A heavy-capacity cushion-tire forklift was experiencing violent "death wobble." At low speeds, the steer tires would suddenly dart left and right violently, shaking the entire truck and nearly throwing the operator from the seat. The shop replaced the hydraulic orbital steering valve, assuming the spool was oscillating. It didn't fix the wobble.
We looked at the mechanical steer axle. The orbital steering valve is a closed-center system; it only moves oil when the wheel is turned. The wobble was a mechanical harmonic, not a hydraulic one. We jacked the front of the truck up and grabbed the top and bottom of the steer tire. We rocked it back and forth. There was massive, visible play in the kingpin.
The bronze kingpin bushings were completely worn out, allowing the spindle to tilt and pivot on the kingpin shaft. As the forklift drove, the slightest bump would cause the steer tire to deflect inward at the bottom and outward at the top. This deflection pushed the tie rod, which pushed the steering cylinder, which forced the orbital valve to react, creating a hydraulic feedback loop that amplified the wobble until the truck was shaking itself to pieces. We pressed out the old bushings, installed new bimetallic kingpin bushings, and reamed them to spec. The steer axle was tight, the feedback loop was broken, and the wobble vanished. Never blame the hydraulics for a shimmy until you've pry-barred the kingpins.
