September 2011, TruckingInfo.com - Feature
The scene was a frozen lake in central Sweden in the early 1980s. On the ice in the midst of powdery, freshly fallen snow sat a municipal snow-plow truck, a Volvo N10 conventional-cab dumper fitted with a V-plow up front and a wing plow on its right side.
It was a three-axle truck, but its rear-most axle was raised and the tires on its single drive axle were chained.
"Now we will do some plowing," one of our hosts from Volvo Trucks said. "You will see how much traction the 6-by-2 truck has." There was sand in the dump bed and about 22,000 pounds on the drive axle, and its differential was lockable.
The 6x2 axle layout was and is the standard in Europe, and for a good reason: One drive axle carrying enough weight will push the truck or tractor over pavement better than two of them, because each axle of a "live" tandem will have less weight and less traction. Yet North American truckers bought 6x4s - a foolish practice, the Swedes thought, except for off-road work. Thus this demonstration.
I was among a few editors who drove that plow truck, and I was impressed at how well it worked. The two plows slammed aside snow that was 2 and 3 feet deep. Drive wheels spun some, and force from the wing plow sometimes kicked the truck's rear end slightly to the left, but it was very manageable. On a road tractor, a 6x2 works even better, the Swedes said.
In the late 1990s, Freightliner came up with a highway-type 6x2 that worked automatically. Jim Hebe, now with Navistar but then Freightliner's president, recalls that it was dubbed the Airliner Plus, as it used the builder's own air-ride suspension. A tractor's ABS controls sensed wheel slip and immediately bled air from the dead axle's bags; this shifted some weight to the single drive axle, which bore down harder on its wheels and tires. Otherwise, weight was equalized between the live and dead axles.
At a demo near Mount Hood, Ore., the Airliner Plus's traction was shown to be superior to that of a 6x4 tandem. We in the press reported on this, but Freightliner sold very few of the systems.
Over the years I've talked with truckers who said that yes, the setup does work - if they can flip a switch to lock the diff on the drive axle, and can raise the dead axle to put more weight on the driver. This is forbidden by some state authorities, but doing it automatically at least partially removes the legal obstacle. Raise the dead axle off the pavement while empty and turnpikes charge less in tolls, one guy told me.
The 6x2 has other advantages: Fewer gear sets means lower weight, by 300 or more pounds, and also reduces parasitic friction. Frank Bio of today's Volvo Trucks says he's seen reports of 6x2s delivering 3-5% better economy than 6x4s. Upfront cost is less by thousands of dollars, though resale value suffers because the 6x2 is an oddball spec. But special 6x2 drive systems that are convertible to 6x4s are now available. At trade-in time, you remove the dead axle, bolt in a new forward drive axle, interaxle diff and driveshaft, and off you go to the used-truck lot.
At the Mid-America Trucking Show show earlier this year, Meritor showed off a "concept" 6x2 system that, like the earlier Freightliner product, does the weight transfer automatically. It's a system whose time has come, representatives said. We'll see.From the July issue of HDT.