Driveline vibration can be felt in the cab and it can result from incorrect driveline angles, damaged U-joints or an unbalanced condition.

Driveline vibration can be felt in the cab and it can result from incorrect driveline angles, damaged U-joints or an unbalanced condition.

Noise, Vibration & Harshness is not the name of a truck-chasing law firm. NVH is something manufacturers try very hard to engineer out of their products, but it’s often like a game of whack-a-mole. Once you solve one problem, another shows up.

Take the effort to reduce noise levels inside the cab. Engineers do everything possible to eliminate the obvious sources of noise and vibration – they design new cab and engine mounts, they isolate the frame and the powertrain from the cab as much as they can. Then, with the newly quieted cab, drivers start noticing the squeaks and rattles they were couldn’t hear before because of engine and wind noise. 

Consider the possible sources of NVH: engines, tires, suspensions, cab squeaks, wind noise, drivetrain, brakes and more. Truck and engine makers and component suppliers test endlessly for NVH, seeking to isolate the source of the noise or vibration from the cab if they can’t reduce or eliminate it at the source.

Some sources of NVH are third-party sources. Tires, for instance. More aggressive tread patterns, such as lug or open-shoulder tread, tend to be noisier than rib-tread tires. The tire noise doesn’t make its way into the cab through physical contact with the cab, but by being a source of noise that happens to be in close proximity to the driver.

Tires and wheels can also be a source of vibration, such as when they are unbalanced.

“Drivers feel the vibration either through the steering column and steering wheel, in the case of a steer tire problem, or through the cab floor and seat in the case of a drive tire problem,” says Steve Ludwig, senior product development engineer at IMI, developers of Equal and Equal Flexx tire balancing products. “Many fleets will balance tires only when they get a driver ride complaint, but fleets should try to identify the source of the problem while they are working on it. Sometimes a seat vibration can result from a driveline problem like a damaged U-joint or driveshaft that has thrown a balancing weight. Also, tires running in an unbalanced condition can develop irregular wear, which can exacerbate the vibration.”

Or the vibration drivers feel could be a problem related to the recent trend toward downsped drivelines. While engine downspeeding has proven to be a reliable method for improving efficiency, additional challenges in vehicle dynamics affecting NVH have surfaced as a result. Torsional vibrations produced by the engine (the firing of each cylinder at relatively low engine rpm) are transmitted through the transmission, driveshaft and axles, and can be felt in the cab as low frequency vibration.

Engines have long been a source of noise and vibration, but current designs are much quieter thanks to structural and material improvements.

Engines have long been a source of noise and vibration, but current designs are much quieter thanks to structural and material improvements.

To reduce this effect, drivetrain supplier Dana is producing a next-generation driveshaft optimized for engine downspeeding with design characteristics it claims enable it to be balanced to one half of the current industry standard.

Jim Holman, Dana engineering manager, is intimately familiar with NVH, its impact on ride quality and the challenges faced by suppliers to reduce its effects.

“Suppliers and original-equipment manufacturers have been able to stay ahead of NVH quality for new vehicles,” he says. “But fleets need robust diagnostic and repair procedures to resolve driver complaints.”

Holman is the chair of the S.6 Taskforce leading revisions to the American Trucking Associations’ Technology & Maintenance Council recommended practice, Troubleshooting Ride Complaints (RP 648), which covers driver feedback on overall ride quality. At 57 pages, it’s one of TMC’s more detailed RP documents and contains a mind-numbing number of possible solutions to driver ride complaints.

Interestingly, a likely culprit in many driveline vibration complaints can be incorrect driveline angles. These aren’t necessarily an OEM problem, either.

“When NVH issues occur in the field on a truck, it is often the result of a driveline angle issue,” suggests Shane Groner, director of field business development at Eaton Vehicle Group, North America.

Drivers have been known to tamper with their tractor air suspension leveling valve in the mistaken belief that lowering the ride height will produce a smoother ride. That’s generally not the case. Instead, altering the driveline angle places additional stress on the U-joints, which can instead create vibration, and may ultimately lead to a failure.

“The driveshaft and U-joints are part of a complete drivetrain system, from the transmission to the drive axles,” explains Meritor’s director of product line management, Karl Mayer. “Driveline angles are determined by the supplier and installed by the OEM in accordance with those requirements for correct operation and long service life. Arbitrarily resetting the driveline angles can create problems.”

The fact that drivers occasionally attempt to rectify a perceived ride problem by adjusting the ride height suggests there is actually a problem. Driver comments on the problem could range from rough or pounding ride, to stiff and unyielding on rough road.  TMC’s RP 648 suggests several possible culprits, including front and rear suspension.

“Suspensions play a key role in reducing noise and vibration,” says Jason Heath, product manager for powered suspension systems at SAF-Holland. Suspension systems can obviously help minimize road inputs into the tires, such as potholes and bumps for example, he says. They also can help mitigate inputs from the drivetrain. “Not all suspensions are equal when it comes to isolating N&V and some manufacturers do a better job than others,” he says. “In general, when dealing with N&V from road inputs, air suspensions tend to perform much better than rubber ride and leaf spring suspensions. Although uncommon, four air spring suspensions tend to do a better job than two air spring suspensions and it would be my first choice if I wanted to have the most N&V isolation.”

Beyond spec’ing a suspension designed to address NVH, RP 648 says barring an obvious defect such as a broken spring or non-functioning shock absorber, possible solutions could include incorrect air springs, worn or seized spring shackles, damaged bushings, loose crossmembers, incorrect ride-height settings or even an overload condition or improperly inflated tires.

Troubleshooting ride complaints can be like working through an electronic fault-code tree. A hint of a problem could lead the

technician down several paths. RP 648 provides a comprehensive troubleshooting guide to ride complaints, many of which focus on suspensions, frames and various running gear. 

Transmissions and axles contribute to the mechanical cacophony in their own way, often in the form of a harsh high-frequency whining sound. That noise is produced by metal-to-metal contact between the gear faces in the various rotating parts within the components. Since transmissions sit directly beneath the cab, they can be perceived by drivers as the noisiest.

The clutch plays a role in dampening the torsional vibrations or oscillations mentioned earlier, and thus helps reduce some of the noise they produce.

“Newer clutch designs have a softer damper rate, with longer travel to handle the high torque output from today’s engines,” says Ryan Trzybinski, product planning manager for Eaton Vehicle Group. “Other vehicle specification factors such as direct drive versus overdrive or single versus tandem axles are critical considerations that can affect the specific clutch damper needed and its ability to mitigate vibration.”

Crown and pinion gears as well as the differential gears in the axles produce noise of their own, but it’s often drowned out by tire noise. If you install quieter rib-type tires, the axle gear noise may become more obvious.

Recent innovations in gear manufacturing processes have produced more finely polished gear faces in an effort to reduce the sliding friction between gear teeth in transmissions and axles. This saves fuel but also results in quieter operation, which is a bonus for drivers.

Meritor’s NVH dyno in Cwmbran, Wales, measures and charts the sounds and vibrations brakes make, helping engineers design quieter systems.

Meritor’s NVH dyno in Cwmbran, Wales, measures and charts the sounds and vibrations brakes make, helping engineers design quieter systems.

Engine NVH

The busiest single system on the truck, engines are also the greatest source of noise, vibration and so-called harshness. Geartrain noise, cam noise, combustion noise, etc. are all part of an engine’s sound signature. Some consider it annoying, while others revel in the sound, just like their fondness for the smell of diesel exhaust.

That said, engineers are producing quieter engines, and some of that progress is an offshoot of trying to improve fuel efficiency and reduce weight. Combustion noise has been greatly reduced by the advent of multiple fuel injection events for a single power stroke. Rather than the big blast produced when all the diesel enters the combustion chamber at once, the fuel burns more efficiently and with less noise when it is injected a bit at a time.

Engine construction techniques and materials have made a difference as well. Disappearing are the flat iron wall of the crankcase, which to some extent behaves like the cone in a loudspeaker, resonating with the combustion sounds of the engine and flexing slightly under the shock. This amplifies the sound to a small degree. Newer engines have vaulted crankcases, as is the case with Cummins new X12 engine, which are said to be stronger, lighter and produce fewer resonant vibrations.

Paccar, among others, uses compacted graphite iron in engine construction. 

“With shorter and thicker graphite particles than standard gray iron, CGI offers stronger adhesion between the graphite and the iron, giving the material 75% more tensile strength and 200% more thermal resistance than gray iron,” explains Kurt Swihart, Kenworth marketing director. “This allows for reduced wall thickness without compromising strength and optimizes the stiffness of the block and head, resulting in lighter and stronger cylinder heads and blocks with less vibration and noise.”

With combustion noise vastly reduced, what remains is gear noise. In fact, on some newer engines, the gear noise is almost as prominent as the combustion noise – which is actually a compliment.

“Paccar’s MX-11 and MX-13 engines use an optimally balanced crankshaft positioned low in the block to help reduce engine noise,” says Swihart, adding, “A rear-mounted gear train and a floating oil pan reduce engine torsional vibrations and noise to provide drivers a quieter operating environment.”

Cummins’ newest engine, the X12, features a sculptured block design, which allows unnecessary mass to be removed from the block without giving up structural rigidity. The block is reinforced by an innovative network of supports, which also helps reduce noise.

“The sculpted block along with multiple injection events makes the X12 run smoother and much quieter than its predecessor,” said Steve Reedy, program director for Cummins’ X12 engine, speaking at the X12 unveiling in July. 

Wind noise is a diminishing problem. Aero advancements in pursuit of fuel economy are creating quieter cabs.

Wind noise is a diminishing problem. Aero advancements in pursuit of fuel economy are creating quieter cabs.

Cab and chassis NVH

Brakes are one of the greatest contributors to the presence of NVH in trucks. Drivers expect brakes to operate quietly, but they can become distracted and frustrated by noisy, vibration-prone brakes or brakes that grab, leading to lurching stops. Engineers have a handful of colorful terms relating to the noises brakes can make, such as groan, moan, howl, squeal and others. In engineering terms, each sound has a place on a frequency spectrum and a list of possible causes.

Worn or heat-damaged linings/pads and drums/rotors can squeal and chatter toward the end of their service life, and worn components such as bushings in S-cam brakes can lead to harsh application behavior. Those, however, are mostly maintenance issues. New, fresh-from-the-factory brakes seldom misbehave. OEMs and brake manufacturers have put a lot of energy and resources into reducing NVH in commercial brake systems.   

Meritor, for example, recently completed a $42 million expansion and refinement of its air disc brake manufacturing facility in Cwmbran, Wales. Much of that money went into testing equipment that also benefits North American lab and testing facilities in Troy, Michigan. That project included the installation of a state-of-the-art NVH dynamometer, the first of its kind in the world.

“Customers won’t tolerate noisy brakes,” says Dietmar Knoop, director of engineering for Meritor’s European brake operations. “Eliminating brake noise has become one of the biggest challenges for brake makers.”

Still with running gear, items like shock absorbers and suspension bushings, which are usually good for the first life of the vehicle now, occasionally need attention. At stake are ride and handling issues that can contribute to additional vibration and harshness. 

Shock absorber performance degradation or outright failure is not always obvious, but drivers can certainly feel the difference between one that works and one that doesn’t.

“Shock absorbers use a fluid flow restriction principle to increase the amount of energy absorbed by the ride control product,” says Robert Nossal, product engineering manager at Gabriel Ride Control. “Among other aspects, fluid flow restriction ensures that drivers do not feel aggressive rebound events as strongly. When that is no longer functional or the shock absorber is physically damaged, ride quality will suffer.”

More than just a comfortable place to park your backside, seats are a critical line of defense in isolating drivers from bumps and jolts and whole body vibration.

More than just a comfortable place to park your backside, seats are a critical line of defense in isolating drivers from bumps and jolts and whole body vibration.

While the cab itself is usually not a net generator of NVH, it’s certainly affected by other systems and components. Wind noise and window and door seals that aren’t tight can create an annoying work environment. Newer model, highly aero-styled cabs are dramatically quieter than the classic square cab and hood trucks because the air flows smoothly over the cab surface, rather than bombarding the cab with turbulent air.

Driving experience with four recent Class 8 on-highway offerings, Kenworth’s T680, Peterbilt’s Model 579, Western Star’s 5700XE and the new Cascadia from Daimler Trucks North America, aptly illustrate that point. Each is very aerodynamic, each underwent extensive NVH evaluation, and each has nearly no wind noise at highway speed, thanks to improved door seals, smoother transition points across various body panels, and improved sound damping and insulating material in the cab structure. 

“We analyzed and tested to determine areas where even low levels of noise could enter the T680 cab and sleeper,” Swihart says. “This extensive fine-tuning process helped us to pinpoint exactly where to place sound-insulation materials to optimize decibel reduction.”

Efforts to improve the ride quality and driving environment led Freightliner to redesign both the cab and engine mounts to reduce vibration and improve noise isolation, says Kary Schaefer, general manager for product marketing and strategy at DTNA.

“Noise and vibration travel through those mounting points and into the cab itself, impacting the driver environment,” she says. “The redesign eliminated a lot of that. We also incorporated a new engine tunnel cover design using Quiet Steel technology to keep engine noise out of the cab.”

Daimler now offers a new cab insulation package using 3M’s Thinsulate product, which is known for its thin structure and very good thermal and acoustic insulating properties.

Of course, everything discussed in the previous 2,000 or so words kneels before this final item: seating. The driver retention properties of a quality seat are obvious, but seating really is where the butt hits leather, if you’ll pardon the pun.

“The seat is the driver’s last line of defense against the hazards of whole-body vibration experienced all day every day by drivers,” says Jason Rhoads, engagement and marketing specialist with Minimizer. Better known for floor mats and fenders, the company has partnered with seat builder Isringhausen to market a premium driver seat. “This goes beyond just being comfortable while driving. Vibration is well known to cause all sorts of problems that can cause lasting pain and discomfort.”

Because drivers come in a wide range of sizes, body type, weights and even leg lengths, a good seat should be highly adjustable and customizable to each individual, Rhoads says. “Features such as the length of the seat pan, lumbar support and the height of the seat relative to the floor are all important things consider when looking at seating.”

To understand what NVH actually is, given its near absence in today’s trucks, just climb into a 20- or 30-year-old truck and take it around the block. You’ll be painfully aware in a very short time of just what we’re missing today. 

About the author
Jim Park

Jim Park

Equipment Editor

A truck driver and owner-operator for 20 years before becoming a trucking journalist, Jim Park maintains his commercial driver’s license and brings a real-world perspective to Test Drives, as well as to features about equipment spec’ing and trends, maintenance and drivers. His On the Spot videos bring a new dimension to his trucking reporting. And he's the primary host of the HDT Talks Trucking videocast/podcast.

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