Striking a Balance

In many cases, the push toward air-ride suspensions on semi-trailers has been driven by shippers tired of opening the doors to find products snug on their pallets but damaged because of a poor ride. Nowadays if you want to haul delicate freight — from computers to potato chips — there’s hardly a choice. Spec air.

But the benefits of air-ride come at a cost compared with traditional steel springs, and not only in money and weight. The price of a smoother ride is vigilance. For many fleet managers, early experiences with air-ride were memorable for all the wrong reasons, like leaking height-control valves and punctured springs. Newer, tougher parts helped reduce these problems, and today a ruptured air bag probably has more to do with the way it’s been used and maintained than how it was made.

BITS AND PIECES

A typical air-ride suspension consists of hangers, trailing arms, and, at one end of each trailing arm, a rubber air spring (also called a “bag”). A shock absorber mounted near each bag helps control rebound. The bags (four on a tandem) are tied together pneumatically, rather than with mechanical walking beams. Compressed air moves rapidly and automatically through the lines to equalize the load. 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.

Manufacturers offer an array of options with their suspension. Several integrate a slider, axles, and brake components — a complete trailer undercarriage ready for installation. Some sell pneumatic controls for the slider pins, and gauges that use the pressure in the bags to measure the load on the axle — a great tool for ensuring you’re not running illegal weight. Special air systems with long air bags can be used to adjust the trailer to different dock heights, an important feature for drop vans designed to handle light, high-cube cargo.

AIR CARE

The lifeblood of any air-ride suspension is air, after all. John Lewis, vice-president of fleet and safety for SLH Transport in Kingston, Ont., one of the first major dry-van carriers to switch from steel springs to air, says you can reduce your suspension maintenance costs if you take time to remind drivers and technicians to keep the air system clean and dry. Valve components are vulnerable to moisture and other types of contamination. If a bag fails (or freezes up), don’t be too quick to blame the suspension. Ask yourself when the air tank was drained last.

Bill Spence, a consultant and former maintenance manager at Ryder Canada, agrees the integrity of the air system is a major issue.

“Dirt is the death of the valve. Use a primary filter to keep the system clean and dry,” says Spence, noting that a filter will also help protect the antilock braking system.

Sometimes air-related issues involve air quantity, not quality. Consider the phenomenon of “dock walk.” This is the tendency of a detached trailer to move forward slightly when a loaded forklift bangs aboard.

This happens on some suspensions because with no air supply, air bags drain and the suspension squats under the sudden weight. Even though brakes are applied, the axles and tires rotate slightly and pull the trailer away from the dock.

A variation of dock walk is “gravel walk.” Loaded flatbed trailers parked alone on dirt or gravel have been known to move forward slightly, digging their landing gear pads into the ground and collapsing them. This, too, results from bags deflating and squatting and axles and tires rotating slightly.

Some suspensions have devices to prevent dock walk. These are built inside or outside the bags to keep the bags from compressing as they deflate. Sometimes, though, these devices will not disengage as the trailer is loaded; the result is a solid-mount suspension that completely negates the air-ride’s ride.

Another source of trouble is uneven pavement at the base of the dock, says Lewis. “The suspension is sensitive to uneven surfaces,” he says. “The bogey gets twisted in such a way, you might have a situation where the block will be hitting on one side and not engaging.” Regardless, check to see how any air-ride suspension you may consider will disengage its anti-dockwalk device.

Some suspensions will not move away from a dock but will squat down as their bags deflate. This can make loading awkward. Of course, squatting, dock walk, and gravel walk do not happen if the tractor stays attached. Its brakes and air system keep the trailer from moving and its bags aired up.

It’s true an air-ride suspension is more expensive than one with leaf springs, and heavier — although newer systems on the market have dramatically reduced the overall weight. But the long-term benefits more than outweigh the initial costs. Besides, if you’re a company looking to attract a wide variety of customers, air-ride is important.

Also, don’t forget that when considering resale value, it’s always good to go with what’s popular.

“You have to look at the overall picture not just the purchase price and what it costs to maintain,” Lewis says. “How many more customers did you get? How many drivers have you retained? How much revenue is attributed to that? When you look at the whole picture, I don’t think it’s more expensive at all.”

AIR WHERE?

While the trend to spec air-ride on dry vans in growing, manufacturers have successfully produced and marketed air-ride for severe-service applications for a long time. There are situations when air is needed in combination with springs on heavy loads for lift axles and sliding axles to conform with local weight laws.

However, trailers that go into rough terrain still tend to have heavy-duty leaf springs. These include end dumps that haul rock and rubble to landfills, and those venturing into deep construction pits.

One reason these truckers stick with springs is that with a huge mass, the payload will ride smoothly even when the tires hit a bump because the springs will flex. There are still many who feel the softness of the air-ride can also be counterproductive in vocations like off-road logging with no weight limits or with high loads. With the lack of roll stability, the cushiness of air-ride can squish down on one side of the suspension and lift on the other. If there’s instability sideways on the suspension the whole trailer can tip over.

“What you really have to watch is when you’re hauling something like steel or logs with a diminishing load and you’re pulling half the load off from one side or the other,” says Bill Spence, former maintenance manager with Ryder Canada, and now a maintenance consultant. “That will cause the trailer to dip and roll stability becomes a concern. There, you have to go with springs.”