Heavy haulers that transport loads above 120,000 pounds gross combination weight (GCW) must carefully specify their trucks for both durability and power. An over-spec’d truck may reduce fuel mileage and increase acquisition costs, while an under spec’d truck may not be sufficient move the load, or may increase maintenance costs. The challenge is to achieve the right balance between the truck’s job requirement and expected annual mileage to help produce the lowest operating cost per mile.
2010 engines: SCR or EGR
This year that challenge is increased with 2010 federal engine emissions standards in the United States and Canada. When it’s time to purchase new trucks, these standards may necessitate some changes to the heavy hauler’s current equipment configurations. Heavy haulers can choose an engine aftertreatment approach that utilizes selective catalytic reduction (SCR) technology in combination with exhaust gas recirculation (EGR), or an in-cylinder approach through increased EGR.
Both technologies use EGR to circulate a portion of an engine’s exhaust gas back to the engine cylinders and a diesel particulate filter (DPF) to remove particulate matter from the exhaust. A critical difference is the amount of exhaust gas recirculated back to the engine; the enhanced EGR approach uses a significantly higher level of recirculated exhaust gases. SCR also mixes a reactant — most commonly a solution of urea and de-ionized water known as diesel exhaust fluid (DEF) — with the nitrogen oxides (NOx) in exhaust gases. The exhaust then passes through a decomposition reactor, where the DEF reacts with the NOx to convert it into nitrogen and water.
Increased EGR reduces NOx by boosting the amount of exhaust gases in the engine cylinder, then slowing and cooling the combustion process and burning off pollutants. The increased heat created with the enhanced EGR approach requires greater engine cooling capacity. Increased EGR also requires more fuel to be injected into the DPF for active regenerations. SCR doesn’t rely on engine heat to treat emissions, so SCR-based engines offer the advantage of higher fuel economy. Since SCR doesn’t narrow the engine’s maximum speed range for optimum efficiency, or its “sweet spot,” to attain emission reductions, fleets also can still maintain fuel economy at lower or higher engine speeds. It’s important for heavy haulers choosing SCR to consider DEF tank capacity and placement. Also, not all SCR technology engines are the same. An aftertreatment catalyst using copper zeolite is much more efficient than one with iron zeolite at reducing NOx at normal engine operating temperatures, adding up to two per cent fuel economy compared to engines using iron zeolite
Heavy haulers rarely run trucks with engines under 15 litres. Most choose engines rated at 475 hp and 1,750 lb-ft of torque and higher. Engine cooling is always a serious consideration. As these trucks can spend extended periods pulling a heavy load up hills at slow speeds with little air circulation, the radiator package is critical. Dual cowl-mounted air cleaners keep such big engines breathing easy, rather than under-hood air cleaners
Wheelbase & frame rails
The truck’s wheelbase is an especially important consideration when spec’ing a heavy hauler. Local length and weight regulations generally dictate the ideal length. Some US states closely follow the Federal Bridge Formula, which can dictate the number of axles required and how they’re spaced. However, any longer-than-necessary wheelbases reduce maneuverability, which is critical for negotiating heavy haul tractors on crowded job sites.
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 two inserts can be obtained. 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 must be chosen carefully to ensure a good balance between cruise speed and low gearing. Startability of 15 to 20 per cent is recommended 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 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.
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, planetary axles with capacities up to 150,000 pounds may be spec’d.
Traction needs will also dictate axle choices. Wheel differential locks or a cross lock on at least one drive axle is recommended. 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. A versatile configuration is a 22,000-pound steer axle, 46,000-pound tandem drive axles, and a 20,000-pound steerable pusher.
And fleets running lift axles may want to consider upgrading their braking system. A four-channel antilock brake system is standard, but a six-channel system is recommended 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 will accommodate extra-legal loads.
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. Bit wider 445 section tires maximize tire width and improve flotation over loose surfaces.
Pay attention to steering geometry and wheel. Many manufacturers may install a single steering gear with an assist ram but dual steering gears on front-axle ratings 16,000 pounds and above are better. Heavy haulers often find themselves on job sites maneuvering at slow speeds, which places a lot of pressure on the steering system. An oil cooler on the steering system maintains safe operating temperatures under demanding conditions.
Suspensions and transmissions
Rear suspensions on heavy haulers have historically been mechanical types, but air suspensions are also popular. The ride is better and drivers have more operational flexibility. By lowering the air suspension, a driver can back und
er and pick up a lowboy trailer instead of using skid ramps and ramming into the trailer in order to couple it to the tractor.
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. Heavy haulers typically choose an 18-speed manual, but in very heavy applications, options include a two-speed auxiliary transmission or two-speed rear axle. 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 lbs. GCW, but above that auxiliary transmission is recommended.
Finally, heavy haul truck operators should look at driver performance-related items to help them gain operational efficiencies. Since heavy haulers often deal with large, oversize loads, look at specifying 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.
Pick 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 seek a more cost-effective solution than paying for the extra length of a big sleeper. An extended day cab might be the right choice for heavy haulers who don’t not need a sleeper, but are still looking for a little extra room in the cab. To enhance truck productivity and the driving experience, consider adding a navigation system with communication, diagnostics, and infotainment technologies.
Samantha Parlier is Vocational Marketing Manager for Kenworth Truck Company in Kirkland, Washington. Contact Samantha at firstname.lastname@example.org