Walking Beam Suspensions

Those of you who have been around the truck maintenance business for a long time have seen plenty of walking beam suspensions. These were the first popular tandem-axle designs. The suspension incorporates two equalizing or walking beams, one on each side of the truck, that pivot at the center point onbushings, usually made of rubber. The walking beams are connected by a crossover tube and each beam is supported by a leaf-spring pack or rubber load cushions to absorb road shock and rovide stability. The design distributes weight evenly over the two rear axles and provides the articulation needed to keep all the wheels on the ground over uneven terrain. Hendrickson Truck Suspension Systems' RT series, with its characteristic leaf-spring pack above the walking beam, was the industry standard on vocational trucks and tractors for more than 70 years. The design has been updated and remains a solid performer, both for on and off road work applications.

Spring packs are mounted on saddle assemblies and are connected to spring hangers at the front ends with spring eye pins. The rear ends of the springs have no rigid attachment and move forward and backward as the spring deflection changes. Periodically check all leaves and spring clips for cracks and other damage. Also, inspect each frame hanger. Replace any that have developed cracks or have severe rust. Hendrickson recommends that high mileage springs with one or more leaves broken below the No. 2 leaf be replaced with a new spring assembly of the same part number. Replacing both springs will assure ven spring deflection. Main and wrapperleaves are available as service replacements. The part number is stamped on the spring clips. An extended-leaf version of the walking beam suspension, with longer top leaves and an additional rear spring hanger, provides two stage spring action for a smooth ride and good stability with a full load, partial load or when running empty. When empty, the extended leaves contact outboard spring pads to lower the deflection rate of the springs for asoft ride. With a load, the springs contact the main pads and provide a higher deflection rate. On Hendrickson RS series suspen- sions, four rubber load cushions are used in place of leaf springs. The cushions, available in several durometer hardness ratings for a range f heavy-duty applications, should be inspected for cuts and swelling at least once a year. Cushions cut by the retaining lips of the saddles or frame hangers might not be of the correct hardness or configuration for the truck's application or may require rebound control. A rebound ontrol stop kit is available from Hendrickson. Cut or damaged cushions should be replaced, says the manufacturer. Lateral support is provided on Hendrickson vocational suspensions by transverse rods, which transmit lateral loads directly to the frame. This keeps the axle centered laterally and controls axle walkout during cornering.

Longitudinal torque rods help maintain optimum driveline angles, restrain the rotation of the axle housing while braking and acceler- ating and absorb leaning and cornering forces. Torque rods should be inspected routinely for looseness and for excessively worn bushings. Hendrickson's HN series vocational suspensions combine an equal- izing beam withtheVariRate Spring System, which features diagonally- mounted rubber bolster springs, a Hytrel auxiliary spring and shock absorbers. In the empty or lightly loaded condition, the rubber springs act in compression and shear for a smooth ride. As the load increases, the springs compress and stiffen for more stability without affecting ride,says Hendrickson. When fully loaded, the auxiliary springs make contact to provide more stability. Integral shock absorbers are designed to eliminate tandem axle hop and smooth the ride further.

4 Spring Suspensions

Four-spring suspensions, which have been standard on many on- and off-road trucks over the years, incorporate a multi-leaf steel spring pack at each wheel and give good service and excellent articulation. Multi-leaf suspensions incorporate long spring packs that serve as equalizing beams and work like a walking beam suspension to distribute the load to all of the wheels. But in recent years no single suspension type has grown in popularity as quickly as air ride.

Air Suspensions

Air suspensions have become the norm for road tractors. Inflatable, rubber cushions absorb road shock as much as 50% more effectively than multi-leaf springs. According to the Firestone Industrial Products Company, its air spring products are found on more than half of the vehicles equipped with air suspensions on the road today. Truck manufacturers offer proprietary air suspensions as well as those supplied by Hendrickson, Neway, Reyco and others. Neway claims to have the heaviest capacity rating up to 52,000 Ibs. with its AD 252 Series for on- and off-road applications. Reyco claims to have the lightest tractor air ride suspension on the market with its 240AR model. The design utilizes a unique spring beam that reduces the number of components and features two-piece torque arm bushings for easy maintenance, says the company.

Air Springs

An air suspension typically uses two air springs or air bags per axle. Bags are available in single, double and triple convoluted types and reversible sleeve models for on- and off-road applications, says Firestone, which also supplies aftermarket parts. Since air bags are prone to damage, periodic inspec- tion to detect tears, holes, abrasion and other damage is good policy. Misalignment can result in over-extension of the bags, which causes them to pull away from the metal top plate or from the piston at the lower mount. The air bags are secured to the frame rails with mounting brackets ahead of the axles, providing a trailing characteristic that keeps the suspension aligned and stable. Each bag supports one end of a trailing arm;the other end is attached to the frame. The suspension is designed to move, enabling the axles to turn around the trailing arms' center of rotation.
Air suspensions include a height- control system to respond to sudden changes in ride height and shock absorbers to issipate axle rebounds and add stabihty. Track bars and torque rods with rubber bushings restrict the lateral movement of the axles and maintain pinion angles for reduced spline and U-joint wear. Transverse rods add roll stability and reduce axle stress. Worn bushings in these rods can cause excessive roll and sway and lead to premature air spring failure.

Shock absorbers are an important part of any suspension system and must be in good working rder to keep tire wear to a minimum. Feel the shocks after the truck has been operated to tell if they are working proper- ly; they should feel warm. If a shock feels cold to the touch, disconnect it at the bottom bracket and work it up and down to check the resistance and to determine if it should be replaced. Look for oil leakage, too. All shocks will "mist" and a film of oil will be seen below the skirt. Real oil leaks are tell tale signs that shocks need to be replaced. Some service specialists recommend replacing a set of shocks after 250,000 miles.