SUSPENSIONS: Still on a diet

As any trucker knows, weight is a big deal: Shaving pounds off a vehicle helps improve fuel economy and most importantly makes it possible to carry more freight, thus generating more revenue.

As any trucker knows, weight is a big deal: Shaving pounds off a vehicle helps improve fuel economy and — most importantly — makes it possible to carry more freight, thus generating more revenue. That's why truck manufacturers continue to focus on driving weight out of their products, and why developing ever-lighter suspensions remains a top priority in truck design circles.

After years spent slimming suspensions down, however, the challenge at this point is finding new ways to cut weight, explains Frank Bio, product manager for Volvo Trucks North America.

“Over the years, we've been pursuing what I call an ‘evolutionary’ process to get weight out of not just suspensions, but the entire tractor as a whole,” he explains. “Now, however, we may have maxed out on potential weight savings in both areas, so we need to move from ‘evolutionary’ to ‘revolutionary’ design efforts. My feeling is the only way we're going to get more weight out is to completely re-look at the job a suspension must perform and see how we can do it better.”

DIMINISHING RETURNS

According to Tom Smith, chief engineer-suspension for International Truck & Engine Corp.'s new ProStar highway tractor line, “We are indeed rather quickly reaching the point of diminishing returns in weight savings from suspensions; the days of even 10 to 20 pound savings are all but behind us. Now we're down to achieving three- to four-pound savings.”

Yet even dropping a pound or two out of a suspension, with corresponding savings from components connected to the suspension, can make a big difference to a trucker's bottom line, he emphasizes.

“Giving them more available payload — even just 50 pounds — really adds up over time,” Smith notes. “Fifty extra pounds a day compounded over weeks of operation runs right into a carrier's revenue stream, and those dollars fall right to the bottom line.”

WHY STAY SKINNY?

Saving money is the primary reason that reducing vehicle weight will remain a hot topic for some time to come, says Christoph Hofmann, director of product strategy for Freightliner Trucks.

First, several segments of the industry, notably bulk haulers and tanker fleets, remain extremely weight sensitive and can more readily turn extra payload capacity into cash for their business. The other big issue revolves around emission regulations, he adds, because meeting the 2007 and upcoming 2010 emission targets is adding weight back into trucks the form of extra components such as turbochargers, exhaust gas coolers and diesel particulate filters.

“Bulk haulers are traditionally the most weight sensitive segment for Class 8 trucks. However, approximately 20% of the Class 8 linehaul market is weight sensitive, in part due to the technology needed to meet emission mandates,” he says. “As a result, customers are demanding lighter weight trucks in general, with suspensions being a part of the larger issue.”

Any customer who is weight sensitive will seek lighter suspensions and lighter components, says Hofmann, because any pound you remove from the truck directly correlates to an increased pound of payload capacity. “That's an advantage that will continue to pay off for the entire life of the truck, load after load,” he notes.

These are not the only benefits, however. Trucks that weigh less require less horsepower to accelerate and climb hills. That decrease in horsepower requirements means lower fuel consumption, resulting in improved fuel efficiency.

“Spec'ing lighter weight suspensions and other components can result in improved fuel economy and decreased ton-mile costs,” Hofmann notes. “Roughly each 1,000 lb. weight reduction can mean up to a 1% increase in fuel economy. While a lighter weight suspension will not achieve a 1,000 lb. weight reduction by itself, every pound in weight reduction counts, and that pays off over the life of the vehicle.”

Steve Keifer, director of marketing for Hendrickson International's truck suspension division, points out, however, that a gain in payload capacity remains the primary reason for reducing vehicle and suspension weight.

“Whether they are on-highway or vocational carriers, fleets that generate revenue from available capacity are going to keep trying to shave off the pounds, and reducing the suspension weight is the quickest way to do that,” he explains. “Fuel economy gains from weight reductions are really going to remain a secondary benefit.”

MATERIALS & DESIGN

The keys to reducing suspension weight revolve around using lighter materials, such as aluminum or different grades of steel, in conjunction with new designs that affect not only how the suspension is constructed but also how it's attached to other components, such as the axles and frame rails, says International's Smith.

“Just moving to fabricated steer axles and away from forged axles really gave us huge weight savings,” he says. “We've also gone from three-leaf suspensions down to a single or mono-leaf design; that's also really helped pare down the pounds.”

DURABILITY STAYS

Yet engineers are keenly aware that no matter what materials or design approach they use, suspension durability and performance must be maintained. “The primary function of a suspension is to insulate the vehicle from road and driveline vibrations, characteristics that are usually expressed as ride, handling, stability and vehicle longevity,” says Smith. “When you start taking weight out, those characteristics can get impacted, with durability being a big one.”

That's a huge worry since the suspension needs to protect other components on the truck. “That's what a suspension does. That's why you've seen the rapid rise of ‘torque neutral’ rear suspensions in recent years,” he says. “They don't allow driveline vibration to alter the pinion angles of the tandem rear axles. And maintaining those angles translates into less wear and tear on the axles, resulting in potentially longer life and less maintenance.”

As an example of how suspension design can do this, Jim Bechtold, assistant chief engineer for Kenworth Truck Co., points to the AG400 multi-link air suspension for KW's Class 8 trucks, which has a flexible sway-bar arm design.

“This configuration allows cross-articulation, letting axles adjust naturally to changes in the road without binding components and linkages, thus reducing the strain in the suspension components when the vehicle is operated on uneven road surfaces,” he says. “It also virtually eliminates maintenance, as no lubrication is required, and also reduces weight by using high-strength, heat-treated aluminum castings.”

MUTUAL SUPPORT

As an example of how suspension design improves the life of other components, Bechtold points to the parallel orientation of the AG400's top V-linkage and swaybar arms. “This ensures constant pinion angles as axles move up and down, which prolongs the life of U-joints, axles and transmissions,” he adds.

Jerry Warmkessel, highway products marketing manager for Mack Trucks, emphasizes that despite the desire for weight reduction, durability, which is perhaps the most critical characteristic of any truck suspension, must be maintained.

“Durability is a primary concern for fleets; they also want suspension systems that require minimal maintenance while providing excellent ride characteristics and driver comfort,” Warmkessel says. “While weight is always a concern since the lighter the truck, the more payload capacity there is available, we must constantly balance weight against durability.”

Engineers at Peterbilt Motors Company add that there is no room for compromise in the performance of suspension components. Parameters such as roll and lateral stability, spring rate, ability to withstand brake windup loading and ride comfort cannot be sacrificed to gain weight savings, they point out.

In terms of future design, weight reduction efforts will focus on a number of areas in and around the suspension, according to Gerry Remus, marketing manager for vocational suspensions at Hendrickson International.

“We're looking at the brackets and crossmembers that attach the suspension and axles to the truck; that's an area where 5- to 15-pound weight savings are still possible,” he says. “We'll need to look at those areas closely as the next round of emission changes in 2010 is sure to add more weight to the vehicle, such as urea storage tanks for SCR [selective catalytic reduction] systems.”

FUTURE FOCUS

Hendrickson's Keifer adds that the core elements of truck suspension design in the future will include: life cycle cost, further weight reduction, improved performance, i.e., ride, handling and stability, plus better serviceability.

“We can't sacrifice durability in our search for weight savings,” notes International's Smith. “There's a certain weight threshold we must meet to maintain durability and vehicle safety.”

For the future, the focus will be how to maintain durability and performance with the least possible weight. “That means using more exotic materials, new engineering designs and integrating the components together better,” Smith explains. “Electronics are going to play a big role in making suspensions more adaptable to road conditions, giving them the ability to change their shape to better conform to road inputs. That's where we are heading in the days ahead.”

Lightening the ‘07 load

Steve Jansen, regional fleet manager at leasing firm First Fleet Corp., will tell you that removing weight from Class 8 tractors got a whole lot more important this year.

“Trucks are heavier due to ‘07 emissions. Why? Well, weight got added from the engines, cooling systems, diesel particulate filters and frame,” he says. “The average weight increase we've seen from 2006 to 2007 for a tandem-axle day-cab equipped with a 14- to 15-liter engine is 360.7 lb.”

Speaking during the company's annual Fleet Management Conference in Ft. Lauderdale, Jansen says a major weight saving spec is to use an aluminum cab versus a steel model, thereby saving 200 lb. Going with just three instead of four batteries saves an additional 35 lb., while using a plastic instead of steel battery box shaves off another 21 lb. Changing your exhaust stack arrangement can remove anywhere from 23 lb. all the way up to 95 lb. if you go from twin stacks down to one, he says. Also, using aluminum hubs on the front and rear axles removes 20 and 100 lb., respectively.

Even small changes can add up. As Jansen notes, shaving just an inch off a tractor's frame rails saves 4 lb. per inch.

Gary Ciapetta, marketing director for Hendrickson International's trailer suspension division, adds that suspension selection is crucial for weight savings. He notes that Hendrickson's front air ride suspension, the Airtek, can save up to 120 lb., but how much weight is actually saved depends on the size of the steer axle tire used by the fleet.

Another Hendrickson product, the HTB torque box suspension, uses a parallelogram design to eliminate torque rods, saving 250 lb. Matched with an HTB rear suspension, it would save a fleet 700 lb. total.

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