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A driving wheel on a steam locomotive. On locomotives with side rods, including most steam and jackshaft locomotives, the driving wheels have weights to balance the weight of the coupling and connecting rods. [10][11] The crescent-shaped balance weight is clearly visible in the picture on the right.
There were four types of disc driving wheels for steam locomotves. Each was made by a different company and each varied slightly from the others in their appearance. The four types were:
Geared steam locomotives such as Shays, Heislers, and Climaxes do not have a standard wheel arrangement classification system. Instead of being classified by wheel arrangement, they are instead classified by their design and their number of trucks.
The three types of wheels: The first thing to recognise is that steam locomotives have three basic types of wheels; Leading Wheels, Driving Wheels and Trailing Wheels. All locomotives have Driving Wheels but not all have leading or trailing wheels.
Wheels at the front of the locomotive [3]: 62 to guide the front driving wheels around curves, and minimise yawing at higher speeds with the attendant risk of derailment. The truck has some side motion and is equalised to the driving wheels (41).
In using the system, the French counted the axles, the Germans also counted the axles but used letters only for the driving axles instead of numbers and the Americans and British counted the number of wheels. This table shows how the classifications work.
The 4-8-8-4 steam locomotive has four leading wheels, two sets of eight driving wheels, and four trailing wheels. They were designed to haul heavy freight in mountainous regions. This locomotive had hinged frames to allow them to negotiate curves.
