Taking the Suspense out of Suspension Selection

Keith Gelinas

When it comes to maximizing a refuse vehicle's effective life, few specification decisions are more important than selecting the proper suspension. Yet this most critical component remains one of the industry's most misunderstood. So much so, in fact, that many refuse fleet managers rarely bother to participate in the suspension decision. That can be a costly mistake.

The suspension does more than simply cushion bumps; it plays a major role in a vehicle's handling and control, both on the road and off. It also impacts driver comfort, payload capacities, truck stability in severe service conditions, vehicle traction on uneven terrain and the maintenance required to keep the truck running smoothly, regardless of conditions.

Refuse fleet managers should acquaint themselves with specific performance attributes when evaluating suspensions. The following guide will help operators match their performance preferences with the suspension that best meets those needs.

Ride Quality, Loaded: Ride quality, under both loaded and unloaded conditions, is especially important for refuse operations, where comfort often is directly related to driver retention. Ride quality is affected by three factors: * vehicle weight, which includes the truck's weight and payload;

* spring rate, which is determined by the amount a spring will deflect or the amount of force required to deflect it one inch; and

* damping, usually by shock absorbers, affects ride quality by minimizing bounce as the vehicle hits bumps, potholes and other irregularities.

When loaded, the increased weight resists the truck's vertical movement as it passes over bumps. Because good suspension springs are designed for optimal performance at rated load, loaded ride quality can be dramatically better than an unloaded ride.

That same spring system should be designed to deflect and stiffen as payload is added. The springs also should provide additional deflection when fully loaded to absorb bumps. This not only benefits ride quality, but it also helps keep tires in contact with the ground for improved traction and handling.

Ride Quality, Unloaded: Even though refuse trucks generally have large, heavy bodies, vehicle weight still changes between empty and loaded conditions. That change can affect ride because unloaded ride quality primarily is a function of spring deflection and damping.

Heavy-duty truck suspension springs typically are very stiff (high spring rate) to provide a good vehicle ride while carrying heavy payloads. The combination of low vehicle weight and high spring rate can produce harsher ride characteristics when the truck is running empty or lightly loaded.

Empty loads also may adversely affect traction and handling. For instance, drive tires are more likely to lose contact with the ground as the truck encounters bumps. With some suspensions, this may cause the axles to bounce up and down. This phenomenon, known as "axle hop," can be minimized with shock absorbers.

Durability: Another key factor in suspension selection is durability. For refuse applications, a suspension should be highly durable. A good suspension manufacturer will rely on a variety of advanced design tools, such as computerized modeling and finite element analysis, to identify and fortify critical areas of the suspension to ensure durability.

Articulation: A refuse vehicle's mobility is, in many ways, determined by its articulation. Articulation provides traction and maneuverability, which are especially important for traveling over obstacles.

While suspension articulation is an important consideration, axle travel and vehicle mobility over rough terrain are almost always limited by vehicle design, such as the position of axle stops. In addition, fuel tanks, air tanks and other equipment can reduce ground clearance. For acceptable traction, the suspension must articulate to the extent allowed by vehicle design. Any suspension articulation beyond that is of no value.

Stability, On-Highway: On-highway stability is a measure of how well the suspension resists vehicle rolling or swaying at normal road speeds and an important consideration for refuse operations due to the large number of quick stops and tight curves that are encountered.

Suspensions with high spring rates when loaded and wide spring centers generally deliver good stability. Another important factor for on-highway stability is axle parallelism. When traveling over rutted or uneven road surfaces - on highway or off - axles can be pulled out of parallel, causing the truck to pull or wander. Your suspension system should be capable of maintaining the parallel relationship between drive axles.

Stability, Off-Highway: Off-highway stability is a measure of how well a suspension resists tipping in low speed operations, particularly while loading and unloading. This is especially important in high center of gravity (CG) conditions such as while loading debris (especially front-end loaders) or negotiating sloped, unstable surfaces.

Suspension stiffness - spring rate and spring center - is the principal factor affecting off-highway stability. Consequently, it is best to specify suspensions with high spring rates when loaded and wide spring centers to bring about the greatest resistance to swaying and tipping during high CG operation.

Maintenance: Most suspensions have a number of wear points, such as greasable pivot points, wear pads and various bushings that require periodic lubrication or replacement. Some suspensions also require retorquing or replacement of axle connection hardware. Still others may be equipped with shock absorbers that might need to be replaced according to the manufacturer's recommendations. Keep in mind, however, that more "low-maintenance" suspensions are entering the marketplace. They are a solid investment, requiring less work and money over time.

Weight: Payload capacity should be a top priority, and the suspension weight can play a significant role. In the past, there always has been a strong correlation between strength and weight. The reasoning was that "a heavy suspension is a strong suspension." However, this is not true today; systems are available that weigh less, yet maintain a high strength-to-weight ratio.

Do: * Create a phone list of the manufacturer's of each component on the vehicle.

* Understand each warranty's terms and conditions; some warranties may expire before others. Some also require lube intervals, approved lubricants and recommended maintenance practices.

* Take advantage of the point-of-sale interview by asking the dealer/ salesperson about the warranties when spec'ing the vehicle.

* Keep an updated warranty tracking sheet that lists each component manufacturer's warranty terms.

* Understand how each manufacturer's warranty works. Some have different filing and reimbursement procedures.

* Keep warranties valid by performing original equipment manufacturer (OEM) and component supplier recommended scheduled maintenance.

Don't: * Spec a component on price. Rather, spec the component that best suits the application and will result in the lowest life cycle cost.

* Expect warranties to cover all components that fail.

* Wait until there is a problem with a component.

* Assume the warranty will be covered anywhere you take the vehicle for service. Rather, take the vehicle to its nameplate dealer during the OEM warranty period.