This article shares information on the use of the 6x2 rigid truck configuration overseas. The 6x2 offers a potential way of reducing costs and improving efficiency over the 6x4, although it is worth noting New Zealand legislation requires at least two motor-driven axles when operating a combination vehicle with a GCM greater than 39 tonnes.
A report from www.transportengineer.org.uk suggests that 6x2 tractor units are the workhorses of the UK’s road freight industry. A 2013 Confidence Report produced by the North American Council for Freight Efficiency (NACFE), updated in 2017, found that the uptake of 6x2s in the US market is steadily increasing as truck manufacturers and OEM suppliers improve their products and product choices. There are suggestions that by 2027, 30% of all new class 8 vehicles in the US may be 6x2. In the 2017 update of the NACFE report, the study teams were able to group 6x2 configurations into three generations, as shown in the accompanying table produced from the report. The generations tend to illustrate the impacts of changes in technology rather than design improvements.
Styles and operation
In the UK, the predominant 6x2 is the fixed mid-lift or pusher axle where the nondriven axle is in front of the drive axle, whereas in the US, tag axles fitted behind the drive axle are more common. There are advantages and disadvantages of both styles. Common advantages of the 6x2 include lower tare, providing the opportunity for an increased payload, lower capital cost, an average 2 to 3% reduction in fuel use compared with a 6x4 engaged in the same work, improved stability under some operating conditions, and with fewer components, 6x2 axles need less maintenance, resulting in lower operating costs and reduced downtime. In terms of disadvantages, when lifted, tag axles do lengthen rear overhang, inefficient load positioning can increase tyre wear with potential for loss of traction – although technology is rapidly overcoming this – and resale is often less compared with an equivalent 6x4. In a tractor and semi-trailer application with the non-drive axle lifted, weight transfer onto the kingpin may result in the kingpin becoming overloaded. For the mid-lift variant as used in the UK, disadvantages cited include decreased manoeuvrability in tight spaces, and increased maintenance and tyre scrubbing on driven axles if load sharing is not accurate. Driver perception of 6x2s is often poor, citing safety and traction as significant issues. The NACFE report suggests that some of this negativity could in part be due the in-service representation of 6x2s not matching the 6x4, and many drivers not suitably trained in their operation, therefore forming opinions based on populist industry views.
|1||6x2 with tag axle, no loadshifting technology, manual differential locks.||2 to 3% fuel savings; weight saving of 130 to 180kg compared with 6x4s; issues with accelerated tyre wear and traction under certain conditions, resulting in poor driver perception.|
|2||6x2 with tag axle, manual or automated load-shifting, traction control, engine operating parameters adjusted to reduce low-speed clutch engagement and engine brake torque.||Load-shifting and traction control mitigates traction issues; limiting torque can improve tyre wear significantly.|
|3||6x2 with liftable pusher axle, automatic load-sensing/ load-shifting, traction control, engine operating parameters adjusted to limit speed/brake torque.||An option for fleets expecting to carry less than 28,000kg at least 30% of the time, giving the potential for an additional 2% fuel savings; improved traction and tyre wear compared with Generation 2 under lightly loaded conditions; a heavier front axle, suspension and tyres and other equipment may optimise this configuration.|
In an evermore competitive transport and distribution environment, careful consideration of all options available should be undertaken as more margin is preserved through the accumulation of multiple initiatives. In New Zealand today, all of the major European and most Asian OEMs offer sophisticated 6x2 catalogues.