6x2 drive axles

[kscarbel2]

Fleet Owner  /  May 31, 2016

In today's fuel-focused fleets, nothing on a truck is off the table – no matter how deeply ingrained that component, system or technology might be.

A good case in point are 6x4 drive axles.

These axles have simply been the standard drive configuration for decades now. Until recently, most fleet managers scarcely gave them a thought: They simply were the way things were.

But there are no Scared Cows left in trucking today. And so the 6x4 axle suddenly finds itself being reconsidered by long-haul fleets looking for every edge they can get in the constant war to improve fuel economy.

6x4 axles evolved as a technological solution to a very specific problem: Trucks are big. They are heavy. They don't always operate on ideal surfaces. And it takes a lot of torque and tractive effort to get them rolling down the highway. 6x4 axles, with four wheels transmitting engine torque to the roadway, were an ideal way to insure that trucks could get moving efficiently in a wide range of surface conditions.

But that extra tractive effort comes at a price: More internal moving parts means increased friction and parasitic power loss. Extra weight is a given.

But 6x2 drive axles do away with many of those extra components are are lighter as a result. According to the North American Council for Freight Efficiency (NACFE), which recently released a Confidence Report endorsing them, 6x2 axles can deliver up to a 2.5 percent increase in fuel economy in the correct applications.

Therein lies the catch: 6x2 axles aren't for everyone. In many applications, especially urban and vocational ones, an extra set of drive wheels can make all the difference in the world between a productive day for your drivers and a horrible one.

6x2 axles are best suited for long-haul operations, but stories about drivers having to call tow to move a truck just 2 or 3 feet because the drive axles slipped off the pavement and can't get traction in loose dirt or gravel. Snow and ice can be problematic, too. Even on good road surfaces drivers have to be coached to avoid over-aggressive starts: The lack of two additional drive axles means all the drivetrain's torque ends up concentrated in just two wheels. Fleet managers say it's not uncommon to see veteran drivers "boil" the tires the first time they drive a 6x2-equipped truck because they're used to the axles being able to handle all that torque at once. The trick is to coach drivers to use a light throttle to manage that torque flow more effectively when getting a 6x2 rig rolling. But once the rig is under way, overall road performance is on par with 6x4-equipped trucks.

Driver training is critical to avoid potential traction problems. Many fleets note that while drivers tend to prefer 6x4 axles, 6x2 systems perform just fine if handled correctly. Moreover, if fuel bonuses are part of a driver's pay plan, then 6x2 axles are going to help pad that next paycheck – a fact that can make drivers more forgiving of their tractive shortcomings.

From a maintenance point of view, fewer internal drive components means less work for technicians. But you can expect to see accelerated drive tire wear. That's because even if your drivers are handling the rigs correctly, 6x2 dive tires see a lot more wear than when their workload is shared with an additional two wheels.

Switching to 6x2 axles requires an adjustment for fleets – although, as noted, it is primarily the driver who bears the burden of that change. Still, the challenges of dealing with 6x2 axles are nothing a seasoned driver can't easily handle. And, as NACFE has verified, the fuel economy benefits are real. And as any truck designer will tell you, finding a solid, 3 percent fuel economy boost isn't as easy as it once was.