is over the tow ball.
As dual axle trailers are generally heavier, they will have a higher tow ball load on the rear of a vehicle, which can cause the vehicle to lose weight over the front wheels, affecting braking and steering performance. The added weight on the rear axle can also affect suspension stroke, so the ride may become harsher as the vehicle’s suspension can’t work as effectively. Weight distribution hitches can help overcome these issues.
So, whatever trailer/axle configuration you decide on or end up with, you can be certain that if it’s well-designed and loaded effectively, it’ll be safe and comfortable to tow.
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