Liftable 6×2 Axles
Liftable pusher axles are the newest variation of 6×2 axles. They are suitable for fleets with diminishing loads where the full carrying capacity of a tandem axle is not needed all the time.
When operating in the down position, a liftable system performs like a conventional 6×2 system, with one drive axle and one free-rolling axle. When the full capacity of the tandem is no longer needed, the pusher axle is retracted off the road surface.
An additional 1% fuel economy improvement over non-lifting 6×2 systems is possible with liftable 6x2s.
Benefits
- Fuel Savings of 1% over traditional 6x2s.
- Better traction in certain situations through positioning of the drive axle behind the fifth wheel.
- Improved tire life on the free-rolling axle tires as well as other tires.
- Improved drivability due to the axle in the up position giving the truck an effective longer wheel base.
- Lower tolls since tollways typically charge by the axle
- Improved fleet image by providing a visible cue to the fleet’s commitment to improved fuel economy.
Challenges
- Increased cost and weight. The pusher axle requires a more expensive drop center axle.
- Additional component costs. Some fleets spec a heavier capacity front axle along with higher rated kingpins, bushings, and shock absorbers, plus heavier front suspension components and higher capacity tires.
- Legal restrictions. Utah does not allow liftable axles that are not steerable and most of Canada has restrictions.
- Not for all duty cycles. They are best suited for fleets with diminishing loads where the full carrying capacity of a tandem axle is not needed all the time.
- Unsure of residual value. As a very recent development, there is no residual value comparison available.
Common Fleet Strategies
Driver training and understanding of liftable 6x2s is critical.
Training needs to focus on:
- Helping drivers understand how the system functions
- The best way to handle liftable 6x2s in challenging conditions
- Techniques for handling poor traction situations
Fleets must implement a thorough communications and training plan to offset such biases. Major points that should be addressed include:
- A more efficient fleet is at an advantage over its competitors, which is a benefit to all employees. Technologies that the industry has not yet widely adopted, such as 6x2s, helps build that advantage.
- There is no evidence that 6x2s are less safe. Fleets that have implemented 6x2s report no safety issues. In fact, many believe they are more stable in slippery highway conditions.
- Traction issues can be addressed with the right combination of training and technologies including load shifting, traction control, and locking differentials.
Available Systems
Inn 2015 Volvo Trucks North America rolled out its Adaptive Loading product. The Volvo-driven technology developed with Link Manufacturing drives the rear axle in the tandem thereby making the front axle free. An electronically controlled axle lift for this forward axle offers load biasing in poor traction or actual lifting of the free rolling pusher axle clear of the pavement for additional fuel savings over those achieved with the regular 6×2 configurations.
Mack Trucks introduced the same product under the Lifting Pusher Axle name.
Hendrickson Truck Commercial Vehicle Systems’ originally announced the OPTIMAAX system in 2014 as a 6×2 solution with a liftable axle in the forward tandem position. Hendrickson has since refined the OPTIMAAX and will announce the launch OEM in 2017. A Hendrickson developed controller will manage the lift function as well as load biasing for traction. The system is expected to provide performance and benefits similar to Volvo’s Adaptive Loading with the added flexibility of availability at multiple truck OEMs.
Conclusions
Major study conclusions include:
- Drive tire wear on 6x2s will not achieve parity with 6x4s.
- Fleets that 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 have a much better chance to realize the ROI potential of 6x2s.
- Good and consistent driver communication and training is critical with this technology.
- The baseline (traditional 6×4 systems) is also improving. High-efficiency drive axles reduce the potential efficiency gains of a 6×2 system albeit at a greater cost.
- Residual values for 6x2s remain a problem.