There are a variety of factors that affect truck performance. Chief among these are vehicle configuration, total vehicle weight, vehicle cruising speed, trailer type, trailer gap, tire type and driver driving habits. Consequently, there is no single drivetrain solution.
However, critical to any truck's productivity and lifecycle costs - residual values, fuel economy improvements, reduced maintenance, decreased driver training, etc. - is the proper spec'ing and maintaining of the drivetrain.
Basically, the drivetrain, also referred to as the powertrain or powerplant, is composed of a group of components that generate power and deliver it to the road surface to make the vehicle move. All the components must be compatible, otherwise, vehicle performance is not going to be optimized. When drivetrain components don't work in harmony, there can be premature component wear and failures, resulting in increased vehicle downtime and maintenance costs.
Proper spec'ing and maintaining of the drivetrain begins with having a complete understanding of the vehicle's intended application and operation. Vehicles should not be mixed in applications. A two-axle pickup and delivery truck cannot perform the same duties as a tandem-axle truck spec'd to haul construction materials, for example. A long-haul truck spec'd to operate in the flat Midwest will not perform well in mountainous terrain or as a city delivery vehicle. Fire apparatus needs high horsepower and torque, whereas a city pickup and delivery vehicle doesn't.
A lot of frustration can be avoided by using the right truck for the right job.
For proper component specification, the truck dealer and the original equipment manufacturers (OEM) must know the truck's purpose and application, including maximum loads; maximum grades; road surface, terrain, routes and operating environment; duty cycles; annual mileage; and percent of load going versus returning. Also important are the details of the truck specifications, such as make, model, height and width, and gross axle weight rating, gross combination weight and gross vehicle weight.
Truck dealers and OEMs also need to know desired road speeds (maximum and cruising), engine horsepower and torque, tire specifications, transmission ratios and step sizes and axle ratios.
The three key factors in drivetrain specifications are startability, gradeability and fuel economy, all of which are impacted by the aforementioned factors.
Engine suppliers and truck OEMs have software tools and simulation programs to help improve drivetrain performance. Components ought to be checked for compatibility. This will prevent trucks from being built with axle ratios or transmission gearing that is not ideal for the intended application.
To get some guidance on engines and drivetrains, Fleet Maintenance Magazine visited with officials at ArvinMeritor. Headquartered in Troy, MI, ArvinMeritor is a premier global supplier of a broad range of integrated systems, modules and components to the motor vehicle industry. The company serves commercial truck, trailer and specialty OEMs and certain aftermarkets.
Be sure to understand the specific application and vocation of each vehicle, stress the ArvinMeritor officials. The right components need to be selected from the start. Components that are not properly matched will not work efficiently together to produce the desired performance and economy.
What's more, trucks spec'd with components that do not fit the vocation and application the truck will be used in will undoubtedly end up with costly repairs and downtime in the future, they say.
High grades and poor road surfaces, gross combination weights, quantity of start/stops, etc., all impact a vehicle drivetrain's performance and life. It does no good to save a few dollars up front by underspecing components, point out the officials, because these components will fatigue, causing the truck operator to pay for them in downtime and repairs later.
They note that warranty coverage of components is dictated by the vocation.
An understanding of the contribution of each individual component of the drivetrain will help as well, the ArvinMeritor officials say.
• Engine - The engine is used to generate enough power to provide the performance and fuel economy specified by the truck owner.
The officials recommend identifying the engine's "sweet spot" - the most efficient range of the engine chosen. The balance of power and performance does not necessarily have to mean a sacrifice, they say.
• Clutch - Its function is to connect and disconnect the power from the engine to the rest of the drivetrain. A properly adjusted clutch - making full contact - will eliminate loss from the engine and reduce heat.
• Transmission - The purpose of this component, simply put, is to transmit power to a driver mechanism. A manual transmission transmits power via an input shaft which splits the power (torque) inside the transmission box evenly between two countershafts and provides road speed via selected gears. The selected gears will be in reduction - direct or overdrive mode - to provide the road speed needed.
A direct-drive transmission will take less energy (parasitic loss) to move the power through the transmission. However, the direct drive may not have the top drive gear needed for a desired road speed.
An automatic transmission basically changes gear automatically, using a torque converter in lieu of a clutch. The torque converter multiplies the engine's torque.
An automated transmission combines features of manual and automatic transmissions. Automated shifting is controlled electronically (shift-by-wire) and performed by a hydraulic system or electric motor.
Engine torque and rpm, along with the load and desired road speed and driver issues, will determine the best transmission model.
• Driveshaft - This component, also known as the driveline, transmits torque and rotation. It is used to connect other components of a drivetrain that cannot be connected directly because of distance or the need to allow for relative movement between them. The driveline's mass should be effectively minimized so that energy from the engine is not lost, advise ArvinMeritor officials. The proper diameter of the shaft and length are factored in with engineering assistance from the OEM and driveline manufacturer.
• Rear drive axles - Their function is to take the engine power and turn it at 90-degree angles to drive the wheels and both wheel ends while allowing differentiation of wheel speed. Drive axles are ratio dependant on the vocation, application, cruise speed and transmission ratios selected for the job.
There will be some parasitic loss in the gears. However, a properly specified axle ratio that works in conjunction with the engine and transmission will give the desired road speed.
Axle ratio is the number of driveline revolutions necessary to turn an axle one time. By way of example, with an axle ratio of 3.50, the driveline will turn 3.50 times for each revolution of the axle. The higher the ratio, the more torque and the slower road speed for a given engine speed.
Common ratios for axles these days, according to ArvinMeritor officials, include 2.64 and 2.79 with a direct drive transmission and 3.21, 3.36 and 3.42 with an overdrive transmission.
If purchasing a stock truck, they stress the importance of being certain to match the truck's exact application to the right, or even special, componentry - a dump truck versus a pickup and delivery truck versus a linehaul truck.
Further, they recommend checking to be sure that the truck's gross vehicle weight rating and/or gross combination weight rating is sufficient for the operation's vocation. Also, be certain torque input does not exceed the axle's input rating.
Refer to the axle manufacturer's applications guidelines or contact the axle manufacturer, say ArvinMeritor officials. It is much better to do this before a truck is ordered, rather than after a failure and the warranty is denied.
For special applications, the truck's frame strength and proper gear capacity need to checked to make sure it can handle the gross vehicle weight. Some applications and suspensions require a thicker axle housing wall for added durability. Automatic traction control or driver-controlled differential lock may be required for off-road applications.
• Transfer cases - These direct power to both ends of an axle through separate driveshafts. Transfer cases are used for more than conventional power split and driveline drop. Many single-speed units are selected to provide additional features, such as PTO mountings to drive vehicle accessories, such as pumps, winches, etc.
Dependent on torque requirements, many transfer case configurations are selected to improve driveline angles and/or ground clearance, the officials say. Various transfer cases offer flexibility to reverse engine rotation into vehicle designs, as well as have feature options such as park brakes, power torque control and torque differentiation between front/rear axles.
Here again, ArvinMeritor officials counsel referring to the axle manufacturer's applications guides or contacting the transfer case manufacturer for the correct on-off-highway usage requirements to avoid a failure and the denial of a warranty for a condition that voids the warranty.
• Front drive axles - Similar to a rear drive axle, it is important to match these axles with the exact application of the vehicle. Determine whether the axle will be used in drive full-time or part-time. If used with the front axle engaged more then 20 percent of the time, a differential transfer case should be specified, suggest the officials.
ArvinMeritor officials also point out a number of other factors to be aware of with regard to engine and drivetrain performance and life:
• Vehicle and drivetrain performance can be impacted by improper use and specification of engine and driveline retarders. The amount of power and torque developed by such retarders needs to be calculated when specifying a drivetrain. Be certain not to use more brake retardation than is specified by the component and vehicle maker.
• Wide-base tires can affect bearing life. The officials recommend consulting tire dealers and manufacturers on these issues.
• Use lift axles properly to save on tire wear and to extend life of the drive axle. Overloading the drive axle - resulting in cracked housings - can be caused by too much vertical load and not employing the tag or pusher axle. One tip is to look for the sticker(s) on the cab door to understand all load limits.
• When spec'ing standard hubs (not the long-life package), it is a good idea to insist that the manufacturer install inner and outer bearing components from the same manufacturer.
• Know the difference in specifying and using a driver-controlled differential lock (DCDL), automatic traction control (ATC) and inter-axle differential (IAD) lock. A driver's behind-the-wheel performance can extend drivetrain life through the proper use these devices.
The driver-controlled differential lock positively locks the wheel differential to provide improved traction under adverse road conditions. The differential lock is controlled through an electric switch or air valve mounted in the cab.
The locking mechanism is air operated to engage a mechanical clutch that locks the wheel differential. It is spring operated to disengage the lock and permit the wheel differential to operate normally.
Automatic traction controls are integrated with antilock brake systems and electronic engine controls in such a way as to prevent loss of stability and control caused by excessive drive-axle wheel slip.
The inter-axle differential lock is a driver-controlled, air-actuated traction device that allows for speed differences between the forward and rear tandem axles, while providing equal pulling power from each axle of the tandem to help ensure that the vehicle maintains traction.
• Match tires on tandem wheel ends. For example, placing an older steer tire in a drive wheel position can make the inter-axle differential work harder and cause undue wear. Also, tires with different tread depths or air pressures can lead to a premature loss of axle life. Another result of mismatched tires is elevated lubricant temperatures and thus shorter lubricant and axle life.
• For longer drivelines, adhere to proper applications and load ratings. Using the proper lubricant and maintenance intervals will also help maximize driveline life.
• Checking and adjusting driveline angles will also maximize the life of the driveline, as well as other drivetrain components. Do not overload. Specify components that match the engines maximum torque, including higher torque when the engine is re-rated for resale.
• If purchasing a stock truck, make sure that the components are rated for the gvw and gcw ratings of the intended application. If the vehicle needs to be modified, get the OEM's approval for the proposed changes.
• Utilize an air dryer to reduce contamination of the air system. With newer engine oils, a coalescing desiccant cartridge is recommended.
• If spec'ing wide-base tires, be sure to check hub specification to avoid severe strain on the axle housing. Use of two-inch offset wheels can reduce outer wheel bearing life. Premium wheel bearings should be used with wide-base wheels along with pre-set hub packages. Consult the manufacturer's guidelines as needed.
• In some applications and operations, synthetic axle and transmission lubricants can provide advantages over petroleum-based lubricants. In particular, there are also some physical differences that make synthetic oils appealing. Because synthetic lubricants are typically lower viscosity fluids, they result in lower dynamic operating temperatures and that, in turn, results in longer lubricant life.
Synthetic axle and transmission lubricants also typically contain anti-wear additives and rust, oxidation and corrosion inhibitors which work to reduce component wear and promote longer component and lubricant life.
• When spec'ing components, pay attention to the weight. Lightweight components, such as aluminum carriers and Meritor SteelLite X30 brake drums can take significant pounds out of a drivetrain. Since the components are lightweight, it takes less energy to move the truck, resulting in improved fuel economy.
While many lightweight components are premium items at the front-end of the sale, the initial cost will be paid back through the life of the component through lower maintenance costs and less downtime, note ArvinMeritor officials.
• Be mindful of contaminated lube conditions, often predicated by moisture in the spring of the year. Consider conducting regular oil analysis to manage the vehicle performance. A good rule of thumb: check for leaks, lube levels and oil appearance.
• Brake over adjusting causes improper functioning of an automatic slack adjuster (ASA). If something is causing the unit to over-adjust that is external from the ASA, do not replace the stroke-sensing ASA unless it's mis-applicated. Check for the cause of problem - correct application (length and piston color), air system compounding, chamber bracket flexing, camshaft twisting, brake lining compression, etc.
• On a hydraulically-actuated clutch, it is important to note that the tips of the release yoke are in constant contact with the release bearing at all times. This contact may result in polishing, which is not detrimental to clutch operation. However, ArvinMeritor does recommend lubricating this contact area during routine PM maintenance. Refer to the OEM maintenance procedures, the officials advise.
• Keep in mind that "clutch adjustment" restores the designed clamp load to the clutch assembly and also restores the original angularity at the yoke tip/release bearing mating surfaces. Improper angularity can cause binding, increased pedal effort and possible early component wear.
While the driver through his driving habits remains the most important factor is fuel economy - good or bad, drivetrain specifications impacts fuel efficiency in heavy duty vehicles. All drivetrain specifications directly effect how efficiently the energy output of the engine is transferred to the tires. The correct specs can help ensure that the engine operates at its optimal speed for any given operation.
Reducing the rotating mass of drivetrain components and reducing the amount of heat generated within drivetrain components both help to improve energy loss from the engine to the tires, say ArvinMeritor officials. The appropriate specs for the vehicle application and operation will result in the engine consuming the least amount of fuel for the road cruise speed. Drivetrain component ratios should target the bands of the engine manufacturer and work as a true system for maximum fuel economy.
The officials recommend that fleets consider the following operating factors when specifying drivetrain components for maximum fuel efficiency:
• What kind of freight will the vehicle be transferring and how much does it weigh?
• Where does the vehicle operate geographically - mountains, snow, and ice?
• What percentage of the time is the vehicle on the highway?
• What is the most important: performance or fuel economy, or the best of both?
• What size tires and tire brand?
• Are automated transmissions of interest?
• How important is resale? Are you willing to pay up front for a premium specification?
• What will be the startability, gradeability and gearing of the vehicle to run at the optimum engine rpm for maximum fuel economy at road cruise speed?
Optimizing the drivetrain for reliability, long life and reduced ownership costs is a matter of devoting the necessary time and attention to the details, conclude ArvinMeritor officials. The payoff from properly spec'ing the components and making sure they are well-matched with one another and are used and maintained correctly is higher vehicle productivity and low lifecycle costs.