When the payload gets weighty or the road gets rough, a truck’s cargo-and, to some extent, the driver’s tailbone-will be thankful that you “spec’d heavy” when you chose a suspension for your trailer.

While some operators might define a heavy-duty trailer suspension as one starting at a rating of 25,000 pounds per axle, and others will say it’s not until you jump to 30,000 pounds, most agree it’s one of the two.

As with power-unit or straight-truck suspensions, the choices come down to air-bag or mechanical designs (primarily leaf-spring or “walking-beam” models in this latter category). Simply put, air springs are squat, durable rubber bags, about eight to 10 inches in diameter, that are inflated by the truck’s compressed-air system and work as cushions within the suspension to provide a smoother ride than is often available from steel-leaf springs.

Leaf springs are of three different basic designs: flat-leaf, taper-leaf, and parabolic-leaf. While each style offers its own shock-absorbing features, they are all built in a stack of several progressively longer leaves (top-to-bottom within the suspension) that are usually formed in a bow or arc, located above and parallel to the axle.

As weight is imposed on this pack, it flattens out and the friction between the moving leaves also contributes to an overall absorption of road impacts that would otherwise be transmitted from the tires and wheels up to the trailer’s chassis and contents.

A walking-beam suspension is designed for tandem axles and joins the two axles on each side of the trailer using either a beam-itself often in two pieces, joined via a pivot-or a leaf-spring pack so the axles remain equidistant from each other, and the load on each wheel-end is similarly equalized.

An air-bag suspension typically offers a smoother ride for the cargo, and the driver up front can even benefit because less shock is transmitted through the trailer nose and fifth wheel into the cab. Some claim that air-ride suspensions add to tire life, and an air system can certainly equalize loaded weight between spread axles better than leaf-spring designs. Air is necessary, too, where lift axles are employed.

For their part, leaf springs-whether of steel or the newer plastic/fiberglass composites-offer lower cost and lighter weight. Historically, a leaf-spring suspension was standard with a trailer manufacturer because of its relatively low cost-but that looks to be changing.

“We’ve standardized on air-ride for the past seven years,” says Duncan Sinclair, sales manager at lowbed manufacturer Paron Trailers, London, Ont. “Back in the Eighties and earlier, I’d say 80% of the lowbeds where I then worked were spec’d with spring suspensions. That’s literally gone to zero here at Paron since the early to mid-Nineties.”

Sinclair also notes that operators of aluminum-frame trailers like the fact that air-ride also smooths out the bouncing and rattling when running empty that, over time, can cause a lot of structural problems with this lighter-weight equipment.

Mind you, air-ride has its limits for a heavy-hauler or on/off-road environment. The max rating is 30,000 pounds, while mechanical suspensions can be designed for 40,000-pound loads and beyond. (Note that many jurisdictions don’t like to see more than 22,000 pounds per axle, so highway ops at higher ratings may require special permits.)

“A spring suspension can take a certain amount of overload and the operator won’t really notice anything unless they really over-do it and a leaf breaks,” Sinclair explains. “With air bags, you can only put so many psi of air into them from your truck’s system. Once they’re at that max, if you put too much weight on them the whole trailer will squat right down as the bags deform.”

At Arne’s Trailers in Winnipeg, sales manager Alex Maciejkow likewise confirms that the vast majority of orders for his company’s flatdecks, lowbeds, and dump trailers are specifying air-ride.

“We’re at least 90% air-ride,” he says. “Our platform-trailer customers mostly haul heavy machinery, and just about everybody wants 30,000 pounds. We order off-the-shelf Hendrickson HT Series suspensions for air-ride, and Reyco leaf-spring units for clients who want a mechanical suspension. It’s pretty straightforward-we don’t have to do any alterations or customizing.” At suspension specialist Hendrickson, director of marketing Gary Ciapetta notes that there are various ways to achieve extra-heavy-duty capabilities on a trailer.

“We’ve had people buy what’s normally a truck suspension, like our walking-beam product, and put it on a trailer,” he says. “And some of the very-heavy-GVW applications are just for use on private roads such as within a steel-plant complex, logging lands, or what have you, so users can create vehicle configurations that don’t have to be ‘street-legal.'”

He adds that there’s a whole other approach to handling heavy weights on trailers instead of (or in addition to) spec’ing a 25,000-pound or higher-rated suspension: you just add more axles. You need to fully understand what jurisdictions you’ll be running in, and what their weight regulations are, per axle.

Ciapetta also points out that, no matter what rating your suspension may have, it’s meaningless unless all the other related aspects of the system-brakes, tires, wheels, axles, etc.-offer an equivalent capacity.

“Remember: the weight that goes on the trailer’s vehicle plate is the lowest common denominator, as it were,” he says.

PLAYERS AND PRODUCTS

A vast number of firms across North America produce trailer suspensions (with or without an integral axle) in numbers ranging from thousands a year to a couple dozen. What follows is a mere sampling of these OEMs, with highlights from their current product line:

Chalmers Suspensions International, Mississauga, Ont. Specialists in rubber-spring suspensions, the company’s trailer-oriented product line includes the tandem-axle 700 Series, single-axle 500 Series, and air-ride 900 Series.

Eveley International Inc., Hamilton, Ont. Eveley’s offerings are highlighted by the air-ride Roadair 2000 Plus, but the firm also produces leaf-spring suspensions.

Hendrickson Trailer Suspension Systems, Canton, Ohio; Brampton, Ont. The HT Series of air-ride suspensions ranges in capacity from 20,000 to 30,000 pounds for primary suspensions, and to 50,000 pounds for sliders. The integrated Intraax air-ride includes the axle, and is currently rated up to 25,000 lbs, while company sources indicate a 30,000-pound version is imminent.

Holland-Neway Muskegon, Mich.; Woodstock, Ont. The RLU-250 series of air-ride suspensions, intended for platform, bottom-dump, and livestock trailers, is rated at 25,000 pounds, and features a two-pin bushing design for improved roll stability and vehicle tracking.

Hutchens Industries, Springfield, Mo. The 900 Series of single-point five-leaf spring suspensions is expressly designed for logging, heavy-haul, and dump operations. Capacities range from 36,000 to 60,000 pounds, while a lightweight three-leaf unit is available in 44,000 and 50,000-pound capacities.

Meritor Automotive, Troy, Mich. The RHP-11 Highway Parallelogram air-suspension system includes integrated axles, brakes, and slider. This suspension is available in capacities from 36,000 pounds to 50,000 pounds

Reyco Granning (owned by Tuthill Transport Technologies), Springfield, Mo., and Grimsby, Ont. In air-ride, the 30,000-pound Model 74AR and 30AR suspensions are offered, while leaf-spring on/off-road models include the 21B (max. 26,000 pounds), Model 74B (30,000 pounds), Model 960 (up to 52,000 pounds), and the Model 770 (36,000 to 62,000 pounds).

Ridewell Corp, Springfield, Mo. The air-ride monopivot RAR-240 series comes in 25,000- and 30,000-pound ratings and offers a weld-on axle seat that requires no U-bolts. On the mechanical side, the RDX-204 independent tandem-axle unit is rated up to 100,000 pounds, and the rigid-walking-beam tandem-axle RWB-206 maxes-out at 150,000 pounds.

Timbren Ride Control Products, Ajax, Ont., Niagara Falls, N.Y. Their Cushion Ride trailer suspension kit includes the firm’s AEON hollow-rubber spring system that supports the trailer when empty and assists any original leaf spring when loaded. These kits come in 10,000-pound and 16,000-pound versions, and these capabilities are added to the basic rating of the vehicle’s originally installed suspension.