Optimum Knob Diameter

Abstract

When frictional resistance is heavy, the optimum knob diameter is about 2 in. The curve relating turning time to knob diameter is roughly U-shaped, and diameters outside the range of 1 3/4 to 2 1/2 in. are significantly inferior to the 2-in.-diameter knob. When frictional resistance is reduced to a moderate level, the curve becomes essentially flat in the region from 3 1/4 in. (the largest diameter tested) to 1 in., but rises steeply and significantly with decreasing diameter at diameters smaller than 1 in. (i.e., 3/4 in. and 1/2 in). There is an indication that if friction were reduced to a very low level, the curve would flatten out still further to include in its flat region all practical diameters smaller than 1 in. Evidence is cited in support of this indication. Evidence is also presented to the effect that the above findings are valid over a wide range of rotary inertias. Reach time was independent of frictional resistance, but increased with decreasing diameter at diameters smaller than 1 1/2 in.