Four- and five-axle trucks have a twin-steer tandem axle set at the front. This enables the truck or tractor unit to carry more weight. Rather than only being able to legally support 6000kg, 7200kg or 7600kg, which is the rating for single standard, single large or single mega tyres, respectively, the maximum axle mass can be 11,000kg regardless of the types of tyres, as shown below.
Each axle has single tyres and both axles are connected to the same mechanism so that they steer in the same direction. In the diagram below, S = Standard, SL = single large and SM = single mega. You can find out about what this means in our maximum axle mass course as the implication for a truck operator is that the additional tare weight of the extra axle can be negated by any additional weight capacity, especially if the vehicle is already operating at or near the maximum limit.
Each steering axle has its own fluid reservoir that must be checked in the pre-trip inspection.
In terms of driver comfort, twin steer axles can reduce the see-saw effect that you can get on 3-axle trucks with a single steering axle.
Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above
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