When it comes to spec’ing a suspension system for a heavy-duty truck, two obvious questions come to mind: what am I going to do, and where am I going to go? Although there are other things to consider, it is ultimately your truck’s application and operating conditions that are the foundation of any spec’ing decisions, to ensure support for the load and stability for the truck.

Naturally there are many manufacturers and many models to choose from, but all suspension systems still fall into one of three categories: air-ride, spring and rubber load cushion. All three systems have their pluses and minuses related to ride quality, cost, strength, ease of maintenance and durability, but some systems are simply not appropriate or necessary for certain applications.

If you are an owner/operator spending long hours on the highway, personal comfort is likely going to be a top priority. And given the stiff competition for drivers these days, driver comfort has become a top priority for fleets as well. It is no wonder then that the air-ride suspension is by far and away the top choice for almost all highway applications.

“My guess is at least 90 to 95 per cent of all new equipment sold now comes with air-ride,” says Claude Sauriol, a spokesman for Reyco Granning Suspensions. “Obviously the specific application matters, but for highway or street uses, you would rarely sell on springs. Fleet operations managers have to keep their guys happy, so cargo has really become a secondary consideration.”

(Not so for some shippers, many of whom insist on air-ride trailers to keep their goods intact.)

“For applications within the city, like delivery work, springs are usually fine. That’s unless the cargo is especially sensitive – like bottled soft drinks for instance – or the operations manager is overly concerned about driver comfort,” says Sauriol. “But the bottom line is, air is a better ride. Plus, air-ride can increase the life of the entire vehicle because less road shock is transferred to the other components”

There is a price to be paid, however, for fewer bumps. Leaf springs are often the standard offering from truck manufacturers, while air-ride is offered as an upgrade. Most truck manufacturers now offer their own air-ride systems, but there are also systems available from OEMs like Reyco-Granning, Hendrickson, Ridewell and ArvinMeritor, to name a few.

Another concern with air-ride systems has been weight. But for the most part, says Frank Maly, ArvinMeritor’s project manager-suspensions, those problems have been overcome.

“In the past, people’s concerns almost always centered on weight and cost,” Maly says. “But with new designs and components available now, some air-rides are actually lighter than standard springs. And while air still costs more, the price has been reduced dramatically over the years. So the reasons to stay away from air have really gone away.”

Because strength and durability are such important concerns when it comes to suspensions, cutting weight from the system has always been a tricky proposition. But competition in the marketplace forces suspension manufacturers to constantly tinker with their designs, looking for ways to shave pounds without compromising quality.

“The average air-ride suspension on the road today weighs about 300 lb. – it was twice that 10 or 15 years ago,” says Jerry Steele, vice-president of sales for Ridewell Corp. “But it is like a person losing weight: the first 10 lb. are easy, then the next 10 are a lot tougher, and after that it is tough to lose even 1 lb.”

Steele says his company is working on a design that will integrate the axle and the suspension, with the goal of trimming as much as 100 lb. from a single combined unit. Reyco-Granning recently introduced a unit that features spring beams that are twisted inward like a hockey stick, thus bringing each air spring closer together. That small design change eliminated the need for a heavy crossmember that once provided lateral support. Other efforts to shave pounds centre on suspension materials.

“Integrating components can sometimes allow you to use thinner metal for some components,” Steele says. “If you can add strength by cutting down on welds, you might find you can get by with half-inch instead of three-quarter. Some premium suspensions use quarter-inch steel on some components, but it would be difficult to get thinner than that and maintain integrity.”

Good, old-fashioned steel is still the standard right now, but the future of suspension materials is anybody’s guess, Steele says. “Eventually, the only way to get lighter may be with, who knows, titanium or cast aluminum or a combination of steel and aluminum. But I would say a suspension like that would be at least five or 10 years away.”

But which suspension to choose? With a number of OEMs offering a range of air-ride designs, as well as those offered by truck manufacturers, how do you know which make and model is right for your application?

At this point, more specific considerations come into play, such as appropriate axle seat groupings, pinion angles, carrying capacity, ride height, and matching different axles to different trailers. Frank Haselden, vice-president, maintenance/compliance for TST Overland Express (and winner of this year’s Volvo Canada Fleet Maintenance Manager of the Year Award), is responsible for spec’ing components for his company’s fleet of 490 tractors and 1,245 trailers.

“It’s tough. If you talk to the salesmen, everybody has the latest and greatest,” Haselden says. “A lot of the time, weight is an issue, because a heavier suspension will cut back on your payload. We don’t tend to max-out on weight, so we can afford to run with a little heavier, more durable system and not pay a penalty.”

Overland completed the switch to air-ride on its entire fleet five years ago, Haselden says, a move mainly motivated by customer requests. Another issue was maintenance.

“All of our deliveries are timed,” Haselden explains, “And what would often happen is we would get drivers calling in and saying they had a broken leaf. An air leak is just easier to deal with.

“Air springs are nice and light to deal with. You just jack the vehicle up, undo four bolts and it’s out. With leaf springs you have to take the wheels off, undo the U-bolts, then physically lift the spring out, and it could weigh anywhere from 75 to 150 lb. That usually takes two guys, and there is the possibility for injury there”

Ease of maintenance is also a factor Haselden uses to differentiate between air-ride systems. He is interested in knowing how easy, and expensive, it is to replace suspension components like bushings. By talking to other fleet managers and mechanics in the industry, he finds out how durable certain systems are and what designs are prone to maintenance problems.

Sometimes a spec’ing decision may turn on a particular problem the fleet or owner/operator is encountering. In Overland’s case, that problem was dock walk. Once drivers backed up to the dock and dumped the air from the system, Haselden says, the trailers tended to creep away, which would cause problems when unloading the last few pallets from a load.

“To solve that, we went to suspension systems with pins, or dock-walk legs that snap into place and prevent the creeping,” he says. “We were also concerned that drivers would forget to recharge the system before they leave. But this way the system remains charged all the time.”

As would be expected with a newer system, the bulk of the advances in suspension technology in recent years have been made in air-ride. It was not that long ago, recalls ArvinMeritor suspension project engineer Jim Eckelberry, when an air-ride system could weigh as much as 300 lb. more than a comparable mechanical system. While much of that weight has been eliminated through design changes, he says, future reductions will likely come on the material side.

“In the future, material selection is going to be crucial as we enter th
e composite age,” Eckelberry says. “But right now, cost is a factor because composites are still very expensive. And weight reduction for the sake of weight reduction becomes a losing proposition if the system can’t handle the loads and be durable.”

The story is the same for spring systems.

“Essentially, spring suspension design is the same today as it was 60 years ago,” Sauriol says. “But spring technology itself has come along. There are now new composite springs which are much lighter and the manufacturers claim they outperform traditional steel springs – the jury is still out on that, though.”

While the argument between air-ride and spring suspensions for highway use is pretty much over, the debate still rages for other applications. Leaf springs are still widely used on single- and tandem-axle straight trucks, and are preferred for many vocational applications, especially if there is off-road work involved.

“If cargo really isn’t an issue, like in refuse or bulk materials, mechanical systems can perform well,” Sauriol says. “Some people even argue that springs provide more stability than air, some say air is more stable. There really is no definitive answer. It depends on what you want to do.

“In the long run, air-ride also requires less maintenance than mechanical systems because they have fewer moving parts and connecting parts. And you will hear people argue that air-ride is easier on tires and alignments, but springs can be just as good if they are maintained properly.”

As for rubber load cushion suspensions, their use is more or less restricted to more severe applications. Riding on solid rubber blocks instead of springs or a cushion of air, load cushion systems provide excellent stability for heavy loads. For that reason, such systems are often a preferred choice for refuse, redi-mix, dump and logging applications, both on- and off-road. The rubber cushions come in a range of hardness ratings that can also be tailored to the specific application.

Randy Young, fleet supervisor for Goldie Mohr, an Ottawa-based company that specializes in hauling heavy equipment and other severe service jobs, uses rubber block suspensions on the company’s five tri-axle dump trucks.

“We had the dump bodies on springs, but we found we were always repairing them,” Young says. “They would freeze in the winter and break, or break when the dump was coming back empty and bouncing around. So we decided to go to the blocks. They are very low maintenance and they are more stable when the truck has to go off-road on a construction site.”

Hendrickson has been manufacturing a rubber load cushion suspension since 1954 and currently markets two different products to the vocational market. Sean Coleman, vocational market manager for the company, says along with maximum loaded stability, customers spec’ing rubber block suspensions are also concerned about weight.

“Rubber is lighter than steel, and rubber load cushion can be comparable (within 50 lb.) with some air systems,” Coleman says. “Many vocational applications are paid by the ton. So even if they pay a little more for a good rubber block suspension, if they can save 300 or 400 lb. for payload, they can pay back that investment in a few months. Plus, the driver has more comfort and confidence on the job site.”

Young admits, however, that he plans to go to air suspensions the next time he spec’s a dump truck. “Air is lighter, and it doesn’t pound the truck apart,” he says.

At the end of the day, says Sauriol, spec’ing a suspension system is a bit of a balancing act.

“Suspension design is a world of compromise, and we try to tell customers that,” he says. “You often have to sacrifice on one thing to gain on another. When you sacrifice weight, you might be sacrificing strength and durability. But then you have to determine how much strength and durability you really need or can afford.” n