. Steer axle loads have been on the rise, which means you might want to re-evaluate your steer tire spec.
The 12,000-pound front axle has been the industry-standard on-highway spec for years. Practically any commercial truck tire designated as a steer or all-position radial will meet the 12,000-pound load rating with a comfortable margin at speeds up to 75 mph. But several factors have conspired to push front-axle loads well beyond 12,000 pounds.
OEMs are responding with steer-axle ratings of 13,200 pounds and higher. If you're an-on-highway carrier still using load range G tires on a 13-2 axle, you might be pushing that tire beyond its rated capacity - and you might even be running in violation of DOT's steer-axle weight ratings.
Truck weights in general have been trending upward in recent years. Much of the weight finds its way to the steer axle. There are several culprits, ranging from the Environmental Protection Agency to hours of service regulations - even the price of fuel.
Michelin's product marketing manager, Don Baldwin, says his company noticed the trend in early 2008 as fuel prices spiraled upward. Some customers had to move from load range G to load range H steer tires as they tightened up the gap between the tractor and trailer to reduce fuel consumption.
"Reducing the gap has a marked impact on the aerodynamic efficiency of the truck," he notes. "As they were doing that, they were moving load forward and adding weight to the front axle. That forced them up one load range level on their steer tires."
Bridgestone's director of engineering for commercial products and technologies, Guy Walenga, began seeing steer-axle loads increase as EPA's 2007 emissions regulations went into effect, adding about 400 pounds of emissions-related gear to the truck - mostly close to the front end - on top of what was added in 2004. He expects even more weight be added to many trucks with the 2010 EPA emissions standard, thanks to hardware needed for the selective catalytic reduction being used by most engine makers. The SCR catalyst, the tank for the diesel exhaust fluid needed in the process, and other emissions components all add weight.
"We've heard from some of the OEMs that they see the steer-axle weight - the dead weight of the front-end of the truck - going up," Walenga says. "Overall, truck weight is trending upward at the moment."
But Walenga says the move to higher weight-rated tires is not yet widespread. "It's not happening across the whole market, but we have been asked to make sure we have some choice of 16-ply tires for steer applications for the fleets who want to spec a 13,200-pound axle."
Keith Harrington of Daimler Trucks says even the new hours of service regulations have had an impact on vehicle weights.
"The latest version of the hours of service rules changed the duty cycle of a typical over-the-road truck to where it has become more of a commercial RV," Harrington says. Drivers are spending more time in their trucks, he noted, thanks to the mandated 10 hours of daily rest and the 34-hour restart option, "so they are packing more entertainment devices and appliances to support the downtime. We know, too, that drivers are buying less in restaurants and eating in the truck more often, so they are packing loads of groceries on top of everything else. That has increased the demand for storage space and larger sleepers. That all adds weight to a vehicle."
Harrington says front-axle loads have jumped from around 10,500 pounds pre-2004 to something close to 11,500 pounds post 2010 (fully fueled, no driver, no trailer.) He's speaking generically here, not just about the Daimler brands. Look at the latest steer-axle offerings from various truck makers and you'll see more than a few 13,200-pound axles in the data books.
OEMs are struggling with component placement and limited chassis space, and are moving as much weight back to the drive axles as possible. Adding a driver or two, all his or her gear, and an auxiliary power unit of any description, and you could easily add 1,200 pounds to that already-heavy front end. It will become increasingly difficult, if not impossible, to keep front-axle loads below 12,000 pounds.
From a G to an H
Truck tires typically come in two load ratings: G and H. Load range G is a 14-ply configuration, while load range H tires have 16 plies. The heavier-rated tire will usually add between 700 pounds and 1,000 pounds more load-bearing capacity per tire (see "How Much is Too Much," next page).
Exceeding the weight rating will wear a tire out a lot faster than normal, and increase the potential for failure. There are bead and sidewall endurance issues, and the extra weight generates a lot of extra heat.
"If you run under-inflated or overloaded, the stress on the tire is the same, and the outcome is the same," Walenga explains. "There's no room for inflation pressure error with steer tires. They have to be inflated to the proper numbers to run at maximum load-carrying capacity."
Walenga says steer tires are the only tires on the vehicle that are required to run at full pressure for their load. "If you're running 22.5 lo-pros, they will carry 12,350 pounds at 110 psi. They'll carry just a little under 12,000 pounds at 105 psi. It's absolutely critical that adequate pressure be maintained in the steer tires."
Running overloaded could also net you a pile of fines. Steer axle weight allowances are determined by the rating of the tire and the axle itself. If you have a 13,200-pound axle, but load range G tires, you could be fined. If you're running load range H tires at maximum weight on a 12,000-pound axle, you'll be in just as much trouble. To get the maximum benefit from the front end of the truck, you need tires that match the weight rating of the axle, and vice versa.
A word of caution here, before you get all excited about loading up the steer axle to make more room on the drive tires: Some states have tire contact patch requirements, where weight ratings are based on inches or centimeters of tread width in contact with the road. You'll need to take those restrictions into account before upping the weight up front.
Michelin's Baldwin says he's seeing a large-scale shift to the higher load ranges today, because many fleets are loading heavier to improve ton-mile efficiency.
"I would put it in the category of a trend," he says. "There are some pretty significant fleets now making the shift."
Where's the Wall?
How high might steer axle loads go? Some OEMs offer 14,000-pound axles in on-highway service, but it's not common. Frank Bio, product manager, trucks, at Volvo Trucks North America, doesn't see a 14,000-pound front end becoming a common spec any time soon.
"We're not looking at going to a 14,000-pound axle; rather, we'll be adjusting component location," he says. "For example, we're moving the fuel tanks as far back as we can to take some of the loading off the front axle, but it's surprising how little difference it actually makes. Shortening the wheelbase slightly helps, because the ratio between the fifth-wheel offset and the wheelbase gets to be a better number than if you have a longer wheelbase."
The shorter wheelbase is an option Freightliner is exploring, too. Harrington says fleets are looking at a 226-inch wheelbase rather than 232 inches, and using a 12-inch slider rather than a 24-inch. "This allows for optimization of the trailer gap without exceeding the weight ratings of the front axle and tires," he says.
Harrington says Freightliner's standard configuration is battery box left-hand forward, diesel particulate filter/selective catalytic reduction syst