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File: 1510807436712.jpg (588.61 KB, 2048x1536, so17-1765_13_of_62.jpg)


What's the rationale behind the ratio between the carrying wheels (leading wheel or trailing wheel) and the driving wheels? Like, Pacifics seem to have small leading wheels and relatively big driving wheels, while on this SO17, the leading wheels and the driving wheels are closer in size.

Are the smaller driving wheels because of more curved trackage, poorer quality track, or what?


All other things being equal, smaller diameter drive wheels give more tractive force, but they have to spin at a higher rpm to maintain the same linear speed. Being that there is a maximum safe RPM for any reciprocating engine, this equates to a lesser maximum speed.

Inversely, larger drive wheels give, all other things being equal, lesser tractive effort, but greater potential maximum speed.


File: 1510958256309.jpg (776.7 KB, 1312x984, 163216.jpg)

Well I think the speed-traction characteristics mostly depend on the cylinder stroke in regard to the wheel diameter and not only the wheel diameter. This is effectively a gearbox for the steam loco. But yes, usually freight machines have smaller wheels than the passenger ones and the SO17 is definitely a freight loco. I'm not actually a steam expert.
Heres a living breathing SO18 for ya.

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