Trailer Drawbar Design
A key component of ensuring a trailer is compliant and safe to be used on the road is having evidence in place demonstrating that the drawbar complies with the applicable national road vehicle standards. As part of the Road Vehicle Standards Act, which governs how all vehicles and trailers are complied, more responsibility is placed upon manufacturers to have information in place proving this. A number of Australian Design Rules (ADRs) can be easily demonstrated through Component Type Approval numbers but manufacturers will typically require third party assistance in generating the required evidence to support their drawbar construction.
What makes up a Trailer Drawbar Design
A drawbar is defined within the ADRs as "portion of a trailer that connects the trailer body to the coupling for towing purposes". Drawbars are present on all nearly all types of light and heavy trailers, with semi-trailers the only type of trailer not having a part of the trailer defined as a drawbar. The frame used will vary depending on the chassis and body requirements of the trailer. The most common type of drawbar is an A-frame drawbar, which consists of two main structural members splaying outwards in an "A" shape from the coupling to the main trailer chassis. This design can be used on the majority of on and off road trailers, including flat decks, box trailers and caravans. Some trailers have a single drawbar connecting the coupling to the main chassis section, with this typically seen on boat trailers.
Design Considerations for a Trailer Drawbar
A trailer drawbar must take into consideration both strength and functional requirements. Without the drawbar having sufficient strength, which is determined by assessment to the necessary standards, it presents a safety risk to those operating the vehicle and anyone in the vicinity. Many trailer drawbars fail due to insufficient design for the rating required or poor manufacturing techniques such as welding or drilling holes in high stress areas
Functional requirements include mounting arrangements and dimensional considerations. Many drawbars are mounted beneath the trailer chassis, with the mounting achieved either by welding or bolted connections depending on whether the drawbar has a requirement to be removed. A drawbar's length is also a balance of strength and functionality requirements - a long drawbar induces a higher bending moment which can require the materials used to be of greater profile or grade but without this length key parameters such as rear tailgate clearance when reversing or turning corners can be compromised along with the ability to carry spare tyres, gas bottles or toolboxes in front of the body.
Required Standards for a Trailer Drawbar
Any drawbar must comply with ADR62/02, which provides requirements for drawbars in Clause 14. A drawbar can be physically tested to either the static or dynamic (fatigue) requirements within this clause, with allowance for drawbars to be complied via "approved calculations" provided that both the static and dynamic performance are evaluated through analysis of stresses within the structure.
Within the static performance requirements are individual test criteria to determine the drawbar's structural integrity under loading in the longitudinal (forwards and backwards), vertical (upwards and downwards) and transverse (left and right) directions. The magnitude of this loading depends on the aggregate trailer mass (ATM) of the trailer and whether the drawbar is rigid or can hinge vertically. Dynamic requirements are more complex - up to 3500kg ATM the coupling D-Value determines the load to apply but above this rating a number of additional factors need to be considered, including weight on the tow coupling and axle group along with dimensional properties such as the trailer load space and distance from coupling to the centre of the axle group. A worst case scenario can be assessed for to cover multiple designs, for example a drawbar rated to 3500kg can be used at a lower rating without requiring additional testing.
Our Trailer Drawbar Design Services
BTT Engineering has three main options for providing trailer drawbar design certification for its clients:
Custom Drawbar Design
For individuals or manufacturers looking to have a drawbar designed specifically for their needs, BTT Engineering can arrange for 3D design and modelling, analysis, reporting and manufacturing drawings to be compiled. As part of this option, clients must provide any design requirements which BTT must be aware of when generating the design, including minimum and maximum dimensions, maximum ATM, preferred materials and any other items of note. Upon completion of the design, clients will receive an ADR62/02 compliance report and manufacturing drawings detailing the design.
Analysis/Testing of Existing Design
Many manufacturers will already have a preference as to how they want their trailer's drawbar to be constructed, including proposed length, materials, rating and attachment method. In other situations, an individual may purchase or build a trailer with an already built drawbar requiring compliance. BTT Engineering can review these designs by either calculation or physical testing and assist with design modifications if the initially proposed design does not comply with the required standards. At the conclusion of testing or analysis, an ADR62/02 compliance report will be provided detailing the work carried out and the results in accordance with the RVSA legislation.
Anyone looking to have their designs assessed through computation by BTT Engineering will need to ensure that the relevant information can be provided for analysis. This can be achieved through a number of methods:
3D CAD Files - this is the easiest pathway to having a drawbar assessed as manufacturers with access to 3D CAD modelling software can supply their design in full to BTT Engineering with only minor preparation required prior to analysis
2D manufacturing drawings - supplying 2D manufacturing drawings provides details on how the drawbar is to be constructed but will require modelling by BTT Engineering as a 3D CAD model prior to analysis
Hand sketches - manufacturers/trailer owners with no formal documentation detailing their drawbar design can provide sketches and pictures for BTT Engineering to convert into a 3D CAD model and analysis, similar to the 2D manufacturing drawing path. It is strongly recommended that manufacturers having their drawbar assessed via this method also have 2D manufacturing drawings compiled post-analysis to formalise their design
Site inspection - BTT Engineering can attend on site and take the required dimensions to enable generation of 3D models for analysis. Information such as material properties can be hard to identify on site via visual inspection and may require use of equipment such as hardness and thickness testers. Like the hand sketch option, it is strongly recommended that 2D manufacturing drawings are generated at the conclusion of this analysis.
Supply of Pre-Assessed Drawbar Designs
For clients looking for a fast, easy and cheaper solution to having their design individually assessed, they can opt to purchase any of BTT Engineering's pre-assessed drawbar designs, which are part of our Drawbar Design Guide package. With the purchase of any design comes all the information required for compliance and manufacturing, including both a report and drawings with structural member details issued in the client's name for their use. The Drawbar Design Guide aims to provide designs as simplistic and readily available as possible, meaning that the manufacturer is easily able to add any additional fixtures or equipment alongside the base design.
Our Drawbar Design Guide currently includes steel A-frame drawbars rated from 750kg up to 4500kg. Our designs above 4500kg have been physically tested in-house, negating the need for fatigue considerations and therefore removing restrictions on parameters such as body lengths which are present for drawbars assessed via calculations which need to consider dynamic loading.
Why Choose BTT Engineering for trailer drawbar design?
BTT Engineering has a team with extensive experience with compliance for both light and heavy trailer drawbars. Our team consists of multiple professional engineers with the appropriate qualifications to provide engineering analysis in states requiring engineering registration. We have access to quality simulation software to run assessments via Finite Element Analysis (FEA) and are a registered test facility holding a Test Facility Approval (TFA) enabling us to conduct physical testing to ADR62/02 requirements. Our all-round knowledge of both the theoretical and practical side of trailer drawbars means that we can deliver on customer's requirements in a timely manner and provide maximum value during the process.
As per the Department of Infrastructure's guidance note What is an approved calculation for ADR 62? it is stated that "Calculations must be supported by physical validation testing". BTT Engineering has completed numerous drawbar studies where results obtained via Finite Element Analysis have been further investigated through having physical testing carried out on the same structure, with testing demonstrating that our method of analysis is an accurate representation of results obtained when loading the structure in a real-world scenario. This means that clients having their design assessed by BTT Engineering can have piece of mind that any results obtained are supported by physical evidence and are not a misrepresentation of the design provided.
Frequently Asked Questions (FAQs)
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If you can't obtain evidence of compliance from the design you are copying, you will need to have this assessed.
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Safety chains cannot be validated by calculation as per determinations from the RVSA policy team and must be physically tested. BTT Engineering has safety chain attachment designs available to purchase and also has the capability of physically testing any design a manufacturer may have. More information on safety chain testing can be located here.
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No, without taking into consideration both static and fatigue resistance, a calculation for a drawbar cannot meet the ADR62/02 requirements.
Related services that we can help with
For further information on some of our particular ROVER services, please follow the links below: