Don’t Get Burned When Spec’ing Asphalt Trucks

The Volvo I-Shift and Mack mDrive automated mechanical transmissions feature Rolling Start. It allows drivers to shift the transmission into drive without pressing the truck’s service brakes, eliminating the possibility of creating a bump in the paving surface, which is a critical benefit for customers with paving applications.
The Volvo I-Shift and Mack mDrive automated mechanical transmissions feature Rolling Start. It allows drivers to shift the transmission into drive without pressing the truck’s service brakes, eliminating the possibility of creating a bump in the paving surface, which is a critical benefit for customers with paving applications.

Several different truck configurations are capable of delivering asphalt to a paver. End dump trucks and live bottom trucks can unload directly into the paver. Live bottom trucks have a conveyor system that moves the load directly to the paver without segregation. Belly dump trailers deposit material in a windrow, so the mix must be picked up by another machine, such as a material transfer vehicle, before it goes into the paver.

But the goal with any asphalt delivery system is to maximize the quantity of asphalt delivered to the asphalt paver while minimizing cycle times. The more asphalt that a truck can carry per load, the better – assuming the load can be unloaded into the paver or in front of a material transfer vehicle without disrupting paving operations.

Every truck is limited to a maximum weight it can carry. Limitations are either based on the total Gross Combination Vehicle Weight (GCW) or on each axle and axle group depending upon applicable state or federal regulations. The limits are based on the number of axles and their spacing.

The Federal Bridge Formula is used to calculate weight limits on the national highway system. By using pusher axles, you can increase the payload using the bridge formula. A typical dump truck with three or four axles spread over 20 ft. is typically limited to 51,000 lbs. GCW. A trailing axle offers a greater payload increase using the bridge formula because it increases the overall length of the truck. If the trailing axle stretches the axle length to 36 ft., the truck can have an 80,000-lb. GCW.

But not every state adheres to the Federal Bridge Formula. Lift axles can help achieve maximum productivity depending on state weight regulations. “Pusher and tag axles better distribute weight and help optimize steer axle weights, along with meeting bridge law requirements,” says Alan Haggai, marketing communications manager, Freightliner. “State weight limits often limit effectiveness of multiple lift axles.” Laws in some states make a regular tri-axle, quad dump or Simple 16 truck the most cost-effective option.

Research applicable regulations in all areas you plan to operate before spec’ing the wheel configuration. “In many states, adding lift axles does increase the payload, but local bridge and weight laws can be complex,” says Tim Wrinkle, construction product manager, Mack Trucks. He advises contacting your local dealer and relying on its expert knowledge of local laws, as well as those from neighboring areas where you may operate.

The Case for Super Dumps

Depending upon the location of the paving project, Tucci & Sons has found that in many cases its Kenworth T880 Super Dumps can haul up to 10 percent more asphalt in a single day than the end-dump trailers they replaced.Depending upon the location of the paving project, Tucci & Sons has found that in many cases its Kenworth T880 Super Dumps can haul up to 10 percent more asphalt in a single day than the end-dump trailers they replaced.In areas that do adhere to the Federal Bridge Formula, a Super Dump can be an effective alternative. With these vehicles, a trailing axle increases the overall length of the truck to maximize the payload, yet folds out of the way for maneuverability on the jobsite. The Super Dump can also have up to three pusher axles. This can increase the maximum load to the 80,000-lb. limit.

Super Dumps are capable of hauling payloads from 19 to 26 tons, if you can properly distribute the loads to each axle. A typical tandem dump truck is limited to about a 13-ton payload in accordance with federal bridge law.

Increased truck capacity is important when directly unloading into the asphalt paver. The paver must be stopped each time another truck is backed up. With more asphalt payload, you spend more time paving throughout the work day.

Tacoma, WA-based Tucci & Sons uses Kenworth T880 Super Dumps to deliver asphalt for its paving division. These trucks compliment a fleet of tractors and end dump trailers and transfer trucks. The Super Dumps are spec’d with the PACCAR MX-13 engine rated at 500 hp and 1,850 lbs.-ft. of torque. They achieve a 25-ton capacity by utilizing three 8,000-lb. Watson & Chalin Tru-Track Alumilite steerable lift axles and an 8,000-lb. steerable flying tag axle to meet Federal Bridge Formula requirements and increase the trucks’ GCW to 80,000 lbs.

“Depending upon the location of the paving project, we’ve found that in many cases our T880 Super Dumps can haul up to 10% more asphalt in a single day than the end dump trailers they replaced,” says Tim Tucci, secretary/treasurer and fleet operations manager, Tucci & Sons.

When a driver delivers asphalt using a dump truck and end dump trailer, he must first empty the end dump. Then he gets out of the truck, unhitches the trailer, dumps the load in the bed of the truck and re-hitches the end dump to the truck before he can return to the plant. Tucci says that entire process can take from 20 minutes to half an hour.

“Drivers in our T880 Super Dumps can deliver their loads to the jobsites, dump them and then go right back to our Lakewood or Tacoma plant for another load,” says Tucci. The Super Dumps are also more maneuverable than the trucks and trailers. They measure

40 ft. from the front bumper to the rear steerable axle, while the tractor and end dump combination comes in at 75 ft. This is important when paving in tighter urban areas.

One compromise is maneuverability. With a Super Dump, you may not have the maneuverability of a tandem axle dump truck. “If you are doing a cul-de-sac, you may not be able to maneuver that large truck,” says Chad Semler, director of product marketing for severe service trucks, Navistar. “You may have to use smaller trucks.”

Cut Weight to Increase Payload 

In many applications, a properly spec’d single frame rail will do the job due to the limited off-road exposure of a typical HMA project,. Freightliner's ½-in. thick 11-7/8-in. steel frame is commonly used when a liner is not ordered.In many applications, a properly spec’d single frame rail will do the job due to the limited off-road exposure of a typical HMA project,. Freightliner's ½-in. thick 11-7/8-in. steel frame is commonly used when a liner is not ordered.There are many ways to reduce weight and increase payload. One is to not overbuild the truck.

“From a chassis perspective, you want to have the most efficient truck,” says Semler. “Weight plays a critical role because you want to be able to haul as much weight as possible in each load to minimize your loads. So you want to have a chassis that is strong enough to carry the load, but not overweight either.”

The frame is one area where weight can be saved in an asphalt application. “With our new HX line, we have a 1/2-in.-thick frame rail,” says Semler. “It is a 3.5-million RBM frame rail and takes the place of a very common 10-in. frame rail that would be used in a dump truck application. It is about 13% lighter than a double rail frame.”

In addition, with a standard double rail frame, the reinforcement rail stops under the cab. With the 1/2-in. single rail application, that strength goes all the way through the front of the truck. The single frame rail also eliminates the corrosion jacking that occurs in a double frame rail when subjected to moisture.

Single rail frames can often be used for asphalt hauling. “In many applications, a properly spec’d single frame rail will do the job due to the limited off-road exposure of a typical HMA project,” says Freightliner’s Haggai. “Our 1/2-in.-thick, 11 7/8-in. steel frame is commonly used when a liner is not ordered.”

Mack’s Wrinkle expresses a similar sentiment, noting, “Our most popular frame rail setup on a dump is an 11.1 x 300 mm (7/16” x 11.8”) rail without liner.”

Whether or not the single rail solution will work in your application depends upon a number of factors. “State regulations are going to play a big part in your configuration, which can also affect your frame and how much you can haul. Every state is different,” says Semler.

Another area where you can save weight and boost fuel efficiency is engine selection. Weight and fuel economy both suffer if you spec more engine than the job demands. Asphalt trucks spend a lot of time on the road; therefore, fuel economy has more of an impact on operating costs.

“Spec enough torque to effectively do the job but not more power than needed so as to not degrade fuel economy,” Haggai advises. “A Detroit DD13 between 450 and 470 hp is an excellent choice for most asphalt applications.”

Look closely at the weights of the specific engines being compared, as some engines are heavier than others. “When you go from a 15-liter down to our 12.4-liter, you are going to lose about 500 lbs.,” says Navistar’s Semler. “That 500 lbs. then becomes additional payload that you can carry. The difference going from our 12.4-liter to an 11-liter is much smaller — around 200 lbs. depending upon which 11-liter engine you choose.”

Geography makes a big difference in engine selection, as well. “In some states, if you are running on a really flat surface and you want to maximize payload, there are a few applications where you can get by with a 9-liter engine and shed even more weight for that capacity,” says Semler. However, he cautions that you do give up durability and longevity because the engine works much harder. “But in some cases the economics pays for itself.”

Consider Driver Comfort

Attracting and retaining drivers should be a priority. “Interior creature comforts are key since haulers spend long hours behind the wheel,” says Haggai. “Air ride seats, armrests, tilt and telescoping steering and well-insulated cabs are commonly ordered.”

Suspension choice impacts both stability and operator comfort. “We are seeing air ride suspension spec’d occasionally,” says Haggai. “Some operators prefer the stability of non-air ride units. Freightliner Tufftrack offers a good ride, excellent articulation and stability.”

“Mechanical suspensions are still the most popular in dump models, thanks to the great stability, articulation and maintainability,” Wrinkle notes. For example, Mack’s Camelback and mRIDE rear suspensions offer increased articulation and stability.

“Air ride suspensions have become more popular recently, as they offer a better ride when unloaded,” he continues. “However, these suspensions can be limited in articulation and stability. Driver knowledge and training is also very important with air suspensions, as drivers need to evacuate air bags before dumping the load. Depending on the setup and a driver’s experience, this can slow the dumping process.”

International uses the Hendrickson Haulmaxx suspension. “It is a rubber suspension with a beam,” says Semler. “It is very good for heavy loads without the joust of a spring.” It is also lightweight.

The Haulmaxx suspension has evolved over time. “There have been a lot of advancements in rubber suspensions in the past several years,” notes Semler. “Hendrickson developed a spring that has a hollow section that really helps the unladen ride quality. Once you load the truck, the hollow space disappears and you have the full load on the rubber spring.”

Gear for Success

With automated manual transmissions (AMTs) quickly becoming the dominant transmission in the dump truck market, you will want to ensure the AMTs in your trucks are compatible with asphalt paving applications.

“One of the most important components to consider when looking at efficiency in vocational trucks is the transmission,” says Wrinkle. “Our MP engines are the heart of a Mack truck, which makes our mDRIVE AMT the brains and the enabling element in our fully integrated powertrain that delivers the performance and efficiency customers demand.

“A major limitation in the past has been the availability of transmissions and rear axle ratios that deliver both the startability and performance needed on the jobsite, along with the lower cruise rpm for fuel efficiency on the highway,” he comments. “Mack’s mDRIVE HD 13- and 14-speed AMTs address this problem and enable us to give customers ‘two trucks in one.’ Thanks to the creeper gears, we can spec a dump with low rear axle gearing for great startability on severe jobsite terrain, as well as lower cruise rpms for better fuel efficiency at highway speeds.

“In general, a dump truck equipped with an mDRIVE HD 12-speed AMT already achieves better fuel efficiency compared to the same truck equipped with a conventional automatic transmission, thanks to the inherent efficiency benefits,” says Wrinkle. “That same truck could also see additional improvements in fuel efficiency by spec’ing an mDRIVE HD 13- or 14-speed AMT and selecting the proper rear axle ratios for performance both on the highway and on the jobsite.” For example, the company recommends using ratios around 3.40 to 3.80 on dumps equipped with these transmissions.

Another consideration is asphalt trucks can spend a significant portion of their time backing up to the paver. “A truck spec’d with multi-speed reverse can be important to reverse quickly on the long stretches that you have to back into,” says Wrinkle. “The mDRIVE HD with creeper gears offers up to four reverse gears to get you in position quickly and safely.”

When a truck is pushed by an asphalt paver, you never want to hit the brakes. “When spec’ing a truck that will be pushed by an asphalt paver, you need to make sure the transmission can go from being pushed in neutral to shifting to drive without having to apply the brakes and stop the paver,” says Wrinkle. “Thanks to our fully integrated powertrain, we’re able to provide application-specific mDRIVE features like Rolling Start for our paving customers. Rolling Start allows drivers to shift the transmission into drive without pressing the truck’s service brakes, eliminating the possibility of creating a bump in the paving surface, which is a critical benefit for customers with paving applications.”

But AMTs are only one of the choices available. “The spec must strike a balance between low speed startability and efficiency at highway speed,” says Haggai. “Automatics and automated manuals continue to gain popularity, but many operators still prefer a manual.”

Also consider component layout if the asphalt truck is going to be pushed by the paver. “You have to pay attention to your brakes and your brake chamber locations,” says Semler. Dump trucks that are not in paving applications often have brake chambers that stick out past the tire envelope. “The asphalt paver would actually crash into your brakes.” And it has happened.

“We have an asphalt braking package that flips the brake chambers up and tucks them inside the tire envelope,” says Semler. “When you back up to a paver, there is nothing that sticks out further than the envelope of the tires that would cause a problem with the paver.”

Another consideration for asphalt trucks is an exhaust configuration to accommodate heated truck bodies, where the engine exhaust is circulated though the dump bed. 

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