Tractors With 6x2 Powertrains Save Fuel, But Fleets Still Reluctant to Adopt

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Transport Topics  /  July 11, 2017

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 6x4 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.

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NACFE Updates 6x2 Confidence Report

Jim Park, Heavy Duty Trucking  /  July 11, 2017

The latest Confidence Report release from NACFE shows that 6x2 drivetrains are still viable options for fleets looking for fuel and weight savings, but uptake on the technology is lower than what was predicted. This latest report shows 6x2 technology is evolving as product refinements emerge and fleets continue to work around some of the configuration's perceived challenges.

The North American Council for Freight Efficiency and Carbon War Room today issued an update to the Confidence Report on 6x2 axles, originally published in January 2003. Because of the way the 6x2 is evolving, this is the first time NACFE has convened a new team of experts to re-investigate a technology on which it has already issued a Confidence Report.

“While we found that the fuel savings benefits as well as the challenges from the original report are largely still true, new product refinements are coming to the market that are aimed at addressing some of the concerns fleets had about 6x2 axles,” says Yunsu Park, NACFE study team manager. “For instance, fleets dedicated to 6x2s are improving tire wear by changing the tire models they use and [electronically] limiting [engine] torque when launching the truck. Also, driver training has proven to be a significant part of a successful 6x2 implementation.”

The study team identified three generations of 6x2 products, but focused special attention on Generation III products, which contain liftable pusher axles, automatic axle-load biasing and traction control.

Generation I: 6x2 with tag axle, no load-shifting technology, manual differential locks. This version offered a 2–3% fuel savings along with weight savings of 300–400 pounds compared to a 6x4. Reported issues included accelerated tire wear and reduced traction under certain conditions, which together resulted in poor driver perception.

Generation II: 6x2 with tag axle, manual or automated load-shifting, traction control, engine parameters adjusted to reduce low-speed clutch engagement and engine brake torque. Load shifting and traction control were found to mitigate traction issues, while limiting engine torque was found to improve tire wear significantly.

Generation III: 6x2 with liftable pusher axle, automatic load-sensing/load-shifting (axle-load biasing), traction control, engine parameters to limit low speed/brake torque. The most significant change was a switch from tag to pusher axles, which enabled lightly loaded trucks to operate with the pusher axle raised. This offered tire wear and fuel economy benefits as well as better traction when empty or lightly loaded compared to a Gen II configuration. It's a very attractive option for fleets expecting to carry less than 60,000 pounds at least 30% of the time, offering additional 2% fuel savings potential, improved traction and less tire wear. Heavier front axles, suspensions, tires, and other equipment may help optimize this configuration.

Among the lessons learned by early adopters of Gen I systems are that tire wear is higher when compared to equivalent 6x4 vehicles, with some fleets experiencing a 50–70% reduction in tire life on their drive tires. Fleets that have adopted best practices have cut this penalty significantly, decreasing the reduction in life to 20% on the driven axle.

Additionally, driver perception, particularly as it relates to safety and traction of 6x2-equipped vehicles, has not improved. Many drivers have not actually experienced or been trained on this configuration and often only hear feedback from a negative and sometimes vocal minority.

However, fleets that have implemented a complete Generation II system have found tire wear and traction issues can be managed at a much reduced level and are able to benefit from the fuel savings. Unfortunately for some fleets, the damage in driver perception was done before a full Generation II system could be implemented.

Despite those hurdles, fleets are seeing successes with 6x2.

"Fleets that are dedicated to 6x2 are making them work and have improved their tire wear and traction concerns pretty significantly," says Mike Roeth, executive director of the North American Council for Freight Efficiency, and operation lead of Trucking Efficiency. "We have found that switching to 6x2 is not a simple implementation. It takes a systems approach and some effort from an engineering and spec'ing standpoint as well as a higher degree of driver training.

"We have also found that fleets not wanting to jump through all those hoops are migrating back to 6x4s," Roeth added. "And all the while, 6x4 are improving in terms of weight and efficiency, so that's just adding to the tightening in the baseline comparison between 6x2 and 6x4."

CONCLUSIONS AND RECOMMENDATIONS

The Confidence Report update concludes that while 6x2 systems offer an undeniable fuel consumption improvement, they are still not widely viewed as a net positive by the majority of the line-haul industry. The 2003 6x2 study concluded that adoption of 6x2 axles would double every year. This has not happened in large part because the challenges and consequences of switching to 6x2 were greater than the original study team anticipated. In addition, lower diesel prices have stretched out the payback period.

Many early implementations suffered from very high tire wear combined with poor residual values that more than offset the fuel consumption gains.

However, new 6x2 axles, in particular liftables, are entering the market. They may reduce some of the challenges related to 6x2s but there is limited feedback from fleets at this time.

• Drive tire wear on 6x2s will not achieve parity with 6x4s. 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.
• Fleets should take a system-wide approach and implement the full Generation II package including load-shifting technology, traction control, and engine parameters to limit torque in low gear, at clutch engagement, and under engine braking.
• Good and consistent driver communication and training is critical with this technology. Drivers should understand the overall benefit of 6x2s, how the systems work, and when a manual intervention is beneficial.
• Traditional 6x4 systems are improving, reducing the potential efficiency gains of a 6x2 system albeit at a greater cost.
• Residual values for 6x2s remain a problem due to the reputation of Generation I systems. Fleets that have persisted and implemented Generation II 6x2s do not report a problem with residual values, although these are typically smaller fleets and would not turn over a large number of trucks at once.
• Fleets should gain knowledge of 6x2 tag axle systems and test not only the technology but also their internal processes for managing engine parameters and driver communication and training.
• Fleets that are below 60,000 lbs. GCWR at least 30% of the time should consider testing a liftable 6x2 axle system.

The new report concludes that 6x2s still play a key role in improving freight efficiency, but the benefits are not as obvious as they previously were. However, manufacturers are continuing to work on improving their offerings, and fleets that have invested in 6x2s continue to do so.

“Those fleets dedicated to 6x2s and exploiting the various opportunities are finding they return their investment,” Roeth concludes.

And just to reaffirm that there is indeed lots of life in the 6x2 concept, Hendrickson says it will unveil a "forward-liftable axle" 6x2 suspension system with an as yet un-named OE partner at the North American Comercial Show in Atlanta this September. That, along with Volvo's Adaptive Loading technology, will bring to two the number of 6x2 suspensions using a forward-liftable or pusher axle, giving fleets the opportunity to lift the axle while empty or lightly loaded.   

An executive summary of the report is available, along with the full report. NACFE is now charging a fee of $100 for the full report. More information about the updated 6X2 Confidence Report is available at  truckingefficiency.org.

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Tire wear remains concern with 6x2 axle configurations

Truck News  /  July 11, 2017

WASHINGTON, D.C. – Despite a 2.5% fuel efficiency improvement, an update to a Trucking Efficiency Confidence Report shows that challenges remain for the 6×2 axle configuration.

Highlighting the report’s findings during a conference call July 11, Mike Roeth, executive director of the North American Council for Freight Efficiency (NACFE) and operational lead for trucking efficiency, said there were aspects of 6x2s that they did not fully grasp when they first tackled the issue in 2013.

“The solutions to the challenges of traction, tire wear and driver engagement were more complex than we understood at the time,” Roeth said.

Tire wear in particular is a major consideration for fleets looking to employ the use of 6x2s, but some carriers are finding ways, such as selecting a retread trailer tire for the free-rolling axle, to lessen wear.

“For some fleets that are dedicated to 6x2s, they are making them work and improving their tire wear and traction pretty significantly,” Roeth said, adding that there still is a tire wear penalty with the use of 6×2 axles.

At a 50% wear penalty, the estimated increased tire cost per year for a 6×2 configuration versus the 6×4 is $1,552, assuming 100,000 annual miles.

Despite a 2.5% improvement in fuel efficiency, the report indicates that to overcome the increased tire wear with 6x2s compared to 6x4s, fleets would need to see 4% or higher improvement in fuel efficiency.

“While we found that the fuel savings benefits as well as the challenges from the original report are largely still true, new product refinements are coming to the market that are aimed at addressing some of the concerns fleets had about 6×2 axles,” added Yunsu Park, NACFE study team manager. “For instance, fleets dedicated to 6x2s are improving tire wear by changing the tire models they use and limiting the torque when launching the truck. Also, driver training is a significant part of a successful 6×2 implementation.”

The report categorizes the various 6×2 axle configuration offerings as generation 1-3, with the first putting in place the first 6×2 tag axle, transferring loads with the manual dump out; the second including torque at launch and better selection of tires; and the third providing further refinement, liftable axles and more efforts around load shifting, traction control, and engine parameters.

Roeth said liftable 6x2s help with tire wear and traction compared to traditional offerings, but noted that challenges remain around cost and weight, as the function of lifting the axle comes at a cost.

Roeth added that the adoption of 6x2s is not a simple implementation.

“It takes a systems approach, it takes some effort from an engineering and spec’ing standpoint, as well as driver recruiting,” he said.

Overall, the updated report revealed seven findings: Tire wear on 6x2s will not achieve parity with 6x4s, but can be improved with refinements; fleets should take a system-wide approach to the implementation of a full generation 2 package; driver communication and training is critical with 6×2 technology; 6×4 systems are also improving, which reduces the potential gain of moving to 6x2s; residual values for 6x2s remain a problem (though mostly with the first generation 6x2s and could be diminished with more recent offerings); fleets should gain knowledge of 6×2 tag axle systems; and fleets that haul under 60,000 lbs. 30% of the time should be thinking of using a 6×2 because of the ability to raise the axle.