The right truck

By Brian Lindgren04 March 2008

Heavy haulers needing pusher axles to comply with local weight and axle requirements can choose from

Heavy haulers needing 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-

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

Striking the right balance between the job's requirements and how many on-highway miles the truck is expected to run will produce the lowest operating cost per mile.

One of the primary spec'ing considerations should be the truck's wheelbase – local length and weight regulations generally dictate the ideal length. The Federal Bridge Formula is followed closely in some states and it can have a big impact on how many axles are needed and how they are spaced. 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. 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 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. You can go to wider 445 section tires to maximize tire width and improve flotation over loose surfaces.

But wider tires can come at a price. Steering geometry is restricted and turning circles are limited. Kenworth has taken careful measures to optimize steering geometry and offers among the best-in-class wheel cut with its T800. We recommend a dual steering gear system where other manufacturers may install a single steering gear with an assist ram. Kenworth uses dual steering gears on front-axle ratings 16,000 pounds and above.

Heavy haulers often find themselves on job sites maneuvering at slow speeds, which puts a lot of pressure on the steering system. Kenworth recommends an oil cooler on the steering system to maintain safe operating temperatures under demanding conditions.

Drivetrain selection

To pull the high weights, big horsepower is a given for heavy haulers. Engines under 15 liters are rare and most are rated at 475 hp and 1,750 lb-ft 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. Kenworth developed a 1,780 square-inch radiator and wide hood for the Kenworth T800 to help handle the most demanding heavy haul applications.

Don't forget air filtration. To keep that big engine breathing easy, we recommend dual, 15-inch 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 is 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 we recommend an auxiliary transmission.

Axle issues

The rear axle ratio choice will also affect startability, but choose it carefully to ensure a good balance between cruise speed and low gearing. Kenworth recommends a startability of 15% to 20% 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. In extreme applications when hauling 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. 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.

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.

Traction needs will also dictate axle choices. We recommend wheel differential locks or a cross lock on at least one drive axle. Automatic traction control is an option on antilock brake systems that control wheelspin on slippery surfaces, which can be a big help when starting a load on muddy job site.

Heavy haulers needing 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. Fleets running lift axles may want to consider upgrading their braking system. A four-channel antilock brake system is standard. We'd recommend a six-channel system for anyone running lift axles because it will help prevent flat-spotting of the lift-axle tires.

Frame and suspension

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. The longer the wheelbase and the more axle capacity you add, the more rigid the frame needs to be.

Rear suspensions on heavy haulers have historically been mechanical types, but air suspensions are rapidly gaining popularity. The ride is better and you have more operational flexibility. By lowering the air suspension, a driver can back under and pick up a lowboy trailer instead of using skid ramps and ramming into the trailer in order to couple it to the tractor.

Driver performance

A final issue to consider is driver performance-related items. Since heavy haulers are often dealing with large, oversize loads, the buyer should try to spec as much glass area as possible and plenty of mirrors. Four-way adjustable, cowl-mounted mirrors can be complemented with convex mirrors. The cowl mounting helps because the mirrors are not subjected to countless door slams and stay in adjustment longer than door-mounted types.

Choose low-replacement cost windshields, when available. Most vocational fleets replace at least one windshield side per truck annually. Two-piece flat-glass windshields with roped-in seals can be replaced in 30 minutes for less than a hundred dollars, which can save thousands of dollars over the truck's life.

Many heavy haulers want a sleeper to enable them to run farther without risking logbook hours violations, but want a more cost-effective solution than paying for the extra length of a big sleeper. For those heavy haulers that do not need a sleeper, but are still looking for a little extra room in the cab, an extended day cab might be the right choice.

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