"Some states closely follow the Federal Bridge Formula, which can dictate the number of axles required and how they're spaced," Parlier said. "However, any longer-than-necessary wheelbases reduce maneuverability, which is critical for negotiating heavy haul tractors on crowded jobsites."
To haul heavy loads, frame rails typically need to be reinforced. The amount of reinforcement will depend on the truck's wheelbase and axle capacities.
"An inserted 3/8-inch frame is usually required for most heavy haul tractors, but you can get two inserts," said Parlier. "The longer the wheelbase and the more axle capacity you add, the more rigid the frame needs to be."
The rear axle ratio choice will also affect startability, but choose it carefully to ensure a good balance between cruise speed and low gearing. Parlier recommends a startability of 15 percent to 20 percent for most heavy-haul applications.
"You typically don't want to spec anything faster than a 4.11:1 ratio unless you're running a two-speed rear axle or an auxiliary transmission," said Parlier. "In extreme applications involving bridge decks or oilfield equipment, you may see ratios upwards of 10:1 or even 12:1."
For haulers running long distances at highway speeds, the ratio chosen should be as low as possible without undermining startability. "As a rule of thumb, pick the rear axle ratio for efficiency on the highway, and get the startability required from your transmission ratios," said Parlier.
Durability is another issue to consider when spec'ing the rear axles. "The 46,000-pound axles with heavy wall housings are most common for heavy haul tractors. For extreme heavy haul applications, Kenworth offers planetary axles with capacities up to 150,000 pounds," she said.
Traction needs will also dictate axle choices. "We recommend wheel differential locks or a cross lock on at least one drive axle," said Parlier. "Automatic Traction Control is an option on antilock brake systems that control wheel spin on slippery surfaces, which can be a big help when starting a load on a muddy job site."
Heavy haulers in need of pusher axles to comply with local weight and axle requirements can choose from steerable and non-steerable types. A 20,000-pound steerable is most common, but a 22,000-pound non-steerable is also available if needed. Operators who run non-steerable pushers often have to lift the pusher to negotiate corners or they end up scrubbing the tires. Steerable pushers offer the benefit of improved tire life because the axle will steer through the corner rather than scrubbing the tires. This also reduces the stress on the truck in these situations.
"The most versatile configuration is a 22,000-pound steer axle, 46,000-pound tandem drive axles, and a 20,000-pound steerable pusher," noted Parlier.
And fleets running lift axles may want to consider upgrading their braking system. "A four-channel antilock brake system is standard," said Parlier. "We'd recommend a six-channel system for anyone running lift axles because it will help prevent flat-spotting of the lift-axle tires."
FIFTH WHEEL POSITIONING
Proper fifth wheel positioning is also critical to ensure full use of the rated capacity of all axles, especially the front. "Front axles rated at 20,000 pounds are most common. But a 22,000-pound rating is available for extra-legal loads," Parlier said.
At those kinds of ratings, heavy haulers need wide-aspect front tires not only to handle the load, but also to meet some states' tire-width requirements. The rule is typically 600 pounds per inch of tire width, but it can get as low as 500 pounds per inch width.
"Mounting 425/65R22.5 tires on the front will be good for up to 22,000 pounds on the steer axle in most states," Parlier said. "You can go to wider 445 section tires to maximize tire width and improve flotation over loose surfaces."
Some manufacturers may install a single steering gear with an assist ram. But Kenworth uses dual steering gears on front-axle ratings 16,000 pounds and above to provide among the best system the industry can offer.
"Heavy haulers often find themselves on jobsites maneuvering at slow speeds, which places a lot of pressure on the steering system," Parlier said. "Kenworth recommends an oil cooler on the steering system to maintain safe operating temperatures under demanding conditions."