Suspensions seek to check effects of torque induced by potent engines
Physics tells us that for every action, there's an equal and opposite reaction. Applying that natural law to trucking illustrates how high-powered highway diesels may impact the protective role played by rear-axle tractor suspensions.
This is not a chicken-or-egg conundrum. Over the years, the design of tractor suspensions has certainly kept pace with other developments in vehicle technology and engineering.
However, a steady and growing trend toward greater use of electronic diesels, featuring higher torque at lower rpm, in Class 8 road tractors has put a spin on otherwise well-balanced powertrains.
In the real world of engine torque and suspension dampening, geometry is as crucial as physics. Under acceleration, power-packed heavy-duty trucks force changes in driveline geometry that result in greater torsional vibration.
More than any other component on a truck, the suspension is expected to dampen the harmful energy waves of vibration. What's more, air suspensions for tandem-drive axles should be engineered to control truck ride height.
But when the vehicle's frame rises in response to the greater torque input of a powerful engine, temporary but substantial changes occur in the working angles of U-joints.
Left unchecked, the extra vibration this creates can damage or reduce the longevity of driveline components and create harsh, noisy ride conditions for drivers.
The idea, then, is to avoid incurring expenses from equipment damage and driver complaints by having a suspension that reacts as fully as possible to the extra-torque some engines send to the driveline.
This will become more of an issue if industry estimates on engines hold up. Forecasters already expect higher-powered models to account for 30% of heavy-duty diesels within a few years.
This growth is being driven by several factors, including driver demands for trucks spec'd to their liking and, in some cases, by fleets hauling heavier or fuller loads than in the past. And some truckers prefer more power up front to boost resale value when it's time to trade out their trucks.
By far, the most common air suspension found on rear drive axles is the trailing-arm type. Unless otherwise indicated, all the suspensions discussed here are based on that design.
Leading suspension suppliers, including some truck OEMs, contacted by FLEET OWNER are addressing the issue. In fact, one major manufacturer already markets a comprehensive suspension package designed to counteract the dynamic-torque phenomenon.
Helping light the background on this subject is an SAE Technical Paper authored by Hendrickson Truck Suspension System chief engineer Roger Jable.
The author's discussion stems from testing done with a 75,000-GCW combination consisting of a 6x4 tractor and van trailer. The rig was spec'd with an engine rated 510 hp. at 1,850 lb.-ft. torque, with peak torque occurring at 1,300 rpm, and a 3.55:1 drive-axle ratio.
The truck was shifted so that the engine was under full load at 1,300 rpm in each tested gear. Results were used to determine the effect acceleration from a standing start has on tractor frame lift.
Each time the tractor's clutch was disengaged, the rear air suspension's ride height returned to normal. However, as gears were shifted to re-accelerate the truck, the resulting torque caused the frame to rise again.
The result, reports Jable, indicates that a 3-in. rise in frame height at the axle centerline is enough to throw off driveline angles.
Air suspensions currently on the market feature some sort of height-control system. However, according to Jable, typical height-control valves don't respond adequately to certain "events," including moderate to severe braking and heavy acceleration.
He says these events can be long enough to cause unacceptable vibration, but are short enough that a height-control system can't respond fast enough.
If the vibrations produced are severe enough and are incurred frequently enough, says Jable, they can lead to reduced life for drivetrain components.
According to Jable, how well a height-control system works can be weighed against three indicators:
1. Dead band: range of vertical movement the suspension doesn't respond to.
2. Dynamic height drift: ability to maintain height in normal operation.
3. Response time: time needed to respond to sudden changes in ride height.
As Jable sees it, to keep U-joints from being overtaxed, suspension manufacturers must consider the response times of height-control devices and their ability to react to short periods of dynamic torque input. However, he contends that getting optimum performance is not entirely up to the suspension supplier.
"Accurate suspension design, vehicle design, and vehicle assembly are all required to provide correct static and dynamic U-joint cancellation angles," Jable states. "In some cases, even if the static angles are correct, the amount of driveline angle change caused by suspension dynamics can become excessive and cause U-joint induced torsional vibration.
"Simple changes to the suspension," he adds, "can reduce the U-joint-induced input to the driveline system and reduce the total amount of torsional activity in the driveline."
But before hauling a suspension supplier or OEM on the carpet over this, bear in mind the full jury is still out on how big a threat engine torque now poses to drivelines.
Dan Fuchs, manager of chassis engineering for, says the OEM has "not identified" a need to significantly change suspensions used with high-torque engines. "There may be more of an effect when a vehicle is under acceleration, but it would still be transitory."
Yet he says the truck builder does not rule out a closer look. "Future designs may compensate for higher torque by distinguishing between loaded and unloaded situations. That way, the suspension may more accurately maintain driveline conditions."
Noting that Freightliner works closely with major fleets to scope out such problems early on, Fuchs contends that torque reactivity is mainly an issue of careful spec'ing. "It's important," he points out, "to get the proper GCW rating for the suspension to support the truck's application."
According to Ed Saxman, manager of sales engineering atTrucks North America, careful spec'ing can offset the effect of torque reactivity on various vehicle types. "To help compensate for torque," he advises, "the major axle manufacturers have recommended using 18-in., instead of 15-in., ring-gear axles. Manufacturers have also come up with 40,000-lb. housings to better fit the larger gears.
"A 46,000-lb. rear axle is usually found in vocational applications," Saxman continues, "but some fleets are spec'ing that rating for highway use for its greater beef."
Saxman says Volvo-manufactured highway suspensions rated from 38,000 to 40,000 lb. are typically recommended by the OEM, as is the 46,000-lb. Hendrickson HAS460. He notes that Volvo's T-Ride suspension, developed for severe-duty use, features torque rods that resist the impact of higher torque.
Engine-driven Garrick Hu, vice president of engineering for, sees the torque issue as engine-driven. "It's definitely a concern that's been growing over the last three or four years as electronic engines have become more widely used."
He explains that under electronic control, the torque delivered by heavy-duty engines can "move up," causing suspension springs to "wrap up." That condition can change driveline angularity, thereby inducing more vibration.
"An operator can run into this problem at lower horsepower, too," Hu cautions, "because those engines can share the same torque-rise characteristics as those with higher horsepower ratings."
He says-brand suspensions offer two distinct solutions to torque reactivity. Which one a fleet gets depends on whether it orders a truck with an all-Mack powertrain or a mix of vendor specs.
"Pedigree Macks," says Hu, "avoid the angularity issue since they are engineered with straight-through axle shafts. In addition, the axle carriers are at the top, which is a more forgiving position."
On the other hand, he says that Mack recognizes that air suspensions for highway tractors spec'd with vendor axles will have to be "more non-torque reactive" to maintain correct axle positions under high-torque operation. "The torque effects of electronic engines are a consideration as we design our next generation of air suspensions, which will also be more weight-conscious."
While those suspensions are now under development, Hu reports that the OEM has worked with Hendrickson to make an "interim fix" to improve the torque reactivity of current Mack air suspensions. "With our next generation of suspensions," he notes, "we'll be able to use a more engineered approach to answering the torque question."
According to Dick Von Lehman, product marketing manager ofTruck Co., the OEM's own take on suspension design helps offset torque reactivity. He explains that Kenworth's proprietary Airglide 200 air suspensions are distinguished by their parallelogram design.
Von Lehman says the suspensions, offered on Class 8 KWs with 52- or 54-in. axle spacings, boast a "multi-link" design that allows axles to maintain constant pinion angles as they move up and down. "This improves overall handling, while minimizing torque-induced driveline vibration.
"Kenworth is the only OEM to use controlled-axle geometry to reduce problems from driveline angularity," he continues. "Our system keeps the drive axle from rotating in relation to the frame, which minimizes stress."
"We're evaluating our products in light of today's trend to higher-horsepower," reports Ken Griswold, director of truck suspensions for Neway Anchorlok. "Drive-axle suspensions are typically designed with specific applications in mind."
Grieg Chapman, truck OE account representative, points out that using higher-torque engines in highway tractors may overwhelm the abilities of typical Z-spring trailing-arm suspensions.
However, according to Chapman, Neway's parallelogram-design suspension boasts a rigid-beam trailing arm that "handles engine torque very well." There is a trade-off. "Our suspension costs and weighs a little more than trailing-arm Z-spring units," he says. "But it is a solution for fleets that are experiencing driveline-vibration problems.
"An engine with a 'fleet spec' of 400-hp. equipped with cruise control will crank itself up to the same torque level as a 600-hp. version," Chapman continues. "These engines are getting more popular because they are extremely driveable, which heightens driver retention, and because they offer a higher resale value if reprogrammed for more horsepower.
Watch that Jake "It's also important to bear in mind that high-torque engines provide greater braking ability," he adds. "For example, some Cummins engines produce 600-bhp. of retarding power. That's makes for another tremendous torque input."
Jim Mitchell, director of engineering for Ridewell Suspensions, says the manufacturer focuses primarily on vocational niches by offering dedicated "non-commodity" suspensions.
The Ridewell product mix includes a parallelogram-design air suspension available in single- (4-bag) or tandem-drive (8-bag) trim. According to Mitchell, the parallelogram unit is more resistant to torque than competitive trailing-arm suspensions.
"Our parallelogram design was developed out of the emergency-services market," Mitchell relates. "It is more expensive than other types of suspensions used on highway vehicles, but it has an extremely high capability to handle torque and provides excellent driveline angularity. It's one solution for handling increased power."
According to Ray Mueller, vice president of engineering, Reyco Industries has made changes to its two models of tractor air-ride suspensions to improve their capability to handle driveline pinion-angle changes.
"Higher torque inputs from the engine when climbing hills or accelerating cause the frame to lift and the pinion angle to change," Mueller states. "But the cam-like design of our front hanger produces forces that counteract the torque input into the suspension -- resulting in less pinion-angle change.
"And instead of locating the height-control valve on the axle, we place it near the air bag," he continues. "This way there are no pinion-angle changes as the truck cycles from an empty to a loaded condition."
Mueller says Reyco's most recent improvement is the addition of "instant-response" height-control valves. "This results in a quicker change back to ride height and correct pinion angles during acceleration. We also put 'quick-dump' valves in tandem with the height-control valve on export units of 600 hp. or more," he adds.
In Mueller's view, torque-reactivity problems can be avoided in the first place by proper system engineering. "When OEMs spec our suspensions," he notes, "we set them up to give the driveline angles requested."
"There's no question that we're seeing increased use of high-torque engines -- and their effect on suspension performance," says Keith Gelinas, director of marketing for Hendrickson Truck Suspension Systems. "About 8% of the highway market has moved to the more powerful engines, and that number will only rise in the next few years."
He says Hendrickson offers an integrated solution to torque reactivity via the EDGE (Efficient Driveline Geometry) system available on its HAS Series of air suspensions.
"By maintaining correct vehicle ride height and a level suspension," Gelinas explains, "the EDGE system helps ensure proper driveline angles and U-joint cancellation. The result is longer life for driveline components and less noise and ride complaints by drivers."
Brian Knier, air suspension product manager, says that high torque "creates frame rise and axle wind-up" that change driveline angles. He says the only effect a suspension has on the driveline is its angularity.
Central to the EDGE design is the Hi-Torque shock. "We use a rebound spring inside the shock to keep frame rise in check," Knier says. "In addition, the system uses a height-control valve designed for zero delay. The third aspect of the system is just as important. We insist on proper factory settings of the angles. The idea," he adds, "is to build the system properly, up front."
That last point is one on which all suspension makers appear to agree. Putting something together right the first time around usually saves time, money, and energy later.
It makes sense. Taking action sooner is far better than later -- especially when it results in lower maintenance costs and fewer driver complaints.