Trailer : Truck Mass Ratio and Simple Trailers
By Andrew
Jun 08, 2009
I was asked recently about the legality of a 3-axle simple trailer, versus the more usual 2-axle model. These 2-axle trailers are really useful, but are limited to carrying about a 15 tonne digger. Surely, a 3-axle model could carry an 18 tonne machine?
A simple trailer is like an over-grown car trailer, with a tandem axle set in the middle, and a rigid drawbar that transfers weight onto the towing vehicle’s coupling - usually around 4 tonne.
The gross weight for that tandem axle set is usually 14.5 tonne, as the axles are usually less than 1.3 metres apart. A 3-axle set is permitted to run at 18 tonne.
Clause 4.3(6) of the Dimensions and Mass Rule 41001 specifically limits the Gross Combination Mass for a truck and simple trailer to 32 tonne.
Sections 4.4 of that same Rule specifies that the Trailer : Truck Mass Ratio for combination vehicles shall be 1.5:1 or less - that is, the gross mass of the trailer must not exceed 1.5 times the gross mass of the towing vehicle.
That’s pretty straight forward for, as an example, a rigid truck towing a full trailer - the 1.5:1 ratio sets the maximum trailer weight at 60% of the combination, so if we are running at 44 tonne gross combination mass, the trailer is limited to 26.4 tonne.
This is where I should have checked my logic a little more closely, because my first thoughts for the simple trailer application went something like:
Maximum Combination Mass = 32 tonne
Trailer : Truck Mass ratio = 1.5:1
so maximum gross weight of trailer = 32 tonne x 60%
= 19.2 tonne
typical trailer Gross Vehicle Mass = 14.5 tonne on wheels plus
4 tonne on coupling
= 18.5 tonne
Conclusion: Not much point in having a 3-axle simple trailer, because you can’t utilise the full 18 tonne permitted for a 3-axle set.
Wrong.
Read the words and definition of terms more closely - Clause 4.4 refers to “gross mass”, which is defined as “…the mass of that vehicle and its load…which may be determined by adding the mass on the vehicle’s axles or axle sets.”
Note the subtle difference between these words and the definition for “Gross Vehicle Mass”, which is “…the gross vehicle mass specified…by the manufacturer…”
The 2-axle simple trailer mentioned above would have a manufacturer’s Gross Vehicle Mass of 18.5 tonne - that is, you can load it up to 18.5 tonne total - that is, 14.5 tonne on the wheels and 4 tonne on the coupling - and everyone is happy.
From the Trailer : Truck Mass Ratio aspect, however, things are slightly different.
The Mass Ratio considers the trailer’s “gross weight”, being the sum of the weights on its axles or axle sets - in this case, only 1 axle set, running at 14.5 tonne.
Therefore, for a combination mass of 32 tonne, the truck can run at 32-14.5 = 17.5 tonne, giving a mass ratio of 0.83:1 - well below the permitted maximum.
So, if we consider a 3-axle simple trailer with 18 tonne on its wheels, the truck can weigh 32-18 = 14 tonne, giving a mass ratio of 1.29:1, still below the maximum.
Our 3-axle trailer would have a tare weight of around 4.0 tonne, so the laden weights would be something like:
Mass on axles = 18.0 tonne
plus mass on coupling = 4.0 tonne
less tare weight = 4.0 tonne
gives a payload = 18.0 tonne
Conclusion: Yes, you may carry an 18 tonne machine on your 3-axle simple trailer.
Subsequent note to self: Read those words more carefully.
