It used to be that trailer suspensions were among the simplest moving parts in the business. Trailers themselves were seen as little more than containers on wheels, and a basic steel-spring suspension fitted to the rear would do, perhaps mounted to a sliding subframe to help you distribute the weight better. If the cargo bounced around a bit, well, it was going by truck, for Pete’s sake. What did the customer expect?

Like anything else in life, trailer suspensions aren’t so simple anymore. Sliders are widely used to help balance weight between the trailer and tractor. Weight laws and operational requirements make wider axle spacings and sometimes multiple axles necessary. And shippers from the Yukon to Yucatan are demanding a softer cushion for their freight, while drivers are demanding a smoother ride for their kidneys.

Few fleet managers would disagree that an air suspension could pretty well address all of these needs. Trailer air suspensions typically consist of hangers and trailing arms whose rears are supported by rubber air-springs, or bags, with a shock absorber mounted near each bag to control the rebound (trailer air-ride suspensions closely resemble those on tractors, but without the torque rods). Air lines tie together the bags, and loadings between axles can be equalized by increasing or decreasing the air pressure from the trailer’s compressed air system, which is fed by the tractor. Many systems can be spec’d to mount on a steel-channel subframe that slides, which lets you increase or decrease weight on the axle group by moving the bogey forward or rearward under the trailer’s body. Several manufacturers offer an air suspension integrated with a suspension slider, axles, and brake components – a complete trailer undercarriage ready for installation.

But compared with a lightweight four-spring, the air suspension componentry alone comes with a price in weight (as much as 400 pounds heavier) and up-front cost.

In some corners-household goods, for example-the payoffs of air ride are obvious. Van operators face more of a struggle. Vans are probably the most “cost-sensitive” type of trailer because profits are slim on truckload and less-than-truckload hauls, and the extra weight of the suspension means less payload. Even when more money could be made, most fleet managers spec trailers with an eye toward economy because vans can spend so much time parked at docks and on lots. Flatbeds, too, are prone to sitting. So traditionally vans and flats have been equipped with economical steel-spring suspensions.

Manufacturers have addressed many of these concerns in recent years with the introduction of air-ride suspensions that not only are less expensive, but come to within a few hundred pounds of a steel-spring unit.

Increased volume, greater manufacturing efficiency, and more competition have also caused prices to fall. Moreover, suppliers tout the value of an air-ride suspension at trade-in time, and argue that air-ride can be more trouble-free because air bags are less prone to damage or breaking than leaf springs. And when you consider other factors like tire wear, wear and tear on the trailer, and braking performance, an air suspension looks better all the time.

Indeed, suppliers say air-ride is now ordered on 45% of all trailers, and acceptance continues to grow: it tells you something about the market when Stoughton Trailer drops springs in favor of air-ride as its factory spec on dry freight and refrigerated van trailers.

There are other benefits that don’t translate into direct dollar costs.

The softer ride belies the fact that air-ride can be more stable than leaf-spring suspensions. Air is quickly pumped to the bags to shore them up against higher weight-even if the weight is temporary, as when the trailer is pulled through a corner. That’s why lofty specialty tankers, like those which haul dry bulk commodities, often ride on air. The suspension helps compensate for the vehicles’ high centre of gravity and fights handling instability.

The movement of air among the springs also equalizes the load between the axles and among the wheels. The bags are tied together pneumatically, rather than with any mechanical means. This is why air-ride suspensions have become common on wide-spread axles. Spread tandems, for instance, are sometimes equalized on mechanical suspensions by walking beams. The longer the spread, the longer and heavier the beams. You may as well buy the air suspension and enjoy the ride.

Like any component, you’ll no doubt want an air-ride suspension that will deliver the best combination of performance, reasonable cost, expected maintenance costs, and ease of operation. Here’s a look at some specific makers and models.

HENDRICKSON

Hendrickson Trailer Suspension Systems continues to push the envelope on weight. Its VanTraax HKA 180-the company’s integrated air-ride, slider, axle, and brake system-weighs 1640 pounds, making it the lightest package in its class by about 145 pounds, according to Hendrickson. The HKA 180 is rated to 40,000 pounds in dry-freight and reefer-trailer sliding applications, lighter than the company’s 46,000-pound HKA200 and the 50,000-pound HKA250. Hendrickson says it shaved weight without compromising strength by using new tapered beams, lighter body rails, and a new tri-functional III bushing with a new rubber compound. U-bolts and several bushings have been eliminated.

A new, larger air tank specifically matched to the firm’s K-2 air-ride slider system is designed to increase overall brake performance by accommodating shorter and more balanced length of brake air lines. Two- and three-port single-reservoir tanks and a four-port dual-reservoir tank are available. There are other options, including SureLok, which locks the trailer deck solidly at dock height using stops in the air suspension to prevent floor drop as weight is added; Quik Draw, an air-assist, slider pin-pull mechanism; and Ready to Roll, an air-supply line and fitting that readies the unit for an automatic tire-inflation systems.

HOLLAND HITCH

Known for its fifth wheels and landing gear, Holland Hitch also produces the Holland Binkley AirLite heavy-duty air-ride suspensions. The AirLite is not only designed to save weight, but also to reduce maintenance. The unit’s patented air-beam design is intended to maintain alignment, and it has 26% fewer fasteners that need adjustment among other features designed to cut down on maintenance. AirLite is available with an optional AirLevel system that’s intended to eliminate dock-walk, and Holland’s Quik Release pull handle is standard.

MERITOR AUTOMOTIVE

Meritor Automotive entered the trailer air suspension business in 1997 with the 1785-pound RHP, a highway parallelogram package comprised of ant air suspension, slider, Meritor axles, brakes, automatic slack adjusters, and anti-lock. It’s offered with a 36,000-pound rating for dry vans, reefers, and other sliding tandem trailers with a standard 49-inch axle spacing. Ratings of 40,000 pounds and higher and a full line of widespreads are expected soon.

Meritor’s parallelogram design puts the load-bearing air bags directly over the axles and positions them with upper and lower control arms that are parallel to each other, a design that should eliminate dock walk. In addition, the RHP suspension has an automatic dump valve that exhausts suspension air when spring brakes are set for parking, thus allowing the trailer body and load to lower onto rubber blocks inside the air bags to give a firm trailer floor for loading and unloading. The RHP simplifies axle realignment through the same type of adjustable torque rods used on spring suspensions, and a conventional brake mounting offers easy access to chambers and all system components.

The RHP comes standard with Meritor’s TQ trailer axles, Q-series 16 1/2-by-7-inch cam brakes, and a five-year/500,000-mile warranty. Options include the TP and TB axle series with unitized hub system, 16 1/2-by-7 or -8 Q-Plus brakes, Meritor automatic slack adjusters, and Meritor WABCO ABS.

NEWAY ANCHORLOK

Neway Anchorlok continues to build its NewLite family of lightweight air-suspension/slider packages. Its NewLite NS225 is available in a 45,000-pound capacity rating: that’s 7000 pounds heftier than the company’s NS190 Series, yet the NS225 is comparable in overall weight (bear in mind, the NS190 is 230 pounds lighter than Neway’s previous RLSA model).

Standard on the slider package is Neway’s EZ Align non-welded axle alignment feature which provides axle adjustments from the outboard side of the trailer. No special tools or welding is needed, and the alignment can be performed by one person. EZ Align can also be added on the curb side.

NewLite, which comes fully prepped for Neway’s lightweight EDL dockwalk deterrent system, is available in slider box lengths of 84 and 100 inches, and in widths of either 42 or 48 inches. The slider crossmember spacing allows for two small or one large air-tank mounting. REYCO

Reyco Industries’s integrated slider/air suspension/axle/brake package is the InnovAir system, which comes pre-assembled and only requires plumbing, wheel ends, and rail installation. It features lightweight, seamless tube axles and precision-machined spindles, and a high-tensile steel slider with four-lockpin design which can be configured to body rails from any manufacturer.

Double-convoluted air springs, vertically mounted shocks, and a wide trailing beam centre enhance roll stability, and the suspension geometry is designed to eliminate dock walk. Adjustable torque arms let you align the axles without cutting or welding. Axles are available in 71.5- and 77.5-inch tracks; 49-, 60-, and 72-inch axle spacing; 42- and 48-inch sub-frame widths; and 14- through 17-inch mounting heights. A two-piece spider provides precise brake geometry for longer brake life and proper operation of automatic slack adjusters.

RIDEWELL

Ridewell Suspensions has earned a deserved reputation for toughness in no small part due to the company’s venerable Monopivot 240 Series. Designed for-but not restricted to-lowboys, flatbeds, drop decks, tankers, auto-haulers, and other specialized trailers, the Monopivot is available in 25,000-pound and 30,000-pound capacities (corresponding lift kits are available) and in overslung, underslung, and yoke-mount versions. The preferred mounting method is a welded axle-to-suspension seat; no U-bolts are required. The unique feature of the Monopivot is the clamping of the bushing in the beam hanger pivot. The double-bonded pivot bushing can be installed with common shop tools, no hydraulic presses required, and the eccentric bolt at the left and right-hand pivots simplifies axle alignment by the installer and end-user. Pre-mounted shock absorbers and pre-drilled height control valve bracketry are designed to allow precise installation and proper shock and air spring tolerances. A five-year/500,000-mile warranty is standard.

Incidentally, Ridewell now distributes its suspension products to Canadian customers through a warehouse in Barrie, Ont.

TIMBREN

Ajax, Ont.-based Timbren Industries built its reputation with rubber-ride suspensions-the company’s unique P-Series is probably the lightest independent rubber-ride suspension in its class-and last year introduced its first air-ride suspension for trailers. The TA Series is a tapered-leaf, mono-beam trailing arm suspension available in standard-height and low-mount models. Spring centres of 38 inches on 96-inch units and 44 inches on 102-inch unit promise a wide stance for improved capacity and stability, and the placement of the air spring directly over the axle on the regular-mount suspension means that all road shock is absorbed, the company says. Timbren is working with axle-maker IMT in Ingersoll, Ont., to develop a packaged parallelogram design for a lightweight axle.