Hauling loads above 110,000 Gross Combination Weight (GCW) pounds requires trucks that have been carefully spec’d. They must have the right gearing and power to pull the weight and they have to be durable. But if they are over-spec’d, they may end up costing the operator more in fuel mileage and maintenance.

The lowest-cost-per mile is a matter of striking the right balance between the job’s requirements and how many on-highway miles the truck is expected to run, according to Brian Lindgren, director of vocational sales for Kenworth Truck Company in Kirkland, Wash.

One of the primary spec’ing considerations should be the truck’s wheelbase — local length-and-weight regulations generally dictate the ideal length. “Each province has its own regulations, which may impact the number of axles needed and how they’re spaced,” Lindgren says.

“However, you don’t want to make the truck’s wheelbase any longer than necessary because it reduces maneuverability, which is critical for heavy haul tractors that need to negotiate crowded job sites.”

Proper positioning of the fifth wheel is 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,” says Lindgren. “But a 22,000-pound rating is available for extra-legal loads.

“At those kinds of ratings, wide-aspect front tires are needed not only to handle the load, but also to meet some provinces’ tire-width requirements. The rule is typically 560 pounds per inch [10 kg per millimeter] of tire width. Mounting 445/65R22.5 tires on the front will be good for up to 19,625 pounds on the steer axle in most provinces, and higher in some areas.”

Wider tires can come at a price. Steering geometry is restricted and turning circles are limited. Lindgren recommends a dual-steering gear system but other manufacturers may install a single steering gear with an assist ram.

For heavy haulers, big horsepower is a given. Engines under 15 liters are rare and most are rated at 500 hp and 1,850 lb-ft of torque and higher. Cooling is a big consideration since today’s new engines run hotter than the pre-2007 engine offerings.

“These trucks can spend extended periods pulling a heavy load up hills at slow speeds with little air circulation, so the radiator package is critical,” Lindgren says. “And don’t forget air filtration,” says Lindgren. To keep that big engine breathing easy, Lindgren recommends dual, 15-in.cowl-mounted air cleaners. “Cowl-mounted air cleaners give the engine cooler air than an under-hood air cleaner, and the 15-inch duals have four times lower restriction than the typical single air cleaner.”

The approach to spec’ing transmissions for heavy haulers is similar to that of power: the more, the better. In this case it’s ratios. An 18-speed manual is typical, but in very heavy applications, a two-speed auxiliary transmission or two-speed rear axle are options. In both cases, they double the number of available ratios, allowing improved startability and driveability. A two-speed auxiliary transmission will double the reduction, while a two-speed rear axle will increase the reduction by a third. “A two-speed axle works well up to about 190,000-pounds GCW, but above that I’d recommend an auxiliary transmission,” Lindgren says.

The rear axle ratio choice will also affect startability, but choose it carefully to ensure a good balance between cruise speed and low gearing. Lindgren recommends a startability of 15-to-20 percent for most heavy-haul applications and suggests an operator wouldn’t want to spec anything faster than a 4.11:1 ratio unless he’s running a two-speed rear axle or an auxiliary transmission. Says Lindgren: “In extreme applications where they’re hauling bridge decks or oilfield equipment, you may see ratios upwards of 10:1 or even 12:1.”

For heavy haulers, big horsepower is a given.

For haulers running long distances at highway speeds, the ratio chosen should be as low as possible without undermining startability. Engine maker Caterpillar recommends gearing that will achieve 1,550-rpm engine speed at 65 mph. “As a rule of thumb, pick the rear axle ratio for efficiency on the highway, and get the startability you need from your transmission ratios,” Lindgren suggests.

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,” says Lindgren. “For extreme heavy haul applications, Kenworth offers planetary axles with capacities up to 150,000 pounds, and GCW capacities well over 300,000 pounds.”

Traction needs will also dictate axle choices. “We recommend wheel differential locks or a cross lock on at least one drive axle,” says Lindgren.
Lindgren says heavy haulers needing pusher axles to comply with local weight and axle requirements can choose from steerable and non-steerable types. A 20,000-lb steerable is most common, but a 22,000-lb 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 steers axle, 46,000-pound tandem drive axles, and a 20,000-pound steerable pusher,” notes Lindgren, except where pushers are not allowed.

Fleets running lift axles may want to consider upgrading their braking system. “A four-channel antilock brake system is standard,” says Lindgren. “We’d recommend a six-channel system for anyone running lift axles because it will help prevent flat-spotting of the lift-axle tires.”

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 three-eighths-inch frame is usually required for most heavy haul tractors, but you can get two inserts,” Lindgren says. “The longer the wheelbase and the more axle capacity you add, the more rigid the frame needs to be.”

Finally, rear suspensions on heavy haulers have historically been mechanical types, but air suspensions are rapidly gaining popularity.