If I were a tire, the last place I’d want to be is a drive tire on a 6x2. The North American Council on Freight Efficiency’s Confidence Report on 6x2s, released in January 2014, noted that operators can expect drastically diminished life from tires used on 6x2 drive axles.
“Data from tire manufacturers and several fleets indicates that the usable tire life on a 6x2 drive axle is actually about one-third that of 6x4 drive tires,” the report states. One-third. That’s a lot of tire life to forfeit in pursuit of improved fuel mileage.
What kills 6x2 drive tires? Mostly, it’s torque. But it doesn’t have to be that way.
Defining a 6x2
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%.So what’s causing the wear issue?
“On the driven axle of a 6x2, the torque will all go through one axle as opposed to two axles on a 6x4,” explains Paul Crehan, director of product marketing at Michelin Americas Truck Tires. “Fleets should operate a drive tire that can handle the tractive forces that will be transmitted through one axle as opposed to two in a 6x4 setup. As well, some tread designs may handle the torque of a 6x2 better than other designs.”In reality, a 6x2 behaves much the same as a 4x2 — a single drive-axle tractor. There are tires designed for those applications, such as Goodyear’s G572 1AD, but they aren’t as common as typical long-haul drive tires found on most 6x4 tractors.
Michelin says it offers several tires well suited for single-axle applications that would perform well as 6x2 drive tires, such as the Michelin X Line Energy D or the Michelin X Multi Energy D for optimum fuel efficiency, the XDN2 for better traction or the XDS2 for extreme snow or mud conditions.
A couple of other factors compound the wear problem on 6x2s. The drive tires can suffer tread damage if the drive wheels on the tag-type 6x2 see a lot of slippage in gravel or other rough surfaces.
Tires on the non-driving axle can suffer, too. They will see wear symptoms typical of a free-rolling tire, such as river/erosion wear, diagonal wear (from lightly loaded conditions), shoulder step/chamfer wear and shoulder wear (from tight turns). Interestingly, because the non-driving tire does not experience the torque-related scrubbing, its wear rates might actually be quite slow, but free-rolling type wear may appear sooner than expected.
Many fleets run trailer tires on the non-driving axle to reduce costs. Using a lower cost tire at this position can partially offset the increased drive tire costs associated with the faster wear rates.
“Since traction is not as much of a factor for the dead axle, a rib will suffice as long as it still fits the application,” says Rick Phillips, vice president of sales for Yokohama Tire Corp.
Randy McGregor, fleet manager of Transway Inc., a long-haul truckload carrier based in Holland, Mich., runs a regional steer tire on his tag axles. He says the forces acting on the tag axle tires resemble the forces acting on a steer tire more than a drive tire.
“A drive tire is designed to wear properly under torque loads, not scrub loads,” he says. “The rear axle has no torque on it, but it’s constantly scrubbing, getting dragged around turns. The wear is closer to that of a steer axle.”
McGregor has been playing with tires on his 6x2s for several years now, and has seen the best combination of wear, fuel economy and tire life from a regional type of drive tire rather than a long-haul drive. “Regional drives are designed for lots of starting and stopping and high torque loads,” he notes. “A SmartWay-verified regional tire still does well on fuel economy, but I find it wears better than a really fuel-efficient drive tire.”
Tag or pusher?
The tire-wear discussion is further complicated by the presence of two types of 6x2s: the tag and the pusher-type axles. Most 6x2 offerings are the tag-type, with Volvo being the only OEM currently offering a pusher — and a liftable pusher at that. While NAFCE’s 6x2 report made no distinction between pusher and tag axles with respect to tire wear (pushers were not widely available when the report was published), anecdotally we hear there can be significant differences.
Volvo’s Adaptive Loading option lifts the non-driving axle when it’s not required. This obviously increases the lifespan of the tires on the liftable axle, and there appear to be benefits to the drive tires as well.
However, warns one tire expert, don’t overlook the importance of tire maintenance. “When you lift a dead axle off the ground in order to improve traction, you concentrate all of the truck’s load on the drive axle alone,” says Brian Buckham, general manager of product marketing at Goodyear Tire & Rubber Co. “Because the truck’s load is concentrated on the drive axle, even for a short amount of time, it is important to adhere to established tire maintenance practices, including running with correct inflation pressure.”
Inflation pressure matters
We saved the controversial part for last. Most fleets, we hear, continue to run 100 psi in their 6x2 drive tires because that’s the way they have always done it. But a 6x2 isn’t a 6x4 and may require a fresh look at inflation pressure.
There’s a small, 30-truck fleet called Ploger Transportation based in Bellevue, Ohio, that is doing something really out-of-the-box and getting some amazing results.
Ploger runs several Volvo 6x2s with the Adaptive Loading system. It uses a pusher-type axle that lifts when not needed. The rest of the time, Volvo uses a load-biasing algorithm to keep more weight on the driving axle for optimum traction.
Ploger’s director of research and development, Joel Morrow, is a driver who is out on the road every day, turning 115,000 miles a year or more. It’s his job to monitor the performance of the trucks, and in this case, the tires. Ploger has been testing a Yokohama tire package for several months now on three trucks:
- Drive tire: Yokohama 902L – 445/50/22.5, 20-ply load range L, 85 psi inflation
- Non-driving tire: Yokohama RY407 – 445/50/22.5, 20-ply load range L, 85 psi inflation
- Steer tire: Yokohama 104ZR Spec II – 315/80/22.5, 22-ply load range L, 80 psi inflation You’re reading that right, 85 psi in the tandem axle tires and 80 psi in the steer tires.
Drive tires in tests now are getting an astonishing 30,000 miles or more per 32nd of rubber in the center grooves of the tire. The tires on the lift axle are getting slightly less than the drive tires. Both are tracking to run 400,000 miles if nothing happens to them. “I’m absolutely convinced the lower-than-normal but manufacturer-recommended inflation pressure makes a huge difference,” Morrow says. “At 80 or 85 psi, the footprint is the way the manufacturer designed it to run. It’s huge, and the traction is fantastic, but it doesn’t seem to be hurting our fuel economy.”
For the record, the 104ZR Spec II, 315/80/22.5, is marketed by Yokohama Tire Corp. as a regional and long haul tour bus/motor coach tire. Morrow says because the air chamber of the tire is so much larger than a standard 11R22.5 tire, running only 80 psi meets the load and inflation table minimums for the weight (13,500 lbs.) and provides a very smooth ride. More remarkable is the wear — or lack of it. Morrow also fully expects to see more than 200,000 miles out of the steer tires.
It’s clear that the 6x2 axle configuration has an impact on tires, but it may not be that alone that chews them up. There’s compelling evidence that limiting torque can improve tire wear, and that inflation pressure and tire choice can also influence wear rates.
Jim Park is equipment editor for MTD’s sister publication Heavy Duty Trucking.