TORONTO – It may not be common knowledge, but 6×2 tractor configurations are legal in all of Canada except British Columbia where they are banned outright over traction concerns. Now let me clarify. While “old-style” 6x2s are allowed, or should I say, not prohibited, the newer “Advanced 6×2” configurations are not allowed to operate in two key provinces, Ontario and Quebec. That central-Canada logjam effectively limits widespread use of the Advanced 6×2.

“We are definitely missing out on fuel savings opportunities,” says Bryan Burningham, executive director of Fleet Services at Challenger Motor Freight. “I would have liked to buy 6x2s for our Great Lakes area freight as they are typically flat-land runs with generally moderate winter weather. The technology works so much better now and traction has become less of an issue with the load sensing and traction control options.”

Oddly enough, it’s precisely those load sensing and traction control options that are sidelining the newer technology.

The 6×2 configuration has been around for some time. It was popular in the 1970s and ’80s with drayage and similar fleets that ran bobtail or empty a large part of the time. In the days before air suspensions became popular, the driving axle typically had a steel spring suspension while the liftable tag or dead axle- usually the rear axle- had an air suspension. Drivers had a pressure adjustment valve in the cab they could use to make an attempt at equalizing the weight across the axles when the truck was loaded. The problem was, without a scale, there was no way to precisely equalize the axle loads between the steel springs and the air suspension.

When air suspensions became mainstream, they could be lifted when the vehicle was unladen, and deployed when loaded. Suspension plumbing would ensure equalization took place. In some ways, that was the downfall of the 6×2. Drivers no longer had the option of selectively putting more air pressure in the driving axle to increase traction when conditions warranted. Then in the late 1990s, Ontario launched a series of reforms to its weights and dimensions regulations that, among other things, banned the use of driver-controlled lift axles and suspension pressure settings. Those reforms also demanded that weights between two drive axles be equalized to within 1,000 kg- which is why the Advanced 6×2 is unwelcome in that province today.

It should be noted that Ontario does allow the Advanced 6×2, but considers it a non-SPIF (Safe, Productive & Infrastructure Friendly) configuration. That means it can operate only at a reduced gross weight and with semi-trailer lengths of 48 feet or less. Non-SPIF, 5-axle tractor-semi-trailer combinations are allowed up to 36,500 kg (80,468 lb) gross vehicle weight, while 6-axle combinations may go only as high as 46,000 kg (101,412 lb).

Quebec also restricts Advanced 6x2s for the same reason, and will allow only 13,500 kg (29,900 lb) on the tandem group.

By definition, Advanced 6×2 means it has an electronically controlled suspension that improves vehicle performance in low traction conditions by automatically reducing suspension pressure in the dead axle. That shifts the load onto the driving axle thereby improving traction. The suspension pressure is equalized at all times except when the system senses wheel slippage through the ABS sensors. These systems may also include automatic or driver-controlled differential cross-locks to evenly distribute torque to both wheels on the axle.

Realities and Perceptions

With the next round of greenhouse gas emissions reductions just around the corner, OEMs and fleets will be looking at 6x2s as potential credit earners. With fuel savings potential reported to be in the two-to-four-percent range, 6x2s will be seeing some interest. Too bad for Canadian fleets. Unless regulators in Ontario and Quebec change their minds, the Advanced 6×2 will not be a part of the fuel savings equation.

“We would like to run 6x2s where it would make sense to take advantage of the weight and reciprocating mass reduction and resulting fuel economy,” Burningham says.

Likewise for Derek Varley, manager of Fleet Services at the Mackie Group. “I have to wonder what could possibly be holding us back on 6x2s if it means a good percentage increase in MPG,” he asks. “They worked just fine in the U.S. and in all states, even in the mountains, for years.”

The problem isn’t traction, but regulatory terminology. It’s actually more than terminology; the configuration would be illegal according to Ontario’s SPIF rules. The problem appears to stem from the wording of the rules more than actual concern the 6×2 will leave highways and bridges crumbling wrecks.

Paraphrasing from Ontario’s Highway Traffic Act; a “tandem axle” as defined in Section 114 of the Act is a dual axle whose centers are more than one meter apart … and are designed to automatically equalize the load between the two axles to within 1,000 kg. On top of that, the Act does not allow liftable axles … further defining such an axle as one capable of altering the weight transmitted to the highway.

So, the 6×2, as described above, does not qualify for service in Ontario because the load between the axles is not equalized 100 percent of the time to within 1,000 kg, and is deemed to be a “liftable axle.”

On several occasions over the past three years, industry has approached the National Task Force on Vehicle Weights and Dimensions Policy seeking to modify the National Memorandum of Understanding (MOU) on Weights and Dimensions with little success. Regulators remain unconvinced of manufacturers’ claims that 6x2s pose no threat to infrastructure.

According to Brad Hicks, executive engineer for Government Technical Affairs at Meritor, field tests reveal that typical load-shift events occur only about three percent of the time and last for less than a minute and a half.

“The fact that the system occasionally operates at an unequal pressure [weight] for a very short period creates a problem for us,” he says. “Even in transfer mode, the weight on the driving axle would never exceed the maximum provincial weight limit of 9,100 kg (20,000 lb), and even that is still well under the axle’s actual rated capacity. And, because the system operates like this for such brief intervals, usually only at start-up or at very slow speeds, we don’t believe it poses any significant safety or infrastructure concerns.”

The regulatory barriers to 6x2s seem academic rather than practical.

Only one of the above technologies actually has a liftable axle, however. And interestingly, this technology works in exactly the opposite way to the past-generation equipment the SPIF regulations were put in place to guard against. A liftable axle, in this context, is lifted when there is no or low weight on the axle, thus reducing tire wear and the mechanical drag that hurts fuel economy. The axle is NOT deployed to bear any additional payload beyond normal limits for the truck’s configuration.

And in Volvo’s case, the by-design unequal loading of the drive axles is said to prevent wheel slip and improve ride and handling as well as the steering feeling. At no time does the weight on the heavier of the two axles exceed the axle rating or the permitted axle load. As noted previously, when the load on the axle group approaches its upper limit, the weight is automatically equalized to 50/50 between the axles.

While the technology may not quite fit within current definitions, it does comply with the spirit of the rules–except during the brief periods when axle weight will be unequal due to the traction enhancement feature.

“We would be running 6x2s in some lanes if we had the chance,” Varley says. “I don’t understand why we can’t lift an axle if the trailer only has 20,000 pounds in it. It would save us fuel, tires and brakes. And I certainly don’t understand why they [the regulators] are worried about uneven axle loading that lasts for a minute or so if that weight stays within the legal limits. It’s my understanding that the next round of fuel economy regulations [GHG reductions] could usher in some pretty expensive technology. The 6×2 is here now, it works, and it doesn’t threaten safety or infrastructure.”