6x2 drive axles are emerging in different configurations, including forward liftable axles. Photos: Jim Park

6x2 drive axles are emerging in different configurations, including forward liftable axles. Photos: Jim Park

In its purest form, a 6x2 chassis configuration is a three-axle tractor with power going to just one of the tandem rear axles. Put another way, only two of the six wheel positions are powered. Early versions of the configuration featured a dead axle that went along for the ride until it was needed for carrying capacity. Usually those dead axles were liftable. Some were fitted ahead of the driving axle (called pusher axles), while others were installed behind the driving axle (called tag axles).

Most OEMs today offer what can comfortably be described as 6x2 configurations, but they are far from the 6x2s of old.

The new generation of 6x2s began creeping into the market in about 2010. Already we’re seeing OEMs and component suppliers differentiating their designs through traction control features, automatic load transfer mechanisms to improve traction, and liftable non-driving axles to reduce rolling resistance and tire wear when lightly loaded.

As with most major changes to truck designs, fleets remain wary about 6x2s. Many are reluctant to embrace the technology, despite the promise of lower vehicle weight thanks to the elimination of one heavy drive axle and improved fuel economy through reduced mechanical drag in the driveline.

In fact, 6x2s represent about 4% of the 40,000-pound on-highway axle sales right now, according to Karl Mayer, director of product line management for rear axles at Meritor. “Sales have plateaued. We saw very little growth in the market over the past year, probably because of lower fuel prices. Still, we’re up from about 2.3% market share back in 2013.”

Similarly, Kelly Gedert, marketing manager for Detroit-branded products at Daimler Trucks North America, says the take rate there for 6x2s on Freightliner Cascadia trucks delivered in 2014 is less than 5%.

To help clarify what’s on the market and how the various offerings work, here’s a breakdown of the currently available 6x2 technology by type and approach to the problem.

Advanced 6x2 with automatic traction control

Bendix and Meritor Wabco both produce electronic control systems for 6x2 axle configurations designed to facilitate a load transfer to the driving axle during a wheel-slip event. If wheel speed sensors detect a variation between the two rear axles caused by a loss of traction on the driving axle, the systems respond by exhausting air from the suspension on the non-driving axle. This increases the load, and therefore the traction, on the driving axle. 

Bendix’s system is called eTrac. Built on top of the existing mechanical air suspension control valve, it’s completely transparent to the driver and requires no driver intervention.

“It’s automatic and it activates anytime there’s a wheel-slip situation,” says Mike Tober, product manager for vehicle dynamics at Bendix Commercial Vehicle Systems. “The advantage is the driver doesn’t have to flip a switch on the dash. It activates when it’s needed and only when it’s needed and deactivates shortly afterward — above 25 mph or after 5 seconds.”

Tober says the front and rear suspensions can be isolated, and once air pressure is exhausted from the non-driving axle, the leveling valve increases air pressure in the driving axle to compensate — but only up to the maximum load rating of the axle. “We never overload the axle,” he stresses.

ETrac is available with an optional manual control that’s integrated with the Mud and Snow switch in the traction control system. The driver can manually request system engagement if conditions warrant. If the driver forgets to disengage, that will happen automatically over a set speed or after a predetermined time.

The Meritor Wabco system, called ECAS (Electronically Controlled Air Suspension), is designed to interpret a loss of drive wheel traction and initiate a load transfer to the drive axle to increase traction. Mark Melletat, director of field operations at Meritor Wabco, says the key is to make the transfer happen quickly.

“We use large, high-capacity valves to move the air quickly before the driver gets impatient,” he says. “It takes less than three seconds to fully load the drive axle — up to its maximum capacity. It’s important for the driver to recognize when the system is working and to give it a moment or two to reestablish traction before applying too much power. This is where driver training and awareness become important.”

ECAS also has a manual option, so a driver could engage the system when hooking up a trailer for example, or when pulling out of an inclined loading dock on a slippery surface.

While they function in a similar way, each product offers its own feature set that helps differentiate it in the market. Both eTrac and ECAS are suspension system controllers only and not integral parts of the axle system. Each system is available through specific OEMs and they are often tied to specific axle manufacturers. 

Hard-packed snow can be nearly impossible to escape from if warm tires have melted their way down even an inch or so. The load transfer systems on today’s 6x2 axles place more weight on the driving axle to improve traction.

Hard-packed snow can be nearly impossible to escape from if warm tires have melted their way down even an inch or so. The load transfer systems on today’s 6x2 axles place more weight on the driving axle to improve traction.

Tag axle 6x2

Tag axle systems, where the rear axle in the tandem grouping is the non-driving axle, are offered by Dana, Detroit Axle and Meritor. Dana calls its 6x2 package the Spicer EconoTrek, while Meritor’s offering goes by the name FuelLite. Detroit does not use a brand name beyond the Detroit 6x2.

Dana’s EconoTrek pairs the proven Spicer S170 and S190 single-reduction single drive axles with the new, Spicer S20-045B tag axle. Dana says this axle combination lowers the weight of a comparable tandem drive axle by up to 400 pounds. It’s available with an axle ratio as low as 2.53:1 for downsped driveline specs, and it will accommodate dual or wide-base tires and drum or disc brakes. Dana says the EconoTrek is optimized for use with the eTrac traction control from Bendix.

The Model 6 Detroit Drive Axle in 6x2 configuration is said to be almost 400 pounds lighter than a tandem drive axle. An axle ratio as low as 2.28:1 can be spec’d for downsped applications. The Model 6 is available only with the Detroit Integrated Powertrain package featuring the Detroit DD15 engine and Detroit DT12 automated manual transmission. Daimler Trucks North America uses Meritor Wabco’s ECAS traction control system on both its proprietary Detroit 6x2 axles and Meritor’s FuelLite 6x2.

Meritor’s FuelLite 6x2 tag-axle system features a derivative of the RS23-160 drive axle. Meritor says it’s a more capable axle with a larger ring gear and thicker wall housings for use in a single-drive application. The tag axle is a square-tube axle with an integrated suspension bracket for easier installation. Meritor’s Mayer says the FuelLite 6x2 delivers nearly 400 pounds in weight savings over a standard 6x4 and provides an approximately 2% fuel efficiency increase. It’s available with a drive ratio as low as 2.50:1, and the DualTrac housing allows the option of running wide-based tires or duals interchangeably. Mayer says Meritor will soon release an axle with a 2.31:1 ratio for even greater downspeeding potential.

Currently, all OEMs except Mack and Volvo offer tag-axle 6x2 systems with electronic air-suspension traction control from Bendix or Meritor Wabco. Because of possible vehicle stability concerns arising from locating the fifth wheel behind the drive axle, none of the current crop of tag-axle 6x2 systems offers a lift option.

Liftable forward axle 6x2

But that doesn’t mean you can’t have a 6x2 with a liftable forward axle. Mack and Volvo both offer a 6x2 system with the driving axle at the rear and a liftable non-driving axle in the forward position. Suspension and integrated-axle producer Hendrickson is also developing such a system under the OptiMaax name.

Both systems will keep the liftable axle off the road when the vehicle is empty or only lightly loaded, lessening rolling resistance and improving fuel economy while reducing tire wear. But that’s about where the similarities end. These two manufacturers are taking decidedly different approaches to managing traction.

When it goes into service, OptiMaax will be offered by several OEMs as a factory-installed vendor suspension, with drive axle offerings from the OE data book. Hendrickson supplies the suspension, the liftable axle and the electronic controller that decides when to lift or deploy the axle. Traction control products from Bendix or Meritor Wabco will be installed as per OE agreements.

“Our long-term approach is to integrate the suspension with the OEM at the vehicle level so that it’s truly a component that’s engineered into the system,” says Hendrickson’s director of marketing, Gerry Remus. “Right now we have several placed with fleets to prove the concept.”

Remus sees the liftable forward axle as the technology that will become the 6x2 of the future. A forward liftable axle can be kept up far longer than with a rear lift, Remus says. “We want to have that axle up as much as possible. That’s where we’ll get the real incremental savings in not only fuel consumption but tire and brake wear.”

Hendrickson has designed its own electronic control unit that takes the up or down decision away from the driver. It draws data from wheel speed sensors and suspension pressure transducers to determine when the axle should be deployed. There’s some logic in there that will keep the axle down when the axle weight nears a threshold. Remus says these settings are customizable by the OEM that installs the axle to suit its chassis configurations.

“There is also load sensing between the primary suspension and the liftable axle to ensure there’s balance between the two axles,” he says. “And we will still rely on other industry products that are tied into our ECU to provide load transfer in a wheel slip event, along with the stability control functionality that’s becoming popular.”  

Rather than use a traction control system that reacts after the wheels have already broken traction, Volvo Trucks is using biased axle loading to keep a higher percentage of the load on the driving axle at all times. While empty or only lightly loaded the lift axle is retracted. Called Adaptive Loading, Volvo uses a 23,000-pound drive axle from Dana or Meritor and a 23,000-pound liftable axle with a 9-inch drop center designed and built by Link Manufacturing.

According to Peter Blonde, Volvo Trucks’ aftermarket product manager, the system uses two separate air systems to supply the rear and front axles.

“Sensors on the drive axle maintain the correct ride height for the truck while sensors on both the front and rear suspensions determine the load distribution,” he says.

To illustrate the point, Blonde says when the axle group is fully loaded to 34,000 pounds, the weight distribution will be equal. But with a load of 28,000 pounds, the distribution might be something like 12,000 pounds on the front (dead) axle of the tandem and 16,000 pounds on the rear (drive) axle. The air system controller is a Volvo design based on one that first appeared on the European FH model in 1993. 

“With a well spec’d truck we can load up to 18,000 pounds on the drive axle while loading the steer axle to 13,200 pounds in 4x2 mode [lift axle up] with a gross combination weight up to 53,000 or 54,000 pounds,” says Blonde. “One of the early adopter fleets is running such a set up and he is able to keep the axle up 50% of the time.”

Mack has a similar 6x2 system that can be tied to the Mack engine and mDrive transmission to optimize fuel economy for bulk haulers and similar applications. Three driver-selectable traction control modes are available via a dash-mounted switch.

Spicer AdvanTek 40 Dual Range Disconnect

This axle, announced by Dana at last year’s Mid-America Trucking Show but not yet in production, deserves a category of its own. It’s both a 6x4 and a 6x2. At speeds under 52-54 mph or so, it operates as a 6x4, providing all the traction and torque drivers are accustomed to. Over that set speed, the interaxle drive shaft physically disconnects from the power divider in the forward axle as well as the ring gear in the rear axle, effectively turning it into a 6x2.

As an added benefit, through the magic of planetary gearing and axles with different ratios, the final drive ratio changes from 3.10:1 in 6x4 mode to 2.26:1 in 6x2, giving it a downsped drivetrain with all the associated fuel economy benefits. 

“Depending on your starting point, the DRD axle system can improve powertrain and driveline efficiency somewhere in the range of 2% to 5%,” says Dana Director of Global Product Planning Steve Slesinski. “You’ll also get the downsped 6x2 efficiency at highway speed, and better low-speed performance for startability or when backing a trailer into a dock, for example.”

The Dual Range Disconnect employs a single gear mesh configuration from the engine to the wheels, minimizing efficiency losses associated with tandem drive axles.

Despite the low take rate for 6x2s generally, there are a lot of configurations to choose from. Given the effort OEMs are putting into bringing 6x2s to market, there’s clearly an expectation that they eventually will take off once fuel prices start to climb. Now is a good time to be researching them, while the need to embrace the technology isn’t exactly pressing.

“We have a lot of customers asking about 6x2 now because they are interested in the fuel and weight savings,” says Detroit’s Gedert. “What makes the adjustment difficult most of the time is customers are coming out of a different spec when going into 6x2. Depending on their application and where they are running, some will see the fuel saving, others won’t. It’s very application-specific, and it’s not a technology that will work for everybody.”

The 6x2: Still too many downsides?

While the 6x2 package can save 350-400 pounds and improve fuel efficiency around 2.5% to 3%, the savings may not be enough to overcome some of the downsides for many fleets. The 6x2 Confidence Report released in 2013 by the North American Council on Truck Efficiency reports on some of these risks and challenges:

Traction: The NACFE reports draws conclusions from testing done by Performance Innovation Transport of Blainville, Quebec, that demonstrated decreased pulling distance in traction tests using a standard “truck-pull competition sled.” Results showed distances decreased between 5.4% and 13.5% using various configurations of 6x2 compared to a 6x4. While the test does not represent a real-world situation, it does show the 6x2 did not do as well as the 6x4 in a head-to-head test.

However, the report noted that “many of the conditions where a loss of traction might be noticed with a 6x2 system are likely situations a 6x4 truck should avoid or would otherwise have limited traction in as well,” such as deep snow, deep loose gravel, heavy ice, and even parking lots full of pot holes. In addition, some of the traction shortcomings can be addressed by using load shifting technologies to increase the weight on the drive axle at low speeds.

Tire Wear: Data from tire manufacturers and several fleets indicates that the usable tire life on a 6x2 drive axle is about one third that of the 6x4 drive tires. The increased wear is due to all the driving torque and engine braking torque passing through the single set of tires on the single drive axle. NACFE concludes that tire costs can be offset by using less expensive free-rolling trailer tires or retreads on the non-driving axle and that the fuel efficiency gains offered by the 6x2 configuration more than offset the increased tire cost.

Increased Upfront Cost, Lower Resale Value: The current upcharge for a new 6x2 is between $1,000 and $2,000, the NACFE report says. At the other end, there’s a belief that trade-in values can be as much as $4,000 lower than a similarly equipped and aged 6x4 — but some fleets responding to NACFE’s questionnaire said they had buyers prepared to pay the asking price for their 6x2 trade-ins. At best, that leaves prospective 6x2 adopters in an uncertain position.

Driver Acceptance, Training: 6x2s are different from 6x4s, and if fleets don’t familiarize their drivers with the new technology and make them comfortable with it there will almost always be integration issues. The newer traction control and automated load transfer systems can make the transition easier.

Jason Heath, product manager for Neway Truck, Bus and RV Suspensions at SAF-Holland (which does not presently have any product in the 6x2 space) suggests fleet managers take a long, hard look at how a 6x2 truck will fit into their operation.

  • What are the benefits my fleet will gain by using 6x2 axles?
  • Is there a risk in using the 6x2 axle that we need to think about?
  • Do the benefits outweigh the risks?
  • Are there other technologies that might more cost effectively increase the profitability of the fleet?

“Adopting 6x2 axles should lower your fuel costs, but they won’t offer the same gains across the board,” Heath says. “If your typical application logs high mileage, the payback period for your upgrade is short. If your routes are not as long, the payback period will be extended. Therefore, it is important to evaluate your application to determine your total cost of ownership.”

CORRECTED: An earlier version of this article incorrectly identified Mike Tober with Bendix Commercial Vehicle Systems. We apologize for the error.