Feature: Unlocking the benefits of quad-axle trailers

Quad trailer axle groups are currently permitted to carry 27 tonnes on certain routes and under strict Performance Based Standards (PBS) rules. Since 2016 the National Transport Commission (NTC) has been investigating the possibility of extending quad-axle operations.

The NTC's preferred option is to allow quad-axle vehicles to carry increased mass on higher mass limits (HML) routes previously assessed as adequate for PBS-approved vehicles, without the need for vehicle operators to go through the PBS process.

However, as at January 2019, PBS approval is still a requirement for any quad-axle application, said PBS certifier, Ken Cowell.

With PBS approval, quad-axle groups in B-Double and semi-trailer combinations are limited to 20 tonnes at general mass limits (GML), but can carry up to 27 tonnes on HML routes.

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Over the past seven years, quad-axle semi-trailers in Australia have predominantly been seen hauling containers, although there are some tippers and waste trailers using quad groups – all on restricted routes.

'Super-B-Doubles' with quad-quad axle groups have been successfully hauling two 40-foot containers at 77.5 tonnes GCM in various locations around the country.

The NTC proposal, however, may see wider application of quad-axle groups.

The concept of multiple axles in groups is hardly new. Road authorities all around the world determine what axle weights they will permit, in the interests of pavement durability and bridge security.

In Europe, for example, single-drive-axle loads up to 13 tonnes are permitted, while in the USA and Australia a drive tandem is needed to spread that much weight. Tri-axle groups with single tyres can carry 20-plus tonnes in Europe, but in Australia duals are required to spread that load over a larger contact patch.

Possibly the most bizarre application of the multi-axle rule is in some states in the USA, where eight-axle combinations of single or double trailers behind three-axle prime movers can carry up to 74.5 tonnes (164,000lb).

Of course, such leviathans aren't permitted on the federally-funded US Interstate road network, where the GCM limit remains 36.4 tonnes (80,000lb) on five-axle semi-trailer combinations.

There's method in this apparent madness, according to research conducted by the American Association of State Highway and Transportation Officials and the Michigan Department of Transportation (MDOT). This research concluded that pavement damage is directly related to axle loadings, not gross vehicle weight.

In comparison with the Federally approved five-axle, 36.5-tonner, a Michigan 11-axle, 74.5-tonner combo puts less load onto the pavement. The Interstater has typical dual-tyre-axle loads around 7.7 tonnes and the Michigan mobster imposes only a shade over six tonnes.

Most of the North American highway network is concrete, so the issue of tyre scrub as 11 axles make it around a corner is probably not so critical as it would be in a bitumen/substrate road environment!

We're not likely to see 11-axle, non-road-train combinations in Australia, but if quad-axle group regulations are relaxed we'll certainly see more quad-axle semi-trailers and B-Doubles.

Container haulage was the initial impetus for quad axles, because of the internationally approved weight of ISO containers. Many incoming boxes from overseas are too heavy to be carried legally on existing combinations, meaning that they have to be unpacked. With LCL (less than a container load) shipments that's maybe not such a big deal, but with sensitive, homogenous freight it's a huge problem.

Such a situation was faced by David Tolson of Elf Mushrooms. "Why?" I hear you say: mushrooms weigh bugger all, don't they?

"Mushrooms don't weigh very much at all," David explained to Trucksales, patiently.

"But the imported peat we grow them in is damp and heavy. The best growing medium for our quality mushrooms is from Germany and with the price of freight it makes sense for us to bring in full containers that are legal to be hauled to the ports in Europe.

"However, that 20-foot container often couldn't be carried legally here on a six-axle prime mover and skel combination."

Part-unloading peat to lighten the boxes wasn't an option, given the risk of contamination of this quality material and the costs that would be involved.

David consulted with PBS certifying engineer, Ken Cowell, and chose the 'Blueprint' method as the simplest way to achieve the needed payload increase.

A quad-axle group semi-trailer can have a gross combination mass of up to 50.5 tonnes, with a quad-axle group load of 27 tonnes.

'Blueprint' designs for quad-axle semi-trailer and B-Double combinations were PBS approved some years ago and operators can use them to specify combinations that can have access on PBS networks.

Vehicle requirements include: dual tyres on quad-axle groups; a steerable rear axle with at least +/- 12 degrees steering articulation and an effective centring mechanism; a load-sharing system at least as effective as for a tri-axle group and road-friendly certified suspension. Lift axles complying with ADR 43/04 are optional.

A steerable trailing axle causes almost no tyre scrub when a quad group is turning and, because axle loadings are slightly less in the three non-steerable axles than they are in a conventional tri-axle group, pavement damage isn't increased over that caused by a tri-axle.

Interestingly, it may seem advantageous to fit a steerable lead axle in addition to the mandatory steerable trailing axle, but New Zealand tried that experiment and found that the two steering axles could interact unexpectedly at highway speeds and create severe instability.

Even after gaining Blueprint certification, operators need accreditation under the National Heavy Vehicle Accreditation Scheme (NHVAS) or equivalent mass and maintenance accreditation, and need to have the Intelligent Access Program (IAP) as a potential tool for route compliance.

Quad-axle group combinations can operate only where individual route assessments and individual bridge assessments have been conducted by the relevant road authority and council. Some authorities may be prepared to accept assessment by a pre-qualified bridge engineer, using the authorities' bridge data.

Ken Cowell urges prospective quad-axle operators to do their homework diligently; particularly with respect to route approvals.

Ongoing deliberation by the NTC may see easier compliance and increased route access for quad-axle groups but, until then, watch this space…