There remain sharp transatlantic differences in heavy truck configuration, the most notable being the near universal acceptance of bonneted cabs in North America, in contrast to the full forward-control – or ‘cab over’ – layout accepted for many decades in Europe. Without the imposition of a legal overall length limit for tractor-trailer (i.e. articulated) combinations by the US federal authorities or by individual states, that situation is unlikely to change.
But there are other, less obvious, engineering features taken for granted in European multi-axled chassis, especially maximum-weight tractors, that are a comparative rarity on equivalent US truck models, but which now show signs of being adopted much more widely. A particular subject of lively debate is whether a typical long-haul tractor-trailer rig requires two driven axles. The majority of North American maximum-weight combinations comprise a 6×4 (i.e. double drive) tractor hauling a tandem-axled semi-trailer.
But there are other, less obvious, engineering features taken for granted in European multi-axled chassis, especially maximum-weight tractors, that are a comparative rarity on equivalent US truck models
In Europe, where permitted all-up weights are generally higher than in the US, a single drive axle proves adequate, often on a straightforward 4×2 tractor, typically hauling a tri-axle semi-trailer. Where EU legislation demands six axles in total, implying three on the tractor, a single-drive 6×2 configuration is the norm, though that can come in several variations; the non-driven third axle can be in front of or behind the drive axle, in order to achieve optimum weight distribution.
Double-drive axle bogies are normally specified in Europe only on six- or eight-wheeled chassis – whether rigid chassis or articulated tractors – where off-highway traction is a key issue, most obviously on tippers and other construction industry vehicles which have to negotiate unmade ground. In muddy, sandy and other soft and often unpredictable conditions, for instance waste landfill sites, traction relies on a degree of ‘flotation’, requiring the maximum possible contact area between drive axle tyres and the ground.
In the US, 6×2 tractors are becoming more common, with automatic axle load transfer included in many cases
But for on-highway trucks, the traction requirements are quite different. On hard surfaces which are wet or icy, or on challenging gradients, tyre adhesion depends on maximising the load imposed on the road surface by the driven wheels, in order to ‘bite through’ and gain a better grip. A single driven axle is best placed to do so, provided that, when wheelslip occurs (or is imminent), extra weight can be temporarily imposed on that axle. Accordingly, nearly all European 6×2 and 8×2 chassis embody some form of load transfer device, most commonly relying on deflation of the air suspension on the adjacent non-driven axle.
In the US, 6×2 tractors are becoming more common, with automatic axle load transfer included in many cases. Paccar has just announced the Bendix eTrac system as a line-fitted option on its 6×2 Kenworth and Peterbilt tractors. It points out the (unspecified) cost saving, compared with a double-drive set-up, as well as the all-important tare weight reduction, put at around 230kg, which can be translated directly into payload gain.
Thankfully, the legislators have so far turned a realistic blind eye to the fact that when axle load transfer devices are operative, the load on the drive axle is often likely to exceed, momentarily, the permitted maximum, which in Europe is 11.5 tonnes.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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