I once managed the North American operations for German truck components manufacturer Espar Products, division of J. Eberspaecher. I remember one warranty claim in particular—from a public fleet customer, in which I had determined the product was being destroyed by road shock and vibration when operating off-road.
To find out why, the engineering director came over from Germany. We took him to a heavy truck dealership, where the dealer was preparing a chassis to mount a dump body. The chassis had a Hendrickson walking beam suspension with rubber block springs.
“Where are the springs?” the engineering director asked. We pointed and described the rubber block to him. “Is everything in America so heavy duty?”
It turned out our product wasn't designed with the suspension to resist such forces.
THE IMPORTANCE OF SUSPENSIONS
A suspension's primary purpose is to provide ride quality. It isolates the unevenness of the road (or off-road) surface from the truck's frame. Suspensions absorb shock and isolate vibration so drivers and machinery aren't bounced around and are protected from damage and injury.
Suspensions also keep tires on the road, instead of letting them bounce off the surface—which can cause drivers to lose control of trucks. Tires transfer the forces of acceleration, braking, and turning from the truck to the road. But to do it all, they have to be kept on the ground and in contact with the road under roughly the same load at all times.
Suspensions equalize the gross vehicle weight among all axles. If an axle and its wheels and tires are overloaded, the tires wear prematurely and are prone to blowouts. Axles can break. And without sufficient load on the tires, traction is reduced and the tires can't function properly.
Finally, suspensions determine roll stability. If the roll stability factor is too low, a truck will tip easily and steering and handling may be adversely affected. If roll stability is too high, the ride is rough and road shock transfers to the chassis.
Manufacturers have addressed the roles a suspension plays by offering three types of spring media: rubber blocks, leaf springs, and compressed air.
Rubber blocks, either in compression or shear, are rugged. They operate best when fully loaded. When running empty, though, tires bounce off the pavement for a rough ride.