6x2 axle configurations still warrant consideration

Applications where trucks run at less than 60,000 pounds some of the time or that frequently leave loaded and return at lighter weights should consider 6x2 axle configurations.

Applications where trucks run at less than 60,000 pounds some of the time or that frequently leave loaded and return at lighter weights should consider 6x2 axle configurations.

Fuel efficiency is still possible as new technologies, strategies overcome early concerns

Fleets outfitting their tractors with 6x2 axles are still finding benefits, but adoption of the configuration has been slow according to a North American Council for Freight Efficiency (NACFE) and Carbon War Room update to its 2013 Confidence Report on 6x2 axles.

Still, the fuel-saving benefits of 6x2 axles first found in 2013 are still there, NACFE said, but fleets are finding even more ways to implement the axles, leading to this updated report. The research team found that first generation 6x2s with tag axle, which featured no load-shifting technology and manual differential locks, produced a 2-3% fuel savings and a 300- to 400-pound weight savings, but they also lead to accelerated tire wear and poor traction under certain conditions. This contributed to a poor perception of their performance by drivers, something that still exists today.

But as with any technology, fleets and suppliers are finding solutions.

“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,” said 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 6x2 implementation.”

Updated 6x2s are now on the market and NACFE found those provide better even more opportunities. For instance, with what has been deemed “second-generation” products, fleets incorporating manual or automated load shifting, traction control, engine parameter adjustments to reduce low speed clutch engagement and engine brake torque have found that tire wear can be improved and traction issues limited. So-called third-generation products with a liftable pusher axle, automatic load-sensing/load shifting, traction control, and engine parameter adjustments to limit low speed/brake torque are useful for fleets expecting to carry less than 60,000 pounds at least 30% of the time. These fleets could see a 2% fuel savings with improved traction over generation two products.

“The fuel-savings potential, combined with a skeptical industry on the technology, has spurred some interesting innovations with respect to 6x2s,” said Mike Roeth, executive director NACFE.

This updated report also examined the conclusions based on the first report, and while the fuel savings didn’t change, some other conclusions have been adjusted. For instance, some early adopters of 6x2s found tire life on the drive tires was reduced between 50 and 70%. Those fleets that have adjusted their tire strategies, though, have been able to reduce this decrease to about 20%, the group said.

Driver perception, though, remains a hurdle.

“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,” the report stated.

For fleets considering 6x2 axles, NACFE offered the following advice:

  • Tire Selection: Selecting an appropriate tire is crucial. Considerations to take into account when selecting tires for 6x2 vehicles include:
  • Steer axle: Fleets should choose steer axle tires based on factors such as usage, whether line haul of regional operating conditions dominate, and the potential for scrub wear.
  • Driven axle: Fleets should continue with the same drive tires as they are currently using on a 6x4, and then update the vehicle to optimize torque parameters.
  • Free-rolling axle: Fit both the driven and non-driven axles with the original drive tires used in a 6x4 application, or use an energy trailer tire retread.
  • Traction: Increasing weight on the driven tires is an effective solution when traction is compromised. Fleets that have implemented load shifting technology combined with driver training are able to operate 6x2s in all conditions.
  • Liftable 6x2: Since the initial NACFE 6x2 Confidence Report, the industry has seen the introduction of a 6x2 tandem axle with a liftable pusher axle. Various fleets have cited fuel efficiency benefits of a liftable axle in long-haul duty cycles, as well as better traction and improved tire life and drivability. However, liftable systems do have some challenges vs. standard 6x2 systems such as increased cost and weight.
  • Driver communication and training: Not only do drivers require an understanding of how 6x2 systems function and the best way to handle them in challenging conditions, many drivers start with a strong negative perception of 6x2s. Fleets must implement a thorough communications and training plan to offset such biases.

Talking with suppliers and fleets, NACFE also found that regenerative braking and dual-range disconnect are promising technologies that could boost the efficiency of 6x2s in the future.

“Hyliion has developed an aftermarket system to hybridize tractor-trailer combinations using the trailer as the platform for capturing regenerative braking energy during downhill portions of a trip,” the report said.

Dana, the group noted, is developing a dual-range disconnect system that would “mechanically disconnect the inter-axle shaft and internal gearing of the tag axle when operating at higher speeds.”

The updated report drew some conclusions, but noted that feedback on the latest technologies is limited, making it difficult to determine whether the initial problems, including perception, with 6x2s have been overcome. Its final advice for fleets looking into 6x2s includes understanding that tire wear will not be as good at 6x4s, but by implementing strategies, wear can be reduced. These include using a retread trailer tire on the free-rolling axle. Also, fleets should consider taking a system-wide approach and implementing 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.

Also, driver communication is critical so that drivers understand how 6x2s work and when manual intervention is beneficial. Residual values are also an unknown and 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, NACFE said.

Finally, NACFE recommends any fleet that operates below 60,000 lbs. GCWR at least 30% of the time consider testing a liftable 6x2 axle system.