Driving Wheel - Diameter

Diameter

Driving wheels are generally larger than leading or trailing wheels. Since a conventional steam locomotive is directly driven, one of the few ways to 'gear' a locomotive for a particular performance goal is to size the driving wheels appropriately. Freight locomotives generally had driving wheels between 40 and 60 inches (1,016 and 1,524 mm) in diameter; dual-purpose locomotives generally between 60 and 70 inches (1,524 and 1,778 mm), and passenger locomotives between 70 and 100 inches (1,778 and 2,540 mm) or so. Some long wheelbase locomotives (four or more coupled axles) were equipped with blind drivers. These were driving wheels without the usual flanges, which allowed them to negotiate tighter curves without binding.

The driving wheels on express passenger locomotives have come down in diameter over the years, e.g. from 8 ft 1 in (2,464 mm) on the GNR Stirling 4-2-2 of 1870 to 6 ft 2 in (1,880 mm) on the SR Merchant Navy Class of 1941. This is because improvements in valve design allowed for higher piston speeds.