It's easy to take truck suspensions for granted. In general, they're extremely durable, require minimal maintenance and in recent years have gradually, but substantially, improved ride quality for all types of truck applications. They also play a key role in handling and safety that often goes unnoticed or overshadowed by glitzier technology like electronic stability controls. You might say suspensions are practically invisible to most truck owners, quietly doing their job without fuss or bother.

But that doesn't mean suspensions are being ignored by component and truck makers. Whether it's incremental improvement or innovative leaps, truck designs are constantly evolving and suspensions are an important part of that process.

Take the issue of weight. Emissions systems have added considerably to the weight of most 2010 trucks and tractors. “A major part of that new weight is being seen on the front axle, so customers are going to longer wheelbases, new fifth wheel positions, and smaller fuel tanks, all to keep the weight gain on the front axle to a minimum,” says Sean Coleman, director of sales & marketing for the truck systems group at Hendrickson.

For example, front suspension ratings have typically been offered at 12K, 13.2K and 14.6K lbs., but today Hendrickson is also supplying 12.35K, 12.5K, 13.3K, 13.5K and 14K ratings that can be more closely matched to actual truck applications and eliminate unnecessary weight.

“We're also going beyond more specific ratings, looking at new technologies,” says Coleman. “Replacing the traditional two-leaf spring on the front axle with a new single leaf design can save upwards of 90 lbs. depending on the configuration.” At least one major truck manufacturer has already committed to offering that new mono-leaf front suspension in the near future.

Suspension durability “has always been the price of entry, but recently, requests for weight reduction have become common,” says Erik Binns, product market segment manager for Peterbilt Motors. “Materials are helping there. We've introduced composite front springs that are 106 lbs. lighter than steel, but we're also looking at new types of steel springs with different shapes and configurations to take out weight.”

Every part of the suspension is under consideration, too. Freightliner's proprietary Airliner air ride rear suspension, for instance, has moved to aluminum hangers to reduce weight. “And we've used CAD systems to optimize their strength while minimizing the weight,” says Keith Harrington, product manager of product strategy for Freightliner parent Daimler Trucks North America (DTNA). “We've even replaced the aluminum cone in the air spring with one made of composite to get the weight down.”

While new materials can reduce suspension weights, the cost/benefit ratio has to make sense, according to David McKenna, director of sales & marketing for Mack Powertrain, the supplier of Mack Trucks' proprietary components. “Just making a bracket out of unobtainium might cut 60 lbs., but who's going to pay $25,000 for a lighter suspension,” he asks.

Using current materials in new ways holds the potential “to satisfy that value proposition, providing a lighter suspension that's reliable and has the right cost,” McKenna says. “We want to look at today's materials and make them better before moving to all new materials and designs. There are new ways to make stamped steel products, for example, that will give us components that are stronger and lighter than the fabricated ones used today.”

SYSTEMS APPROACH

That was exactly the approach taken by SAF-Holland when it replaced its vocational Neway AD Series air ride suspension with the ADZ Series, which cut over 220 lbs. per axle from the old design. Moving from fabricated lower arms to ones cast as a single integral module contributed to that weight loss, according to Mark Molitor, vice president of product engineering for the power vehicles systems business unit. A new proprietary system for connecting torsion members to control arms not only took out weight, but also reduced the parts count dramatically while eliminating welds and fasteners for better durability, he points out. And integrating the crossmember into the suspension design also reduced weight and complexity.

In the near future, tighter integration of the suspension with other frame-mounted components also offers significant potential for additional weight savings. “We're certainly looking at the systems approach,” says Molitor.

Volvo Trucks North America (VTNA) is also working on weight-savings integration designs for its proprietary Optimized Air Ride Suspension, according to Frank Bio, product manager-trucks. While “it's too early to talk about the details,” Bio says VTNA is developing ways to integrate the suspension with frame components, perhaps in ways that might surprise many. “Call me in five months,” he says.

Air ride suspensions have virtually replaced mechanical springs for heavy-duty rear axles in over-the-road applications, but new developments have begun making them practical for a much wider variety of applications, including front suspension, medium-duty trucks and, most recently, an expanding array of vocational trucks.

By the middle of 2009, most OTR heavy-duty tractors were available with air-ride front suspensions, either proprietary systems like those offered by Peterbilt and Kenworth, or Hendrickson's Airtek.

The take-rate for front air ride is still low — about 3% by some estimates — and the new emphasis on weight puts them at some disadvantage compared to the more traditional mechanical spring suspensions. But the benefits in ride improvement can be tough to ignore, especially as fleets place more emphasis on driver comfort and fatigue reduction.

Peterbilt's AirLeaf four-bag front suspension improves an OTR tractor's ride by about 20% compared to a steel spring suspension, according to George Maffey, a chassis engineer with Peterbilt. “And there's no compromise on stability or road feel,” he says.

While the adoption of air ride for OTR front axles may be slow, vocational truck users are proving more enthusiastic about new air ride options for their rear axle applications. Construction, on/off road and heavy-haul fleets have long prized mechanical spring suspensions for their ability to stand up to truly demanding conditions.

Air suspensions are broadening their reach,” says Hendrickson's Coleman. “Heavy-duty [mechanical] suspensions do have durability and longevity benefits, but some air suspensions now on the market have closed ground there. Some [vocational air suspension use] is being driven by ride quality as well as vehicle longevity, which is helped by a softer suspension.”

The capability of new air suspension designs is also fostering use of air rides in a broader range of vocational applications. The first generation of Hendrickson's Primaax EX vocational air ride, for example, was not approved for rear discharge cement mixers. “But with advanced development work on roll-steer and geometry changes, we've been able to demonstrate the same stability as older rubber suspensions on rear mixers,” says Coleman. “Same stability, but with a much better ride. ”

Mack, which has offered its Camelback progressive spring suspension for off-road and severe-service vocational applications since 1948, is also looking at air ride for some of those applications. “A highway [air ride] suspension isn't appropriate when you have to handle uneven ground, gross unequal load distribution or moving loads,” says McKenna. Scheduled for introduction in the third quarter of this year, Mack's new vocational air suspension will offer weight and ride advantages over the Camelback, but with similar durability under those conditions, he says.

A redesign of the Neway AD vocational air ride, the new ADZ Series is experiencing strong acceptance in off-road and severe-service applications like logging, oil patch service, transit cement mixers, dump trucks and even fire trucks, according to Molitor. “The ride and its benefits to the equipment are there now, as well as the durability,” he says. “There's about 30% penetration for air ride in that market, and it's growing.”

While air suspensions are making inroads on many fronts, don't count out mechanical heavy-duty suspensions just yet. While Mack's Camelback suspension with its progressive springs has been around since the middle of the last century, its attributes of simplicity, strength, robustness and versatility have kept it a viable choice for many vocational truck users. “Our new vocational air will have weight and ride advantages, but certain customers will never buy an air ride,” says McKenna. While not ready to divulge details yet, McKenna says Mack has some major developments coming within the next 18 months that will address the Camelback's weight and ride disadvantage for those customers.

Developed for the Chinese domestic market, at least initially, Hendrickson's new HUV is breathing new life into rubber heavy-duty suspensions, a design consigned to history by many for its rough ride. The “extreme durability” of the HUV's rubber design is intended to handle the rough operating conditions faced in China, while a new infinitely variable spring rate design “helps greatly with ride and changes that rough-riding reputation of rubber suspensions,” says Coleman. “This design gives you a soft ride when empty and roll stiffness when loaded, and it's also very lightweight.”

Although there's been no official word on bringing the HUV to North America for severe applications, Hendrickson officials introducing the new rubber suspension late last year at the IAA Commercial Vehicle Show in Hannover, Germany, hinted that might be a good possibility in the near future.

MORE ATTENTION

As for future suspension developments, the pace of change may pick up as truck manufacturers can now turn their research efforts away from engine emissions and begin looking at other components.

With new federal stopping distance requirements for trucks taking effect in August, suspension systems may need minor changes to handle any additional braking torque, according to Volvo's Bio. “We'll need to take a new look at suspensions, especially the front suspensions.

Those new regulations “mean larger drums or disc brakes on the steer axle,” explains Coleman. “The added weight and higher torque will impact suspensions, and the brake mountings may need to change, too. We're already looking downstream to make sure we're ready for higher stopping power brakes.”

With electronics invading every major component on modern heavy-duty trucks, “intelligent suspensions are definitely on our radar,” says Molitor. SAF-Holland has already applied electronics to its fifth wheels and is currently staffing up its applied research and development operations. “You may not see it this year, but it's coming,” he says.

Currently, requests for electronic sensors integrated with suspensions are centered about onboard scaling, especially in the refuse industry, according to Coleman. “Future development could bring expanded use of smarter control systems, systems that talk to each other and use that load sensing information,” he says.

Predictably, the future is filled with maybes and could-bes, but there is one certainty when it comes to the future of truck suspensions. “It all comes back to durability,” says DTNA's Harrington. “First and foremost, a truck suspension has to be durable no matter what.”