Why put steam engine speedometer on driving wheels?

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Peter Fox

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I was looking at a picture of Tornado and it has the speedometer connected to the rear, left, driver. The complication is that there's an arm required from the crank pin back to the axle centre. In addition, this can't help access to connecting rod bearings. Why not use the pony truck right under the cab floor?
 
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ainsworth74

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I was looking at a picture of Tornado and it has the speedometer connected to the rear, left, driver. The complication is that there's an arm required from the crank pin back to the axle centre. In addition, this can't help access to connecting rod bearings. Why not use the pony truck right under the cab floor?

If it's attached to a driving wheel it will give a clear indication of wheelslip?
 

pdeaves

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Possibly standard type of design; not all locos have non-driving wheels. Possibly also longevity; attaching to a bigger wheel means slower workings inside the 'gubbins' for any given speed.
 

jamieP

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I was looking at a picture of Tornado and it has the speedometer connected to the rear, left, driver. The complication is that there's an arm required from the crank pin back to the axle centre. In addition, this can't help access to connecting rod bearings. Why not use the pony truck right under the cab floor?

Theres a lot of pipe work etc under the cab making the fitment of a speed there very complicated.
 

edwin_m

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Perhaps because the pony truck moves around laterally on curves so the speedo cable would flex around each time?
 

hexagon789

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I was looking at a picture of Tornado and it has the speedometer connected to the rear, left, driver. The complication is that there's an arm required from the crank pin back to the axle centre. In addition, this can't help access to connecting rod bearings. Why not use the pony truck right under the cab floor?
The rear driver was the norm for steam locos, if attached to a rotating bogie a torque arm would be required to account for the lateral play. BR did experimentally fit one loco with one on the leading bogie in addition to the usual rear driver, this extra cable was to provide more accurate indication at low speed.

It didn't work and afaik BR went back to tradition.

Perhaps because the pony truck moves around laterally on curves so the speedo cable would flex around each time?
Pretty much.
 
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