6×2 Axles

“Today’s emission-controlled engines are heavier. I will need the weight savings of a 6x2 to offset some of that increased weight.”

Fleet Owner

Fleets switching their tractors to 6×2 axle configurations could realize a 2.5% fuel reduction, among other benefits. Since the original Confidence Report on 6×2 axles issued in January 2003 , 6×2 adoption among North America long-haul fleet has increased from approximately 2% to 4–5% in 2016. However, many fleets still view them as a niche offering suited for the southern region of the U.S.
Factors that have contributed to the limited adoption of 6×2 axles include: low fuel prices, higher tire wear, driver perception, residual value, and improved efficiency of 6×4 axles.

However there is reason for optimism with 6×2 technology. Through a combination of best practices and adoption of complementary technologies, fleets have found that 6×2 axles work for their application and are benefitting from the improved fuel consumption.

​One promising new development is 6×2 tandem axles with liftable pusher axles. Click here for more information on liftable 6x2s.

Benefits

6×2 axles offer a 2.5% reduction in fuel use, with our study participants enjoying fuel economy improvements from 1.6% to 4.6%

6×2 axles offer significant weight reductions because a tag axle weighs less than a driven axle. In addition, the full configuration involves removing the inter-axle differential and associated inter-axle driveline components, as well as one differential gear set. A small amount of weight is added back in for load shifting and anti-spin measures. Overall weight reduction is in the 400-450 lb. range.

Including all load shifting and electronics, 6x2s are on a par with 6×4 axles

The fact that the 6×2 configuration allows for a reduction in the total number of driveline components should lead to an overall reduction in maintenance costs. There should be less vibration complaints and fewer U-joint lubes. Overall savings is estimated at $100 per year.

An overlooked benefit of 6×2 axle technology is that it might give the truck additional stability and control when operating in slippery or poor road conditions. With a 6×2 configuration, the tag axle cannot spin out because it is not powered by the engine. This means there will be some level of lateral force available to keep the vehicle stable on its desired path, reducing the potential for a jackknife event initiated by wheel spin.

Challenges

The biggest concern surrounding 6×2 axles is loss of traction or tractive effort. Many of the situations where traction loss may be noticed with a 6×2 system are situations that should be avoided by vehicles no matter what the axle configuration:

• Deep snow
• Deep loose gravel
• Heavy ice
• Parking lots full of pot holes

Traction shortcomings of 6×2 axles can be mitigated with the use of load shifting technologies to increase weight on the drive axles at low speeds. The concern about traction is perceived to be a bigger challenge than it actually is.

Fleets running early versions 6×2 tag axle configurations experienced accelerated wear on their drive tires vs. the same tires used on conventional 6×4 tractor tandems. Reported decreases in tire life range from 50–70% of the wear life on a 6×4 (i.e., one third to one half tire life vs. an equivalent 6×4).

​Some tire manufacturers recently expanded their recommendations on the way 6×2 vehicles should be equipped to mitigate tire wear.

Switching to a truck equipped with 6×2 axles will require some driver training. However, the newer traction control and automated load transfer systems remove a great deal of the concern about proper operation of a 6×2 truck from the driver.

There is no clear-cut view of the effects a 6×2 configuration will have on a vehicle at resale. Fleets that have embraced 6×2 technology indicated that they have buyers interested in purchasing their used trucks. Others believe that 6x2s will sell for upwards of $3,000 less than a comparable 6×4 tractor.

The common belief is that the resale value of 6×2 is rising, as the technology is better understood.

Introduction to 6x2

Technology Description

​The study team identified three generations of 6×2 axles

Generation I: 6×2 with tag axles, no load-shifting technology, and manual differential locks

Generation II: 6×2 with tag axle, manual or automated load shifting, traction control, engine parameters adjusted to reduce low speed, clutch engagement, and engine brake torque

Generation III: 6×2 with liftable pusher axle, automatic load sensing/load shifting, traction control, and engine parameters to limit low speed/brake torque

Available Systems

Common Fleet Strategies

To achieve the best fuel economy with 6×2 axles, fleet purchase them with these other components:

  • Single wide-base tires on the drive axle
  • Ribbed low-rolling resistance wide-base trailer tires on the tag axle
  • A direct-drive transmission
  • A gear fast, run slow rear-axle ratio approaching 2.50 and filled with low-viscosity synthetic lube

​Tire manufacturers have recently expanded their recommendations on the way 6×2 vehicles should be equipped.

Decision-Making Tools

A Decision Guide summarizes the study findings to assist fleets in making decisions about 6×2 axles.

The Payback Calculator estimates the payback in months for end users who adopt 6×2 technology. Fleets input data into the form and the calculator uses that data along with information gathered by the study team to quantify the benefits and consequences likely to be experienced by the fleet in terms of upfront costs and year-over-year costs.

Manufacturers

Currently six suppliers make 6×2 axles.

Conclusions

Major study conclusions include:

  • Drive tire wear on 6x2s will not achieve parity with 6x4s.
  • Fleets that take a systemwide 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.