Axle Selection Techniques for DIY Trailer Builders

Choosing the right axle is crucial for the safety, functionality, and durability of your DIY utility trailer. The axle bears the trailer's load, distributes weight, and ensures proper handling. In this guide, we'll cover axle selection techniques, including dimensions and important factors to consider to help you make an informed choice for your DIY trailer.

1. Understanding Trailer Axles

A trailer’s axle consists of the beam, which connects the two wheels, and it plays a key role in supporting the weight of the trailer and its load. The axle is attached to the trailer frame through the suspension system, which helps to absorb shocks from the road. Common axle types include:

  • Straight Axles: Standard type found in most utility trailers, where the beam is straight across.
  • Drop Axles: The spindle drops down from the beam, lowering the trailer for easier loading/unloading.
  • Torsion Axles: Have a rubberized system within the axle tube to provide built-in suspension, offering a smoother ride.

2. Determining Axle Capacity

Axle selection begins with determining the weight your trailer needs to carry, which is known as the Gross Trailer Mass (GTM). For trailers built in Australia or the USA, compliance with local regulations is essential. As per the Australian Vehicle Standards Bulletin VSB1​, the axle system should be able to carry at least 120% of the load on each axle with the trailer at its Aggregate Trailer Mass (ATM).

  • Light Duty Trailers: For trailers with a GTM under 750kg, a single light-duty axle is typically sufficient.
  • Medium Duty Trailers: For GTM between 750kg to 2000kg, an axle rated around 1500-2000kg should be used.
  • Heavy Duty Trailers: For GTM above 2000kg, you'll need multiple axles or heavy-duty axles rated for 2500kg or more.

3. Axle Dimensions

The dimensions of your axle should match the width of your trailer and support the load it carries. Important dimensions to consider are:

  • Track Length (Hub Face to Hub Face): The total width of the axle, including the hubs, should match the width of your trailer. A typical track length for a small utility trailer might range between 1500mm to 2500mm.
  • Spring Center (Mounting Points): This is the distance between the mounting brackets that attach the axle to the suspension. It must be aligned with the suspension system of your trailer. Typical spring center spacing is between 1200mm to 2400mm.
  • Axle Tube Diameter: The tube size determines the strength of the axle. For example, a 50mm (2-inch) diameter axle tube may be sufficient for light trailers, while a 65mm (2.5-inch) or 75mm (3-inch) axle is recommended for heavier loads.

4. Choosing the Right Axle Type

  • Single Axle Trailers: For most light trailers, a single axle is sufficient. It’s simpler to install, maintain, and less expensive. However, ensure that the axle is rated to support the entire load.
  • Tandem (Dual) Axle Trailers: If you’re building a trailer that will carry heavier loads, tandem axles distribute the weight better and provide extra stability. When using tandem axles, make sure the load-sharing suspension system complies with local standards, especially in Australia, where this is mandatory​.

5. Suspension Systems and Load Sharing

In multi-axle trailers, the suspension system ensures that each axle carries an equal share of the load. The load-sharing suspension is required to maintain this balance and is crucial for trailers that have two or more axles. This not only helps improve tire wear and reduce stress on individual axles but also ensures the trailer complies with the legal standards.

6. Key Compliance Standards

Both the USA and Australia have regulatory standards that DIY trailer builders must adhere to:

  • Australia: Axle and suspension systems must comply with the Australian Design Rules (ADRs) as mentioned in the Vehicle Standards Bulletin 1 (VSB1). This includes specifications on axle placement, load capacities, and suspension types​.
  • USA: In the US, axle standards align with guidelines from the National Association of Trailer Manufacturers (NATM) and the Federal Motor Vehicle Safety Standards (FMVSS), ensuring road safety and performance.

7. Brake Requirements

In addition to axle capacity, brake systems should be factored in during axle selection. For trailers under 2000kg in Australia, overrun brakes may be used, but for trailers exceeding this weight, electric brakes are mandatory and must be operable from the driver’s seat​.

8. Material Selection

The axle tube material is critical for ensuring durability:

  • Steel Axles: These are the most common and provide excellent strength for most applications.
  • Galvanized Axles: For trailers used in corrosive environments (such as marine trailers), galvanized steel axles resist rust and prolong the life of the trailer​.

9. DIY Tip: Measuring Axle Placement

When mounting the axle, ensure the axle is positioned correctly to maintain proper tongue weight (typically 10-15% of the total trailer weight on the hitch). For single-axle trailers, the axle should be placed slightly behind the trailer’s center point to provide stability and avoid sway. For tandem axles, ensure the spacing is even and the load is shared properly by the suspension system.

By following these axle selection techniques, you can ensure your DIY trailer meets safety, performance, and legal standards, whether you're towing across Australia or the USA. Make sure to always reference local guidelines for specific compliance requirements before starting your project.