Twinsteer

The purpose of this webpage is to provide operators with mass limits and access information relating to prime movers or rigid vehicles fitted with a twinsteer axle group. 

For information on B-Doubles or Road trains fitted with a twinsteer, refer to the relevant notice. 

What is a twinsteer prime mover / rigid vehicle? 

Twinsteer is a term given to a hauling that is fitted with a tandem steering axle group.  

The tandem steer axle group is fitted with single tyres to both axles and are connected to the same steering column. The axles are spaced at least 1m apart but no more than 2m apart. 

 

Dimensions for a prime movers or rigid vehicle fitted with a twinsteer axle group 

A prime mover or rigid vehicle fitted with a twinsteer axle group must comply the prescribed heavy vehicle general dimension in the Heavy Vehicle (Mass Dimension and Loading) National Regulation (MDL Regulation) or operate under a notice or permit. Refer to Table 1 

Table 1 Overview of Heavy vehicle prescribed dimensions affecting twinsteer hauling units  

Measurement 

Maximum dimension  

Width 

2.5m 

Height 

4.3m  

Length 

12.5m 

Rear overhang 

3.7m or 60%of the wheelbase  

(whichever is lesser) 

 

Mass limits for a prime mover or rigid vehicle fitted with a twinsteer axle group  

General mass limits  

General Mass Limits (GML) apply to all heavy vehicles. The MDL Regulation provides two GML for a twinsteer axle group, the mass limits are determined based on the fitment of loading sharing suspension. The twinsteer vehicle must not exceed GML unless operating under an exemption under the HVNL (Notice or Permit).  

Note: A hauling unit fitted with a twinsteer axle group, may need to operate at a reduce mass when in a combination in order to comply with a total combination mass limit under the MDL Regulations or a Notice. 

Load sharing suspension means, an ‘Axle Group’ suspension system that utilises hydraulic, pneumatic, mechanical or other means to effect substantially equal sharing, by all the ground contact surfaces of the ‘Axle Group’, of the total load carried by the ‘Axle Group’ and has effective damping characteristics on all ‘Axles’ of the ‘Axle Group’. 

Table 2 GML applicable to a prime mover or rigid vehicle fitted with a twinsteer axle group.  

Table 2 Mass limits of a prime mover or rigid vehicle fitted with a twinsteer axle group 

Twinsteer axle group of a prime mover or rigid vehicle  

Mass (t) 

Steer axle  

Twinsteer axle group without a load-sharing suspension system 

10.0 

Twinsteer axle group with a load-sharing suspension system 

11.0 

Single drive group of a prime mover or rigid vehicle  

Mass (t) 

Single axle group fitted with single tyres with section widths of -  

a. less than 375mm   

b. at least 375mm but less than 450mm  

c. at least 450mm  

 

6.0 

6.7 

7.0 

Single axle group fitted with dual tyres 

9.0 

Tandem drive group of a prime mover or rigid vehicle 

Mass (t) 

Tandem axle group fitted with single tyres with section widths of -  

a. less than 375mm   

b. at least 375mm but less than 450mm  

c. at least 450mm 

 

11.0 

13.0 

14.0 

Tandem axle group fitted with single tyres on 1 axle and dual tyres on the other axle   

13.0 

Tandem axle group fitted with dual tyres on both axles 

16.5 

Tri-axle drive group of a prime mover or rigid vehicle  

Mass (t) 

Tri-axle group fitted with -  

a. single tyres with section widths of less than 375mm on all axles  

b. single tyres with section widths of less than 375mm on some axles and dual tyres on the other axles 

 

15.0 

Tri-axle group with  

a. single tyres with section widths of at least 375mm on all axles  

b. dual tyres on all axles  

c. single tyres with section widths of at least 375mm on some axles and dual tyres on the other axles 

 

20.0 

 

  

Axle mass spacing limits 

In addition to the axle group mass limits mentioned above, the distance between axle groups determines the maximum allowable mass over those axle groups.  

Example 1 Twinsteer maximum mass limit spacing requirement, demonstrates the minimum axle spacing requirements of a prime mover fitted with a twinsteer axle group, required to obtain the maximum GML as mentioned in Table 2 (11t for load sharing)  

Axle spacing mass limits means, the distance between axle groups which determines the maximum allowable mass over those axle groups.   

Example 1 Twinsteer maximum mass limit spacing requirement 

 

 

Networks and approved access areas 

A prime mover or rigid vehicle fitted with a twinsteer axle group are eligible for general access in participating jurisdictions of the Heavy Vehicle National Law (HVNL).  The combination must comply with GML requirement, including axle mass and axle spacing limits . 

General access means, these vehicles don’t require a permit or notice to run on the road networks. These vehicles have as-of-right access to the network unless otherwise signposted. 

Note: The operators must comply with any other road rules or direction of an Authorised Officer including infrastructure load limits. 

 

Additional resources  

Twinsteer prime mover towing a semitrailer  

National Class 2 B-Double Notice  

National Class 2 Road Train Notice