“Today’s emission-controlled engines are heavier. I will need the weight savings of a 6x2 to offset some of that increased weight.”
Fleets switching their tractors to 6x2 axle configurations could realize a 2.5% fuel reduction and a 20-month payback, among other benefits, according to data gathered from interviews, pre-existing studies, and test from seven fleets and two truck manufacturers.
We have also developed a Payback Calculator, and an initial decision-making tool, to help fleets determine whether the best practices for 6x2 performance are relevant or replicable in their own operations.
6x2 axles offer a 2.5% reduction in fuel use, with our study participants enjoying fuel economy improvements from 1.6% to 4.6%, and a 20-month payback. The full details on fuel savings for each fleet test are available in the full Confidence Report, accessible on this page.
6x2 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.
Currently there is an upcharge of $1,000 to $2,000 for a new tractor specified with a 6x2 axle. However, truck makers along with 6x2 axle manufacturers agree that within a few years 6x2 axle configurations will be cost neutral compared to 6x4 axles. The current cost differential is attributed to the fact that the 6x2 system requires a larger gear set. In additional, load shifting electronics are recommended with the 6x2 system, as is the higher-cost 6-channel ABS/traction control. Once production volume increases, these higher-costs should be offset by the fact that the 6x2 axle system does not have an inter-axle prop shaft, inter-axle differential, rear-rear gear set and rear-rear axle shafts. This should result in cost parity between the two systems.
The fact that the 6x2 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 6x2 axle technology is that it might give the truck additional stability and control when operating in slippery or poor road conditions. With a 6x2 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.
The biggest concern surrounding 6x2 axles is loss of traction or tractive effort. Many of the situations where traction loss may be noticed with a 6x2 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 6x2 axles can be mitigated with the use of load shifting technologies to increase weight on the drive axles at low speeds.
Data from tire manufacturers shows that usable tire life on a 6x2 drive axle tire is about one-third of that of a 6x4 drive tire. In the course of our study, a few fleets supplied data that equates to an increased tire cost of $466 per year per truck.
There are several ways to offset this cost:
• Use less expensive tires on the tag axle
• Use retread tires on the tag axle
It is believed that fuel savings from 6x2 axles are 3 to 7 times greater than the increased tire costs.
Switching to a truck equipped with 6x2 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 6x2 truck from the driver.
There is no clear-cut view of the effects a 6x2 configuration will have on a vehicle at resale. Fleets that have embraced 6x2 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 6x4 tractor.
The common belief is that the resale value of 6x2 is rising, as the technology is better understood.
A 6x2 axle configuration has three axles and six wheel-ends, but only one of the rear tandem axles (two wheel-ends) is actually powered by the engine. The non-driving axle often is referred to as a “dead” axle, and the whole 6x2 system is often called a “dead axle tandem.”1
To achieve the best fuel economy with 6x2 axles, fleet purchase them with these other components:
Key finding include:
- While 6x2 axles have made claims of improving fuel efficiency for decades, concerns about lost traction and increased tire wear have prevented widespread adoption of the technology.
- By 2010, early adopter fleets in North America were experimenting with 6x2 axles and specifying them on new vehicles. Manufacturers began designing and supplying 6x2 axles on a by order basis.
- By 2013 6x2 axles were data book offerings at five of the six manufacturers. Manufacturers also are integrating complementary technology such as automatic load shifting systems into the product offering.
Production rates of 6x2 axles average 2.3% of the total over-the-road tractor build.
Key finding include:
- A few fleets started investigating 6x2 axles in the mid-2000s.
- Fleets believed it was best to spec 6x2 on new trucks rather than modify existing tractors
- Traction concerns were the only negative these fleets expressed about the 6x2 axles
- Fleets began purchasing 6x2 axles and using manual dump valves to deal with the traction issues
- Some fleets said their tow bills and accident rates were lower for 6x2 vehicles operating in extreme weather conditions
- Fleets purchased 6x2 packages, which included 6x2 axles, direct-drive transmissions, single wide-base tires, and trailer tires mounted on non-driven axles
- All early adopter fleets said they will continue to specify 6x2 axles
- Tire life is being studied in detail
Key findings include:
- Some fleets have been watching the development of 6x2 axles and are considering purchasing a few for testing
- The need for weight reduction and the adoption of other fuel efficiency technologies has played a role in their decision.
- These fleets indicated they are concerned about traction but have confidence in the automatic load shifting systems
- Tire wear is a concern as is driver acceptance.
Fleet Forum Internet Survey
Key findings include:
- One-quarter of the respondents to a fleet forum internet questionnaire use 6x2 axle configurations and say they have experienced a 0.5-mpg savings and 300 to 1,500 lb. weight savings
- Their biggest concerns are traction, resale value, and driver acceptance
- Those who have not purchased them still believe 6x2s offer significant fuel and weight savings, but don’t think that is enough to overcome traction and driver acceptance challenges
- Half of the respondents who do not have 6x2 axles say they are likely to purchase them in the near future
- Fleets who have purchased 6x2s said it is too early to tell if traction really is a major issue but they are concerned about it
The study team developed several tools to help fleets in making their decision about 6x2 axles.
The payback calculator estimates the payback in months for end users who adopt 6x2 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.
Currently three suppliers make 6x2 axles.
Major study conclusions include: