Early High-speed Locomotive Design
In the 1840s, express passenger locomotive design was focussed on the need for single large-diameter driving wheels of around 8 foot (96"). The wheel diameter is effectively the "gear ratio" of a steam engine. Large driving wheels deliver the high linear tyre speed needed for fast locomotives, whilst keeping the axle bearing and piston speeds low enough to remain within the technology limits of the day.
Later on, increasing engine power would require better adhesion than could be achieved with single driving wheels, but that was not yet a problem at this time.
Along with needing large wheels for speed, stability required a low centre of gravity, and thus a low-slung boiler. The difficulty is that both of these needs are in conflict, requiring the driving axle position to overlap the boiler position.
One solution to this was the Crampton design, where the driving axle was moved behind the boiler's firebox. These engines were relatively long in comparison to their contemporaries and had long rigid frames, sometimes with as many as three carrying axles ahead of the driving axle, for a 6-2-0 wheel arrangement. Cramptons were most popular in France and Germany, but some were also used in England, by companies including the London & North Western Railway (LNWR).
One of these LNWR Cramptons, Liverpool was notably long, with an eight-wheeler (6-2-0) layout and rigid wheelbase of 18' 6". Although fast and capable of working heavy trains for long distances, it damaged the roadbed owing to the long rigid frame.
Read more about this topic: LNWR 2-2-2 3020 Cornwall
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