It's still an oddball spec in road tractors, but some believe the 6x2 — a single drive axle with two powered wheels among six total wheel positions — will someday become more common as truck operators look for ways to save fuel and cut tare weight.
Today, a number of progressive fleets are testing or embracing it, but it's been a long time coming.
When AB Volvo came to America from Sweden in 1981 and ‘82 to help resurrect the products of the bankrupt White Motor Corp., they were astonished to see tractors running up and down our highways with “live” tandems. This was a complicated and heavy way to do the job, the newcomers thought.
When told Americans preferred the 6x4 to get better traction, they insisted it wasn't necessary. The 6x2 tandem with a liftable dead axle worked as well, weighed less and cost less money. That type of 6x2 was common on the highways of Europe and still is.
A couple of years later, Volvo flew a group of truck writers to Sweden to show off their European products and explain how some of those designs were starting to transfer to their American-made trucks. At one point a couple of engineers demonstrated how a 6x2 truck could plow deep snow, letting the journalists drive it on a frozen lake to see and feel how it performed.
The truck, a municipally owned Volvo N10 dumper outfitted with a large V-plow in front and a wing plow on its right side, moved the snow easily, because its dead tag axle was raised and its chained drive tires carried about 22,000 pounds, giving it considerable traction.
Around the same time, a supplier sought fuel economy numbers with a simulated 6x2. Engineers disconnected the tandem's rear axle and locked the interaxle differential so power still went to the forward axle.
On several runs in east Texas, they saw a benefit of barely 1% — statistically meaningless, and something a fleet probably couldn't measure in day-to-day operations. However, recent testing by suppliers and fleets of true 6x2 tandems without the extra gears indicate 2% to 3% better economy, with some results as high as 5%.
In the 1990s, ‘80s and before, many tractors operated by less-than-truckload fleets were 6x2s. Some had lift axles and some did not. With a lift axle, traction was good.
Back then, a driver for Holland Motor Freight out of Michigan said how much he liked the arrangement on his Ford Louisville tractor: “When I run into snow and ice, I just raise that axle and it'll go through stuff that will stop a 6x4 dead.”
However, 6x2s whose non-powered axles always stayed on the pavement were cursed by their drivers in foul weather, because drive wheels spun from insufficient traction and they sometimes got stuck.
Another builder with European ownership didn't give up on the 6x2 idea. In the late ‘90s, Freightliner Trucks came out with its own 6x2.
It was dubbed the Airliner Plus, as it used the builder’s proprietary 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.
However, Freightliner sold very few of the systems and eventually dropped it.
Sodrel Truck Lines, an Indiana-based mail hauler, was running 6x2s in the 1980s and still does today. Back then, its Mack R-model tractors had liftable pusher axles, and Mike Sodrel, the family-owned company's president at the time, said they helped saved fuel.
The fleet still runs the same configuration on more modern Mack day-cab models, of which it now has 230.
“If I can get all the wheels off the ground in a safe and legal manner — that may sound funny — but there's less drag, obviously,” says Vick Morgan, a former driver who's now vice president of safety and operations. “If I can raise the wheels, there's less wear on the tires, and tires are expensive, right?”
Sodrel's system is driver-controlled, based on weight, he explains. An air gauge on the dash tells drivers when they have to lower the pusher because of weight. “When they drop the axle, the thing levels out at a certain number. If the gauge is then above that number, we have to tell the customer that we would have to scale the load before we can take it. We can't be illegal.”
Nussbaum Transportation out of Normal, Ill., is one fleet that is converting to 6x2s today, according to Justin Donley, a former driver and now fleet maintenance manager.
It has retrofitted a Meritor system to 120 tractors. “The fuel mileage is doing real well,” he says. “The only issues are in the winter, when drivers get too heavy on the throttle and the wheels spin. We're training them to ease off on that.”
Driver-controlled dump valves have been added to some of those 120 tractors. Nussbaum has 140 Freightliner Cascadia tractors on order that will have the Meritor 6x2 with electronically controlled dump valves.
In this era of diesel fuel at $4 a gallon and more, fleet executives are trying almost everything to cut fuel use, suppliers say. Tractors have also gotten heavier, primarily from exhaust aftertreatment gear that adds about 400 pounds per vehicle, so managers are trying to trim weight out of other components. Some are looking at the 6x2 to meet both goals.
Eliminating a 6x4 s interaxle differential, second axle diff, and the driveshaft and U-joints between them by going to the simpler 6x2 reduces weight by varying amounts. It can be 350 pounds to 450 pounds, says Steve Slesinski, director of global commercial product planning at Dana Holding Corp.
But heftier differential gears and axle shafts in that single drive axle are needed to take today's high horsepower and torque. That can quickly wear out tread on the drive axle's tires, so they might need blockier tread patterns, or tires need to be rotated often between live and dead axles to even out wear.
Overwhelming popularity of the 6x4 configuration has given it higher residual values. Over the past 10 years, 6x4s have averaged a 6% greater value than 6x2s, according to the Truck Blue Book. For late-model vehicles the difference is 11%, but for ‘06, a 6x4 is actually worth 4% less than a 6x2.
In any case, axle suppliers have come up with modern iterations of the 6x2.
On these modern 6x2s, the dead axle is a tag, placed behind the single drive axle. The tag does not fully raise off the pavement, but pressure in its air springs can be reduced so some of its weight is transferred to the drive axle. Its differential can be lockable, either manually, when a switch is thrown by the driver, or automatically, through electronic controls.
Last year Meritor brought out its FueLite 6x2 tandem and offered it with an electronically controlled air suspension, or ECAS, from Meritor Wabco.
ECAS piggybacks on the electronic controls of a tractor's antilock braking system and traction control to sense when wheel spin begins. The controls quickly dump some air from the dead axle's springs to transfer weight, and traction, to the single drive axle. If a locking differential is used, the system automatically engages it, then disengages above a certain road speed.
Meritor also developed a more advanced 6x2 system called SmarTandem. It comes standard with electronic controls that manage weight transfer to the drive axle and engagement of its locking differential. The diff uses a clutch, so it can be employed at higher speeds than the spline-action diff on the FueLite.
Bendix offers its eTrac automated air pressure transfer system for Bendix ABS-6 braking systems with automatic traction control for 6x2 Class 8 tractors. The system helps drivers of 6x2 vehicles overcome low-traction events by fully automating the air pressure transfer process.
When added to Bendix Smart ATC (automatic traction control), eTrac automatically engages and disengages the vehicle's air bag pressure transfer system during low-traction events. The system transfers pressure from the undriven axle to the driven axle without requiring manual input or driver action.
When the Bendix eTrac system engages, it automatically evacuates air bag pressure, dropping the nose of the trailer. The resulting extra forward weight on a 6x2 tractor helps compensate for the lack of a second drive axle to deliver traction control that is comparable to a 6x4 tractor.
Dana Spicer has a 6x2 tandem called EconoTrek that also uses a tag axle. It's available with a locking differential that's compatible with automatic weight-transfer systems, or the diff-locking and weight-transfer functions can be driver controlled.
Dana also has a “convertible” system that allows truck owners to change an existing 6x4 tandem into a 6x2, or convert a 6x2 into a 6x4 for greater resale value. There's some cost to a conversion, but it answers the 6x2's major financial drawback, that its oddball status hurts residual value.
Residual values are a reality that has helped keep the 6x4 popular. In the last 10 years, a 6x4's average value has been about $5,500 more than that of a 6x2, according to Jessica Carr, an associate analyst at Truck Blue Book. That more than exceeds any purchasing savings a 6x2 might have, and mostly offsets the fuel savings over four or five years of service.
Then again, a 6x2 believer can say that fuel savings pay for any loss in residual value. And there's still the weight advantage.
In sales, the 6x2 is still a minor thing. “It is now in only about 3% of Class 8 trucks,” says Matt Stevenson, general manager for Meritor's North American field operations and marketing. “In five years, that should grow to 18%,” which would be significant.