A common sight in Australia is a three-axle truck with a very long drawbar, pulling what Aussie truckers call a “dog trailer”.
Overseas drivers always ask why waste all that space between the truck and trailer on a drawbar that is up to 4.5 metres long?
The answer is that the long drawbar does not waste space, but more than double the load capacity of the dump truck by adding a trailer.
Designed to jackknife
The length of the towbar is specifically designed to allow jackknifing the trailer at the dump site after its load has been dumped. This exposes the rear of the truck so that its load can be dumped neatly next to the trailer’s heap.
To jackknife the trailer, the driver reverses with careful quarter turns on the steering wheel, until the trailer is parallel to the truck. When both loads are dumped, this parallel positioning allows for a clean exit, without having to pull the trailer over the fresh heaps, or dumping any of the load on the towbar.
Fast metal fatigue
While the long drawbar allows for neat dumping of an extra load, it comes with the drawback of quickly suffering metal fatique along the welds and bolts. This damage appears as very fine cracks which are difficult to spot, but drivers look for fine lines of red rust that shows on critical points after washing the drawbar.
Another disadvantage of the long drawbar is that it adds to the whiplash effect should the driver have to swerve suddenly upfront, which is why the dog trailers are built with stability in mind.
Short-short trucks
But the long drawbar does allows Australians to build short trucks that look like nothing else on Earth, with the sleeper cab often as big as the load bin!
The length of the dog and trailer is limited to 19 metres (compared to South Africa’s 22 metres). The maximum combined mass of these combinations is 43.5 tonnes, although quarry and mines operators often get a permit to exceed this, national limit, loading “Quad Dog” trailers (with five axles), or “Quin Dogs” (with six axles) with many tonnes more.
(Photo credits: Grays Auctioneers, Tactical Aerial Solutions, Ritchie Bros Auctioneers.)
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