By Paul Abelson
If you ask drivers what they want in their trucks, most will place a smooth, comfortable ride right up there with more power.
That's what makes a truck's suspension system so important. Suspensions isolate the cab and cargo from road irregularities. They absorb energy from bumps and jolts and maintain vehicle stability by locating unsprung components (wheels, tires brakes and axles) in proper geometric relation to the sprung or suspended components (chassis, cab, trailer or body and cargo). They affect stability in turns and crosswinds.
Spring suspensions use tempered steel leaves to resist lateral motion. Air suspensions don't have that same stiffness, so they require steel components for stability. Air, in bags or tires, is compressible. Ride height valves allow air to escape if it is under too much pressure, and allow more air to enter if there isn't enough to support the truck at the proper height relative to the axle.
Torque rods help maintain proper axle position so driveline angles are kept within design tolerances. Excessive driveshaft and universal joint angles will create vibrations that, at best, result in a harsh ride. At worst, they break drivetrain components. Because air springs control only the vertical forces, lateral forces (cornering forces, off-camber situations and crosswinds) must be managed by crossbeams, stiff support beams or compliant bushings.
Like any springs, air bags tend to compress, then rebound past their neutral point. They start a cycle, from jounce to rebound to jounce to rebound, which absorbs the bumps in the road but doesn't dissipate it. Without some device to damp that oscillating motion, drivers would begin to experience symptoms of sea sickness before traveling ten miles.
Multiple leaf springs naturally damp the motion through the interleaf friction. Tapered mono-leaf spring suspensions, usually found on steer axles, coil springs and all air suspensions require external devices to damp the oscillations. Those devices are what we know as shock absorbers, although the shock is actually absorbed by the springs. The British call shock absorbers "dampers," which is a more descriptive and apt term.
Shocks damp movement by converting mechanical energy to heat. Oil, an incompressible liquid, is forced through small valves by mechanical action inside a closed tube. One end of the shock is attached to the frame, the other to a moving part of the suspension. A rod pushes a piston, forcing oil through the valves to overcome the oil's resistance. Friction heats the oil, which heats the metal parts. They dissipate heat to the air. That allows us to quickly and easily check shock operation without tearing shocks apart.
Inspection and maintenance are keys to suspension performance. Inspection should start with shock absorbers. According to the Technology and Maintenance Council (TMC) Recommended Practice Guideline RP643, "Air-Ride Suspension Maintenance Guidelines," shocks typically have an effective life of 150,000 miles in on-highway use. They can, however, fail sooner.
You can determine that they are operating properly by visual inspection and by checking temperature. Oil misting is acceptable and common, but streaks of oil indicate a failed seal where the push rod enters the shock tube. Oil-streaked shocks should be replaced.
You can check temperature by touching a reference surface such as a portion of the frame near the suspension soon after the truck stops following a long ride. Then touch each shock tube at the narrower portion, not the dust cover. The shocks should all be noticeably warmer than the reference surface, and all should be about the same temperature. Any shock that is cool is suspect. An infrared non-contact thermometer of the type pioneered by Raytek will make the job easier and more accurate.
The suspension should be checked at every PM interval, about 15,000 to 25,000 miles. Check all fittings and fasteners for tightness. Re-torque as needed. Check bushing condition. Elastomer (rubber or urethane) bushings are maintenance-free but should be checked for cracks and compression ability. Replace as needed. Bushings are not meant to absorb shocks, but to allow temporary misalignment and return of linkages.
Maintenance procedures should include lubrication of all grease fittings. Visually check all air bags. Look for cracks, bulges, chaffing, gouges or distortion. Listen for leaks. If any are suspected, brush on a soap and water solution and look for telltale bubbles. Check suspension hardware for bent torque rods and links, damaged hoses and connection. Check for top plate damage, bag separation or broken valves. These may be symptoms of over extension due to previous replacement with bags too short for the application. Check bags for loose internal bumpers or holes in bag walls, signs of misalignment.
Cabs are also suspended from the chassis, also using air bags and shock absorbers. Coupled with air ride seats, the three levels of air suspensions, seat, cab and chassis, provide comfort with a stability undreamed of just a short time ago. Ride quality can be kept for the life of the truck by practicing good maintenance, preventing problems before they occur or solving them as soon as they appear.
