How it Works: The Mechanical Speedo

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The Chronometric Speedo

The oldest speedometer, introduced about 1900, was of the centrifugal governor (automatic regulator) type - a scaled-down edition of the governors seen on old steam engines. Later came the chronometric speedometer, based on a constant time period measured against the speed of rotation of a road wheel (i.e., distance travelled), thus obtaining the speed.

The chronometric model was relatively unaffected by vibration and for this reason was favoured on motorbikes until the early 1970's. The drive on these older models was usually by a gear on the inside of the front wheel, meshing with a pinion turning a flexible cable.

The Magnetic Speedometer

Originally patented by a German, Otto Schulze on 7 October 1902, the magnetic speedo used a rotating flexible cable usually driven by gearing linked to the output of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel. The magnetic speedo was in almost universal use until the introduction of electronic versions, mainly because it was a relatively cheap and reliable device. The cable was formed from stranded wire, with square end pieces, driven by worm and pinion from the drive (propeller shaft) end of the gearbox. The cable rotated a circular permanent magnet, producing a rotating magnetic flux that induced eddy currents in a spindle-mounted aluminium disc. A stationary field plate completed the magnetic circuit.

The Foucault Effect

An electric field was set up in the disc by the eddy currents and reacted on the rotating magnetic flux. This resulted in a torque, linear with the speed of rotation of the drive shaft, being set up in the disc (Foucault effect). A pointer, moving over an kph/mph scale, was attached to the disc; a light coil spring returned the pointer to zero. Occasionally, a soft iron rotor, spun within a stationary magnet, produced the field. Variations included a cylinder, concentric with the aluminium disc, marked with a speed scale on its edge, moving past a cursor; or a speed scale tape, sometimes with the addition of coloured segments, driven by a cord and pulleys from the disc.

Calibration could be effected by varying the magnetic flux, either by partial demagnetization or by altering the air gap between the magnet and the return circuit. Calibration marks - very small white dots - could usually be seen on the scale. The instrument was usually set to read slightly high so that the driver did not inadvertently exceed the speed limit. An odometer (distance recorder), comprising a series of small drums bearing the figures zero to nine, was usually located inside the speedometer case. The tenth of a kilometre drum was driven by a small worm and pinion drive from the speedometer drive cable; as this drum completed its turn, a pin on its side engaged a slot in a gear driving the unit drum; as the unit drum passed nine and approached zero, the 'tens' drum was engaged, and so on.

A speedo and odo assembly dating from around 1910
A speedo and odo assembly dating from around 1910. It also features a trip meter, very rare at the time.
arly versions of the 'trip' meter had fewer drums, driven in a similar manner. Because of its cheapness and reliability, the magnetic speedometer remained standard equipment for decades, however this type of speedometer had a limitation imposed by the drive cable, which could only be run straight or in a gentle bend. For this reason, larger commercial vehicles used a remotely operated electric speedometer, usually a moving coil instrument driven by the rectified output of a speed-proportional ac generator. The odometer was operated from the vehicle battery, interrupted by a speed-proportional contractor.

Lubricating the Speedo Cable

A later, electronic type of remotely operated speedometer used a road-speed dependent pulse generator to provide a signal source. Then came the digital voltmeter that would display either the road speed or the average speed of a vehicle at the touch of a switch. The magnetic speedometer itself rarely gave any trouble and normally could be replaced only as a complete unit. The drive cable, however, should be lubricated every 20,000 kms, using an SAE 140 oil or an appropriate grease. Jerky operation of the speedometer pointer is a fairly certain indication that the cable needs lubricating, although a damaged cable may produce the same symptom.

To lubricate, it is necessary to remove the cable complete with tube, which is usually secured by .1 knurled screw cap, bolt or circlip at both the gearbox and the back of the speedometer. The cable can then be slid out of the tube and examined; if there are any broken strands, the cable should be replaced, otherwise it should be lubricated and refitted to the vehicle. If the square cable end does not fully engage in the gearbox housing, rotate the cable slightly, maintaining slight pressure. When the square cable end is engaged, tighten the screw cap or other holding device. If the squared end will not enter the speedometer, push the vehicle in gear to and fro to bring the cable into alignment, then secure the speedometer end.

The speedometer unit itself can be replaced; if no workshop manual is available, examine carefully the fixing method before removing the old unit. It may be necessary, on some vehicles, to remove some other part, or all, of the fascia unit to gain access. Remember that, if a car is modified in such a way (e.g., by changing the differential ratio, or fitting wheels or tyres of different diameter) as to change any of the design features related to the original calibration of the speedometer, the speedometer and odometer will then be inaccurate and should be re-calibrated.
Speedo Schematic
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