We all benefit from increasingly long and heavy roadtrain combinations.
With less fuel burned per tonne of payload, there is less environmental damage and less costs that must ultimately be passed on to consumers. However, it is important that there is no trade-off with safety.
Professional drivers and the community quite rightly expect that heavier trucking fleets are just as safe. Automatic Pin and fifth wheel couplings must handle more load on quad roadtrains — yet these forces have never been investigated.
ARTSA Institute led a world-first research project to work out the forces and determine what the future of heavy roadtrain couplings will look like in Australia.
The project was funded by the National Heavy Vehicle Regulator’s (NHVR) Heavy Vehicle Safety Initiative (HVSI) that is supported by the Australian Government. For roadtrains, there are no redundant mechanical systems such as safety chains.
Community safety relies primarily on the structural integrity of the coupling.
The potentially ugly secondary safety measure is that a runaway trailer’s brakes come on automatically when the trailer’s airline connection breaks.
Australian Standards provide engineering formulae to work out the minimum coupling rating.
These formulae, which can be found in AS 2213 and AS/NZS 4968, are based on testing that took place in outback South Australia in the 1980s by the ARRB (now NTRO).
At the time, triple-trailer roadtrains up to 125 tonne were tested on remote roads, and strain gauges were used to measure coupling forces.
In short, the 1980s triple roadtrain tests showed that the forces through roadtrain couplings were about 40 per cent lower than as predicted using the formulas that apply to single trailer trucks.
This was good news. Based upon these results, Australian and European Standards were updated, which allowed the use of regular couplings on heavy roadtrains.
Moving forward to the present day, quad roadtrains in use on Australian roads weigh in at around 160 tonnes, and there are prospects to increase combination weights towards 200 tonne.
Questions linger. Do the 1980s formulas hold up? Can conventional couplings continue to be used on increasingly large roadtrains?
If not, fleets would need to fit a new suite of couplings that are bigger, heavier, more expensive, and not interchangeable with the regular trucking fleet when roadtrains are separated.
The project involved measurement of the coupling forces on quad-trailer 160 tonne roadtrains during routine daily journeys of hundreds of kilometres while underway in the Northern Territory, with part of the route on secondary roads.
ARTSA-i led an industry-wide working group bringing in key personnel from HVIA, ATA, and TIC.
Engineers from the various coupling manufactures in Australia and Europe also contributed their expertise, including Ringfeder/VBG engineers who brought their Swedish coupling integrity testing experience.
The research team pulled together industry support from all corners of the country.
A purpose-built dolly manufactured in Perth by Howard Porter was fitted with a custom drawbar in Melbourne by CIMC.
The dolly was fitted with specially designed hardware and measurement sensors by Smedley’s Engineers, before being transported to Direct Haul’s facility near Darwin.
Both the automatic pin and fifth wheel couplings recorded an array of force, speed, GPS, accelerometer, and road topography data in real time collecting orders of magnitude more data than the 1980s testing program that preceded it.
The challenging fieldwork, then the painstaking task of post-processing, was undertaken by Smedley’s Engineers, with mechanical engineer Dion Simms wrangling the mountains of collected data.
Three separate testing journeys took place. The instrumented hinged drawbar dolly was moved to the front, middle and rear positions in the quad combination for each respective journey, allowing the research team to compare coupling forces at the different locations.
When the data was pulled apart a number of significant findings were revealed, including a few surprises:
• The forces experienced by the couplings were similar whether the dolly was at the front, middle or rear locations within the roadtrain. This was due to dynamic forces and impacts as the roadtrain snakes along the road, including whiplash and shunting. This result was contrary to conventional understanding as reflected in the current Australian Standards, which anticipates lower forces for couplings further from the centre of the train.
• The worst-case peak forces were reasonably well predicted by the existing formulae in the Australian Standards, with forces increasing in proportion to the weight.
• By far the most common occurrence of very high forces was not when the roadtrain was underway at speed, but instead when slow or coming to a stop: braking, shunting, and manoeuvring.
• The highest peak forces were measured while underway at speed over floodways. These peaks corresponded with a sudden pitch of the hinged drawbar dolly.
• Shunting within the coupling connection when braking at any speed resulted in high coupling forces, due to the delay in air braking signal between the front and rear trailers.
• Measured forces were input to proprietary simulation software to calibrate a roadtrain computer model. It was found that even the most advanced simulation software available was severely limited when considering the complexities of the roadtrain combination being driven down a real-life road. Nothing replaces actual field measurement to observe and learn from the complex dynamic phenomena that occurs.
A number of strategies may be considered to minimise the forces through roadtrain couplings without having to make a step-up in coupling size and spec:
1. Electric brake systems, which coordinate brake application for all trailers at once, should be mandated for all roadtrains over a certain mass.
2. Maximum wear tolerances are needed for roadtrain couplings over a certain mass. This may be a lower wear limit than that allowed by coupling manufacturers.
3. Consideration of rigid drawbar dollies to reduce the peak forces due to the pitch – and – shunt forces over undulating terrain.
4. Given that peak forces were similar at all dolly locations, couplings spec should be based on the worst case coupling within the entire roadtrain.
a. Taking this to the theoretical limit based on the formulae within existing standards, the maths reduces to the following simple formula: COUPLING D-VALUE (in kN) = 1.5 X GCM (in tonne).
b. For example, a 160-tonne roadtrain would require a minimum D-Value of 240kN for all couplings. Put another way, a 240kN coupling would be 160-tonne ‘roadtrain’ rated. c. Standardising connections throughout the roadtrain has interchangeability benefits, and makes it easier for drivers, fleets and regulators to check the component specs.
5. An important first step will be to do the work to better understand the damaging peak loads during braking and lower speed manoeuvres.
It is hoped that strategies such as these will enable roadtrain drivers and operators to safely utilise regular coupling sizes on heavy roadtrain fleets into the future.
Wayne is a chartered professional mechanical engineer, an executive director of ARTSA Institute, and was engaged as the Engineering Manager for this research project.