A very narrow aisle (VNA) turret truck was brought in with a complaint of "phantom steering." When driving on the ground, the truck tracked perfectly straight. But when the operator raised the mast to 30 feet to put a pallet away, the truck would aggressively drift to the right. The operator had to hold the steering wheel 20 degrees to the left just to keep the truck from scraping the rack.
The shop assumed the electronic steering sensor was glitching at height, or the frame was bent. We checked the sensor data, and it was perfectly linear. We checked the drive tires for conicity, and they were flawless. The problem was purely mechanical, involving the steer axle kingpins.
We jacked the front of the truck up and grabbed the tire at 12 and 6 o'clock, checking for play. There was zero play-the bearings were tight. But when we grabbed the tire at 9 and 3 o'clock and pushed, the tire resisted, then suddenly "snapped" to one side. The bronze kingpin bushings were bone dry and heavily scored, causing severe binding.
This is called "memory steer." The friction in the rusted kingpin bushings was so high that the steering cylinder couldn't center the wheels perfectly. On the ground, the 12,000-pound weight of the truck and battery forced the wheels to align with the direction of travel, overcoming the friction. But at 30 feet in the air, the center of gravity shifts to the tiny footprint of the mast rollers. The truck's chassis is no longer forcing the wheels straight; the steering becomes incredibly sensitive. The binding kingpin would hold the wheel just a fraction of an inch off-center, which at 30 feet in the air, translated into a massive, terrifying drift into the rack. We replaced the kingpin bushings, greased them properly, and the high-reach steering was instantly perfectly centered again.
