An objective index is developed for the estimation of ride quality during a forklift truck start. After consultation with test drivers,
start-motion quality is defined and three governing parameters selected: engagement shock in the driving direction, vertical
vibration, and pitch vibration. Subjective evaluation ofthe starting motion and objective measurements ofvehicle vibration were
performed on five forklift trucks with various load capacities. The vibration measured at the driver’s seat was changed into a
perceptual amount by using the frequency weighting curves and the fourth power dose method suggested in ISO 2631-1 (Mechanical
Vibration and Shock-evaluation ofHuman Exposure to Whole-body Vibration—Part 1: General Requirements. International
Organization for Standardization). Regression between the perceptual vibrations, expressed as a vibration dose value, and the
subjective rating scores yielded an index equation in the form of Steven’s psychophysical power law: c ¼ 1:912f0:601; where c is
the sensation magnitude ofstart-motion quality and f is the stimulus magnitude ofthe vibration dose value.