Manufacturers love to boast about their big 3.5 and 4.5 tonne towing capacities. Unfortunately, these are mostly untrue.
To understand why, we need to take a step back and understand that there are two types of trailers:
- Pig. Has 1 axle group, which is located near the middle of the trailer.
- Dog. Has 2 axle groups, which are located at each end of the trailer.
The key difference between the two is the weight of the TBM.
Pig trailers have a much heavier TBM because they, in effect, use the towing vehicle’s rear axle for stability. In a pig trailer, the TBM will go up proportionately with the weight of the entire trailer. It needs to, because a light TBM on a heavy trailer is a recipe for an unbalanced rig that is prone to sway, poor braking and vague steering.
Dog trailers, meanwhile, have relatively light TBMs because all of the trailer’s weight is supported by its own axles. It’s not uncommon for Kenworth trucks to be towing a 50-tonne dog trailer that has a TBM of just a couple of hundred kilograms. The TBM is comprised mostly of the weight of the drawbar and connecting hardware.
Why is this important?
When manufacturers quote a 3.5 or a 4.5-tonne towing capacity, they intentionally do not specify whether this is in the context of towing a dog or a pig trailer – because it gives them:
- a legal escape route if challenged
- the ability to hype up marketing through omission
But you, the consumer, will almost certainly look at the towing capacities through the lens of a pig trailer – because that’s the design of most Australian caravans.
As such, you’re stuck with two big problems:
- heavy TBM that increases proportionately with the trailer’s ATM
- TBM tax – an approximately 40% multiplier of TBM on your rear axle, caused by the distance of your tow ball from the centre of your rear wheel
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