Managers at Meritor Inc. think many more truck operators will choose 6x2 axle configurations and will begin asking for faster axle ratios to save fuel and cut greenhouse gas emissions.

Use of 6x2 tandems with only one powered axle, like Meritor's FueLite, could grow to six times the current rate, the company thinks.
Use of 6x2 tandems with only one powered axle, like Meritor's FueLite, could grow to six times the current rate, the company thinks.

In a briefing yesterday at the companys axle plant in Fletcher, N.C., the marketers and engineers said they expect the changes to happen in the next few years as diesel prices stay high and new federal regulations take effect.

"The 6x2, in which only one of the two axles in the rear tandem is driven and the other is a non-powered dead axle, is now in only about 3% of Class 8 trucks," said Matt Stevenson, general manager for North American field operations and marketing. "In five years, that should grow to 18%."

Although the 6x4, with both tandem axles powered, is the overwhelming favorite now, operators will begin accepting the 6x2 because it can save about 2% in fuel and almost 400 pounds of tare weight, said Charlie Allen, general manager of Meritors North American driveline business.

Higher Fruit

The low-hanging fruit already grabbed by builders include reducing resistance caused by aerodynamic drag and tires the two areas that together consume about two-thirds of fuel at highway speeds -- Allen said. But fuel costing $4 a gallon and more has operators thinking of gains of 1 and 2%.

These include friction in gear sets, he said. Meritor has made advances in gear design and finishing to reduce parasitic drag, and the 6x2s eliminating two of the three gear boxes in a tandem has even more effect.

Drivers and truck owners have stayed away from 6x2s because traction is inferior, tire wear on the single powered axle is higher, and resale suffers because subsequent owners cant use trucks or tractors in applications like construction or agriculture than if they had 6x4 tandems, Stevenson said. "A vehicle with a 6x2 tandem thus suffers a resale penalty of about $5,000, studies have shown. However, some fleets say they get a premium for their high-efficiency tractors when theyre traded in four or so years and go back into highway work."

Meritor is trying to educate customers on the 6x2s benefits, and meanwhile has designed products that overcome the traditional objections. For one thing, the 2% fuel savings, which have been proven in fleet tests, can offset the residual-value penalty in four years or less.

For enhanced traction, its current 6x2 product, the FueLite tandem, comes with a driver-controlled locking differential on the powered axle, and some customers order it with a manual dump valve for the dead axles air bags to transfer weight to the drive axle. It also can be bought with a Computer-Controlled Air Suspension, or CCAS, sourced from its partner in the Meritor Wabco joint venture.

Automatic Controls

The CCAS-equipped tandem automatically dumps air from the non-powered axles air springs to effect the weight transfer. A FueLite 6x2 can also have electronic traction control added to the standard anti-lock braking system to augment the mechanical diff lock, Allen and Stevenson explained. In all cases, the axles are installed on air-bag suspensions from suppliers or truck builders.

Meritor is also preparing an advanced 6x2, called SmarTandem, that has a proprietary controller which automatically handles the diff-lock and weight-transfer functions, the executives explained. Automatic locking and unlocking of the differential is smoother and more precise so it can be used at higher road speeds than with the driver-controlled diff lock.

Traction control, which senses wheel spin and gently applies brakes on those wheels, transferring power and torque to those that have better grip, is automatic on both of the products. But the mechanical lock is more sure-footed while starting from a standstill.

SmarTandems electronic controller quickly senses wheel spin and dumps air from the dead axle, adding weight to the driver to aid in traction. This is done in 3 to 8 seconds, and drivers of field-test tractors have commented on how quickly this happens and how well the system works, Allen and Stevenson said.

The dead axle on the FueLite and the SmarTandem is a tag, meaning it follows the drive axle. Other builders also offer 6x2s with a pusher dead axle, which is ahead of the drive axle, a configuration which requires a curved center in the axle beam to allow the driveshaft to pass through. Meritor currently does not offer such an axle, but might if demand warrants.

Heading to Market

The company announced the SmarTandem at the Louisville truck show last March, and will put it into limited production in the New Year. It is more capable and will be priced higher than the FueLite.

SmarTandems fully automatic function might make it more appealing to truckload fleets who train drivers less than others and want to keep drivers out of the operating loop.

"The amount of weight sent to the driving axle is electronically limited to about 20,000 pounds to avoid overloading it," Stevenson said.

In this case, a tandem loaded to the legal 34,000 pounds which is normally split evenly between the axles would carry only 14,000 on its dead axle. The drive axle would be legally overloaded by 3,000 pounds.

But more traction is usually required off public roads, such as coming out of a sloped loading-dock driveway or in a truckstop; if it happens during a snowstorm, authorities have more to worry about than issuing a citation for exceeding axle weights.

Weight transfer, which Allen and Stevenson called weight management, can prevent excessive tire-tread wear on the drive axle, they said. When its more heavily loaded, it cannot burn off rubber as readily.

The drive-axle differential on a 6x2 is more robust than each of the two used in a 6x4, or live tandem. The more massive gears allow the single diff to alone absorb torque stress, even from high-power engines, they explained.

As to purchasing cost, either of Meritors 6x2 tandems when equipped with a locking mechanical diff, electronic traction control and the weight-transferring function might cost about the same as a 6x4 tandem, they said. This will keep the heavier and more complex configuration very popular,

But the weight penalty from the extra gear sets the interaxle differential and the second axle diff -- plus the short driveshaft between the axles, is still considerable. For operators who carry heavy cargoes, like flatbedders and bulk haulers, the 6x2 can add payload capacity, Stevenson said. "The current FueLite can save 387 pounds over a popular 6x4 tandem."

Fast = Slow

Numerically low axle ratios, called fast because they theoretically allow a truck to achieve a high top road speed, have become more popular as truck builders and owners strive to save fuel, Allen said during a tour of the Fletcher axle factory. Fast axles are paired with transmissions with direct-drive top gears, which themselves reduce friction and save a bit of fuel.

Modern trucks and tractors are electronically speed governed to keep engine revs low, a practice that used to be called gear fast, run slow.

Meritors current fastest ratio is 2.47 to 1, and is employed by customers who want a tractor to cruise at 60 or 65 mph with the engine turning only 1,300 rpm or so. But truck makers, under new federal mandates to reduce carbon dioxide emissions, have begun offering even slower engine operation, with cruise at 1,100 rpm.

So Meritor engineers are preparing ratios in the 2.20 range, Allen and Stevenson said. Altered metal alloys and gear designs are being explored to make the upcoming gear sets durable as well as economical.

Modern Plant

The axle plant in Fletcher, about 20 miles south of Asheville in the foothills of the Appalachian Mountains in western North Carolina, was opened in 1980. Meritor has spent $29 million in the last three to four years for new equipment, said its manager, Brian Cavagnini.

Its line workers and support staff use the latest machinery and processes, including dry gear cutting instead of oil-cooled cutting, to ensure production efficiency and highest possible quality. Some of the plants 737 skilled workers have been there since it opened.