The Effects of the Lateral Instability of High Center of Gravity Freight Cars

Abstract

High center of gravity freight cars experience extreme weight shift from side to side as a result of lateral resonance on track with cross-level differences from alternately staggered joints, as well as soft or other local variations in either rail. Dynamic measurements from tests made on test track with controlled 3/4-in. cross-level difference changes illustrate the force and motion magnitudes resulting from resonant and near resonant operating speeds; side bearing loads of 138,000 lb and spring group loads of 100,000 lb, accompanied by center plates separating and wheels lifting. The rotational energy input to the car body can be approximated for a given motion cycle and is proportional to the product of the amplitudes of the track profile and the car body motion. The high lateral (horizontal) forces on the truck at the side bearing and center plate make the truck unstable and cause wheels to lift off the rail on one side. This lateral force at a given end of the car is proportional to the corresponding vertical side bearing load. Freight cars traveling at resonant speed are especially prone to derail on curved track under high wheel-rail friction conditions. Forces and motion generated between the car body, truck, and the track, cause high cyclical stresses and severe wear between components that can shorten equipment life and cause severe track maintenance problems.