* Clearance Price - Final Sale - Limited Quantities Available * This principle is behind the operation of many everyday items, such as the "rack and pinion" system that steers a car. This classic wooden model offers children a basic concept of the laws of physics. Desktop sized with a length of 39 cm (15-3/8") and a gear diameter of 12 cm (4-3/4") outside diameter (11cm (4-1/8") inside grooves of teeth).