If you've ever driven a go-kart, you can appreciate the luxury of modern truck suspensions. In a go-kart, the driver feels every bump, every tar strip, and every pebble on the track through the seat, which is the only thing cushioning shock and vibration from the road. In today's professional vehicles, comfort is critical, if only for the sake of employee retention.
Early vehicles were built without suspensions. Two inventions–tempered steel and the multi leaf spring–allowed the development of suspensions. They cushioned bumps, protecting cargo and drivers. But they were comfortable only when heavily loaded. With light or no loads, the ride was harsh.
Today, almost all new over-the-road tractors and many straight trucks have air suspensions. Air under pressure is a marvelous spring medium. It compresses, rebounds, and can adjust perfectly to any load condition. It provides a comfortable ride and protects loads and equipment better than steel, no matter what the load.
Air suspensions are optional on most ready-mix trucks, pumps, and many vocational trucks. Besides proprietary suspensions, Hendrickson, Reyco Granning Suspensions, Ridewell Suspensions, SAF Holland, and Watson & Chalin Manufacturing offer air suspensions to truck makers.
These suspensions are remarkably rugged, but they still need regular maintenance. The American Trucking Association's Technology and Maintenance Council (TMC) has developed recommended practice RP 643, Air Ride Suspension Maintenance Guidelines, which describes the components of modern air suspensions and suggests proper maintenance procedures, including the following topics.
Shock absorbers limit the jounce/rebound cycle and prevent axle extension travel from pulling the air bag assembly apart. They force oil through a small opening, then slow its return. The oil heats as it moves, converting kinetic energy to heat.
Shocks should be changed regularly. TMC recommends changing them when tires are replaced.
During regular maintenance, look at the shocks. If a dust tube is dented or cracked, replace the shock. Check bushings. Look for loose hardware. Tighten loose fasteners, but don't over-squeeze bushings. Check for oil leaks from worn shaft seals. Don't confuse misting for leaking. Leaks drip down the tube. With misting, heated oil vapor escapes past the seal and forms a light, even coating on the tube.
Regularly check shock absorber heat by touching a chassis member to gauge ambient temperature. Touch the working part of each shock, not the dust cover. It should be warm, but not overly hot. All shocks should be near the same temperature, and noticeably warmer than the reference point. If one is cool, it isn't working. Undo one end and compress. If it moves too easily, replace it.
Suspect worn shocks if you see tire cupping. Off-balance tires will loosen components and deteriorate ride quality through excessive vibration.