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Often, not much consideration is given to the venerable vehicle suspension and its important job of providing good ride, handling and control. This may sound like a simple task, but it is not. Many factors come into play that impact the suspension's proper operation.

Among the suspension's various functions: supports vertical loads - the vehicle's tare (empty) weight and when loaded; transfers loads from vehicle to ground via structural members, axles, wheels and tires; maintains axle and wheel position; provides compliance and isolation from road disturbances via springs and bushings for ride comfort and to maintain tire contact with the ground; and provides adequate stability and cornering stiffness for handling performance and safety. To do all this, the suspension must minimize frame movement and excitation, control vertical movement and lateral stiffness and provide damping to eliminate vibration and oscillation.

Because of the beating suspensions endure, components have a tendency to work themselves loose, tires can wear badly, springs can break, and so forth, causing increased repair costs, a loss of fuel economy and ride complaints from drivers among other things. Much of this can be avoided through proper and thorough suspension inspections and maintenance.

To get some guidance and advice on medium duty truck suspension care, Fleet Maintenance Magazine editor David A. Kolman turned to Hendrickson International, the leading supplier of truck, tractor, bus and recreational vehicle suspensions and springs; trailer suspensions, controls and non-integrated axles; truck and trailer lift axles; and bumpers and trim components.

REAR SUSPENSION

In addition to performing a periodic visual inspection on applicable components per the manufacturer's specification, Hendrickson advises checking for:

• Tire wear : Inspect tires for wear patterns that may indicate suspension damage or misalignment. Replace all worn or damaged parts. Verify proper alignment and correct as necessary.

• Wear and damage: Inspect all parts of the suspension for wear and damage. Look for bent or cracked parts. Replace all worn or damaged parts.

• Fasteners : Look for any loose or damaged fasteners on the entire suspension. Make sure all fasteners are tightened to a torque value within the specified torque range. Use a calibrated torque wrench to check torque in the tightening direction. Replace any worn or damaged fasteners.

• Clamp group: Check torque on clamp group mounting hardware. Refer to the manufacturer's torque specifications.

• Air spring (if equipped): Inspect air springs for chafing or any signs of air spring or component damage. Check the overall condition of the upper and lower air spring brackets for dents, dings, cracks or other damage. Replace all worn or damaged parts.

• Steel leaf spring: Look for cracks. Replace if cracked or broken. Check the front bushing for any wear or deterioration and replace if necessary.

• Crossmember and gussets: Inspect the crossmember and gussets for any signs of loosening or damage. Inspect the bar pin clamp blocks for any signs of looseness or movement. Check all fasteners for proper torque. Replace all worn or damaged parts.

• Frame hanger: Inspect the frame hanger bracket for any signs of loosening or damage. Inspect all fasteners securing the frame hanger bracket to the frame rails, as well as the longitudinal torque rod mounting fasteners. Check all fasteners for proper torque. Replace all worn or damaged parts.

• Height control valve and air lines (if equipped): Check the suspension air system for air leaks. Check all air lines for proper routing and for chafing or pinched areas. Check the height control valve linkage for damage or interference with peripheral components. Replace all worn or damaged parts.

• Shock absorbers: Look for any signs of dents or leakage. Misting is not considered a leak. Misting is very small amounts of shock fluid that evaporate at a high operating temperature through the upper seal of the shock.

• Top pad and rubber axle stop: Look for cracks and/or missing rubber axle stops. The rubber axle stop is exposed to contact forces in extreme jounce conditions. It will be necessary to visually inspect the rubber axle stop for wear at the specified inspection interval.

• Torque box (equipped on Hendrickson HTB suspensions): Inspect the torque box for cracks, damage, torn or shredded rubber or any signs of looseness at the bar pin clamps. Replace all worn or damaged parts. The torque box is a non-serviceable item and must be replaced as an assembly if damaged, or if the bushings are worn out.

• Torque rods: All torque rods must be inspected for looseness, torn or shredded rubber, bushing walk-out and proper fastener torque. If there is metal-to-metal contact in the bushing joint, this is a sign of excessive bushing wear and the bushing needs to be replaced.

FRONT SUSPENSION

Here again, Hendrickson recommends a periodic visual inspection, per the manufacturer's specification, plus checking for the following:

• Tire wear: Inspect tires for wear patterns that may indicate suspension damage or misalignment. The leading causes of tire wear are, in order of importance: tire pressure, toe setting, thrust angle and camber.

• Wear and damage: Inspect all parts of suspension for wear and damage. Look for bent or cracked parts. Replace all worn or damaged parts.

• Fasteners: Look for any loose or damaged fasteners on the entire suspension. Make sure all fasteners are tightened to the specified torque. Use a calibrated torque wrench to check torque in a tightening direction. Correct the torque if necessary. Replace any worn or damaged fasteners.

• Steel leaf spring: Look for cracks. Replace if cracked or broken. Check the front bushing for any wear or deterioration. Replace if necessary.

• Air spring (if equipped): Look for chaffing or any signs of spring or component damage.

• Front and rear spring hangers: Check for excessive movement. Check for cracks or loose mounting hardware and replace if necessary.

• Thrust washers and rear hanger clamp: Look for any signs of excessive wear to the thrust washers and rear hanger clamp.

• Rear spring mount: Check for proper alignment with spring taper and check for proper torque on rear spring mount fasteners.

• Shocks: Look for any signs of dents or leakage. Misting is not considered a leak.

• Clamp group: Check torque on clamp group mounting hardware. Refer to the manufacturer's torque specifications.

• Top and bottom axle wrap liners: Look for any cracking or broken pieces on liners.

• Axle: The axle should be free of any nicks or gouges. Inspect for any cracks or dents.

• Steering pivot points: Check for looseness at all pivot points.

• Operation: All steering components must move freely through the full range of motion from axle stop to axle stop.