Balancing Act: Suspension systems

Dec. 1, 2008
At first glance, the suspension system would seem an unlikely battleground for issues such as capability, durability and driver retention to duke it out. Yet that's increasingly the case, as truckers like Jason Hancock are finding out. As president of Hancock Petroleum, a third-generation family-owned fuel hauler in Lloydminster, Alberta, Canada, Hancock knows his trucks must be super-rugged to handle

At first glance, the suspension system would seem an unlikely battleground for issues such as capability, durability and driver retention to duke it out. Yet that's increasingly the case, as truckers like Jason Hancock are finding out.

As president of Hancock Petroleum, a third-generation family-owned fuel hauler in Lloydminster, Alberta, Canada, Hancock knows his trucks must be super-rugged to handle loads topping out at 120,000 lbs., yet simultaneously must be ultra-plush to attract and retain drivers. “I could put 50 more trucks on the road tomorrow with all the business we've got,” he says. “I just don't have the manpower to do it. That's why I look at doing [whatever I can] to make drivers happy — refrigerators, satellite television, you name it.”

Founded in 1942 by Hancock's grandfather, Hancock Petroleum operates up around Fort McMurry where oil-rich tar sands are extracted and processed — some pretty rough country that's none too gentle on equipment. The company also runs its equipment 24/7 whenever possible by slip-seating drivers, putting over 18,600 mi. per month on its equipment. That means Hancock simply can't compromise on suspension durability in favor of a smoother ride — anathema in terms of trade-offs.

That is, until Hancock decided to try a front suspension package developed by Bellevue, WA-based OEM Paccar Inc., a package designed to satisfy both of those needs at once. Put through its paces at Paccar's test facility, along with 4 million mi. of real-world conditions in Mexico, Canada and the U.S., the company's proprietary front air leaf suspension uses lower spring rate for better isolation of road inputs to gain a 20% improvement in ride. At the same time, it maintains the roll stiffness and handling performance of a taperleaf suspension, says Ken Marko, market planning manager for Peterbilt Motors Co., Denton, TX, one of Paccar's truck-making subsidiaries.

“A key part of this new front air leaf design uses four air springs to support up to 75% of the spring load — meaning, literally, that air is significantly reducing the load on the physical components of the suspension, thus reducing road inputs to the cab, chassis and powertrain to extend component life cycles,” Marko explains. “We're improving ride quality without affecting suspension durability and longevity.”

Hancock is testing this suspension on a new Peterbilt Model 388 with two of the carrier's most senior drivers at the wheel — drivers with 90 years worth of combined experience. So far, the new suspension design is meeting expectations. “My drivers love it. They say it's the smoothest riding truck they've ever driven,” says Hancock, noting that because of this, it's been nicknamed “The Queen Mary.”

Yet that smoother ride has many practical ends, he stresses. “More comfort means less fatigue for my drivers, a critical thing when they are working 15 to 16 hours a day up here,” Hancock explains. “Also, the suspension is designed to handle the higher torques produced by the new air disc brakes we have on the truck and our ‘Super B’ fuel hauling trailers. That gets back to improving the efficiency and longevity of our equipment.”

The trick with suspensions today is to make sure all the pieces are in place — cost, weight and performance — and are balanced off appropriately against one another.

“The trend is more focused toward an air-ride configuration for the front suspension,” says Andy Zehnder, on-highway marketing manager for Kirkland, WA-based Kenworth Truck Co., sister company to Peterbilt and also a subsidiary of Paccar. “Suspension development continues to focus on durability and light weight.”

Yet performance can't be overlooked, explains Tony Musco, manager of suspension systems for Warrenfield, IL-based Navistar. “For example, low weight will get a fleet into your truck because they are looking for weight savings to balance out the gains due to new emissions control systems,” he says. “Performance is what's going to keep them in your truck. You've got to keep your drivers satisfied.”


But erring too much on the side of performance brings up other concerns. “The plusher the ride, the more the nose of the truck drops when you hit the brakes — a very unsettling experience,” Musco points out. It's an issue OEMs and component vendors are keeping close tabs on, with federal regulations in the wings aimed at shortening braking distances.

“Higher braking loads, from air disc brakes or improved drum brake designs, put more twisting force through the axle, the springs and the bushings,” he explains. “So you must include that in your trade-off calculation; how you keep enough stiffness in the ride to manage higher braking force without taking too much away from performance.”

That's one reason why more and more OEMs are rolling out proprietary suspension systems designed in-house, says Frank Bio, product manager-trucks for Greensboro, NC-based Volvo Trucks North America.

“The reason we'll see more proprietary suspensions and axles in the future is that we're trying to control a lot more inputs now, not just vibration from the road, but greater horsepower from the engine and more braking power transmitted through the front and rear axles,” he says. “The suspension is where all of that, including the ride and the handling the driver experiences, comes together. You can't just bolt on components anymore.”

Approaching the suspension as a single complete system — not as individual components such as axles, leaf springs and bushings — is also changing how OEMs work with vendor-supplied products as well, says Bio.

“We work much more closely with our vendors today to optimize their suspension for our chassis models and for specific applications,” he says. “That's because, today, our customers are much more focused on results than on specifications. They want the right weight, the right ride, little or no maintenance, plus durability and longevity.”


Another ingredient being mixed into the suspension system more often today is cost justification, says Sean Coleman, director of sales and marketing for Woodridge, IL-based Hendrickson Truck Systems Group. Everything from weight savings to ride and handling improvements is being passed through the accountant's lens, he says.

“Fleets are more financially astute these days, especially in the current economic climate,” Coleman explains. “That's why from our perspective, weight continues to be the No. 1 factor where suspension choice is concerned: fleets can see immediate payload and fuel economy benefits from weight savings.”

“We are constantly looking to reduce weight without compromising integrity,” adds Jerry Warmkessel, marketing product manager for Mack Trucks, which recently relocated its headquarters to Greensboro, NC, home to Volvo, its sister company. “They want less weight but the same durability.”

Maintenance is also becoming part of that cost calculation as well, he stresses. “There is a trend toward preset, prelubed hubs,” explains Warmkessel. “I think any astute maintenance director knows you cannot totally eliminate suspension maintenance in the highway market. What they are looking for, however, is ‘reduced’ maintenance.”

“The maintenance cost savings are becoming clearer as we accumulate more data,” adds Hendrickson's Coleman. “Better integrated air suspension systems are resulting in 40,000 more mi. of life for steer axle tires, for example. Some fleets are reporting replacing lights in four years instead of two due to the reduction in road inputs. So the suspension is having a much broader impact on the total vehicle than we once thought.”

Coleman stresses that this is only the beginning of the cost calculation process, for more factors lurk just over the horizon. “We've got new design hurdles coming up. We've got higher braking torque we'll have to manage plus the addition of more weight from 2010 emissions control systems and APUs [auxiliary power units],” he says. “Consequently, real estate on the frame rails, weight and packaging are going to become much bigger issues for commercial trucks. More demand for sealed-for-life components, along with the reduction or elimination of lube and pivot points, is on the way as well. That'll require more engineering and collaboration between OEMs and suppliers as we move forward.”

About the Author

Sean Kilcarr | Editor in Chief

Sean previously reported and commented on trends affecting the many different strata of the trucking industry. Also be sure to visit Sean's blog Trucks at Work where he offers analysis on a variety of different topics inside the trucking industry.

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