Camshaft Rocking lever

Stabiliser wheel

Recorder wheel

Integrator wheel

Balance wheel

Rocking spindle

Escapement anchor Single fingers lock wheels until released by cams Pinion driving camshaft through clutch

Pinion driven from mainshaft

'How fast does it go mister?'

'Hang on son I'll just build an unfeasibly complex

The Chronometric speedometer was first built by the Jaeger speedometer company, in Paris, in the 20s. Later that decade, Jaeger Paris set up a division in England, which was subsequently bought out by Smiths Motor Accessories, who made chronos until the late 60s for road bikes and 1974 for police machines. Almost every British bike for 50 years used Smiths clocks

The movement of a chronometric speedometer is an integrating mechanism, using a conventional escapement unit similar to most chronograph mechanisms. So your Smiths speedo is closer In operation to a Rolex than the Nippon Denso on your mate's CB750. At any given time the speedo needle Is either locked in position or moving with the speedo cable. The escape wheel (on the left of the movement) is mounted to a small camshaft and is driven through a spring loaded clutch between it and the driving gear at the base of the shaft. This driving gear converts revolutions of the speedo cable Into motive power to maintain the oscillations of the escapement (the escapement of a watch is that mechanism which operates in a very precise manner in order to release increments of time through the gear train of a watch). In the Smiths speedo the escapement controls the speed of the camshaft and the clutch Is allowed to slip when the driving speed exceeds the camshaft speed.

Next to the camshaft are the three wheels; the integrator (at the bottom), the recorder wheel (centre), and the stabiliser (top) to which the needle Is attached.

Pinion driven from mainshaft instrument

All the wheels can rotate independently on the spindle. The recorder wheel is driven, in one direction only, by two pins on the integrator wheel. The recorder wheel in turn drives the stabiliser and speedo needle (in both directions), through a single pin. The integrator and recorder lv h) P /

wheels both have teeth on the outside and tiny leaf springs underneath. When the leaf springs are lifted, a hair spring causes the wheels to spring back to their starting position.

To the right of the movement is a gear on a shaft who's top bearing is in the end of a rocking lever, which causes the pinion to engage with the teeth on the edge of the Integrator wheel. Three small cams on the main camshaft control the rocking lever and the two leaf springs for the Integrator and recording wheels.


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