Jonathan S. Reiskin |
Highway tractors with 6×2 power configurations have been heralded as an important technology for improving fuel mileage, but fleet owners have been hesitant in selecting the systems because they have a tendency to run through tires more quickly and because diesel is less costly than it was earlier this decade.
The North American Council for Freight Efficiency issued its update on 6×2s on July 11, and noted the systems are now in their third generation of development. NACFE Executive Director Mike Roeth said 6×2s still offer an opportunity for fuel saving — a 2.5% increase in miles per gallon — but successful adoption is more complicated than originally seen in the council’s 2013 report.
“In 2014, we expected pretty quick adoption, which did not happen,” Roeth said during a telephone press conference. He listed diesel prices as a major factor.
The five-year, U.S. retail diesel average price from 2010 to 2014, NACFE’s original test period, was $3.71 per gallon. In contrast, for the year ended July 10, the diesel average was $2.49 a gallon.
Tire wear was also a big part of the NACFE report.
“Drive tire wear on 6×2s will not achieve parity with 6×4s. However, measures can be taken to reduce the accelerated wear. Selecting a retread trailer tire for the free-rolling axle may result in the lowest cost option for fleets,” report said.
A 6×4 is the standard configuration for a power unit, meaning the tractor has six wheel positions, three on each side, and engine power is delivered to the four positions on the back of the tractor.
For a 6×2, the six wheel positions remain, but power goes only to one of the two rear axles, leaving the wheels on the other rear axle to roll freely.
In both cases, the steer axle up front is a completely separate system.
Much of the fuel savings is related to weight reduction. A 6×4 configuration has two differentials to transmit driveshaft power to the wheels, whereas a 6×2 has just one differential, sometimes called a “pumpkin.” A truck differential can weigh about 300 pounds, the report said.
In tracing the evolution of 6×2s, the NACFE report said the first generation features two rear axles in fixed positions. The report said the initial approach had traction problems in certain circumstances, and drivers did not like them.
The second and third generations offer load-shifting by moving the tag axle, which rolls freely, relative to the powered drive axle. When a truck is moving at low speeds, an automated or manual load-shifting system adds weight to the powered axle to improve traction. At highway speeds, the load is split evenly between the drive and tag axles.
For the third generation of 6×2s, the tag axle becomes a liftable pusher axle. If a trailer is empty or carrying a very light load, the pusher axle is lifted off the ground.
The development of the second and third generations also should help with resale values, Roeth said, as the first generation equipment has not been popular on the secondary market.
Roeth said another impediment to further 6×2 adoption is that 6×4 axles keep getting better. New, high-efficiency 6×4 axles lessen the case for 6×2s, he said.
The Environmental Protection Agency and the National Highway Traffic Safety Administration said in their Phase 2 rule on greenhouse gas emissions for trucks that the forecast for adoption of 6×2s rises to 30% in 2027 from 15% in 2021.
Roeth said a number of small fleets are actually leading the 6×2 campaign.
They “continue to make 6×2s work for them,” he said.