Although driven by the same rotating shaft (the cable) the speedo and odo are really separate instruments observing two different characteristics of the shaft motion. The odo 'counts' via a set of gears, the number of rotations of the shaft. The speedo translates the angular velocity or rotation rate of the shaft into an angular displacement of the meter needle. The needle assembly is restrained by a linear spring of known "K" and includes a magnetic disk or iron plate. The shaft assembly never actually comes into contact with the speedo assy, the rotation of the magnet plate induces rotation of the iron plate (Eddies Magnetic inductance stuff). The amount of torque produced is dependent on the rate of rotation or angular velocity of the shaft assembly. This torque is balanced by the retaining spring force producing an angular displacement of the needle. This angular displacement is translated into a reading by the calibrated face of the speedo. Changing the face calibration does not change the angular displacement of the needle. but |
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