SPEEDOMETERS.

Hawera Star, Volume XLVI, 25 June 1927, Page 14

SPEEDOMETERS.

FAiCHLITIES FOR TESTING

There are facilities. for testing several popular makes of speedometers, and tlie tia iver who appreciates the value ox an lamicairaite linstrument shonld satisfy humiself that the recorder is reliable after a year’s use. It is unfortunate that ’little can be done locally with some British speedometers, and there have been cases of failures which have had to be referred to the makers.

Most speedometers are on the optimistic side, but it is not .always wise to rely on this; characteristic in areas where* stop watches are used on the unwary. Thirty miles an hour is tolerated on isome city and suburban roads, and drivers form tlie risky habit of dropping the speedometer to 20 'miles per hour when on a road which is favoured for trapping. If one 'hats a certificate of accuracy for the speedometer, this can be indulged in with .some assurance. As ilonig as speed traps are .set. an accurate speedometer ii.s worth the expense of its poniodiica.l te.sitinig. Errors of four or hive per cent, are common in. new speedometers: fitted, to low-priced cars, and: the 'inaccuracy often letads the owner into a liaise optimism concerning patrol coiv-suniiption.

Some speedometers wo.rk on a centrifugal principal, and here stiffness or partial seizure of some of the working parts may act as a brake, giving an unduly low reading. Some, work by the friction o<f stir against rotating vanes, in which case temperature may cause variations in the reading, while others employ a rotating magnet to drag the painter round. Here the magnets may be slightly too strong or too' weak, while the hair spring which resists the movement of the pointer in this aiul the previous mentioned types may also be either too strong or too weak in action, ■or may even cause a varying eror, so that the speedometer may be correct at some Speeds and inaccurate at others.

Few speedometers possess ,«my means of .regulation comparable to the fast and slow adjustment of a- watch, and it is thus beyond the power ojf the driver to. alter the instrument readings himself after testing over a measured mile with a i.stop-watelli. This is. rather la l weakness in 'speedometers, and the clumsy and often expensive method qr altering the drive ratin' is usually adopted. Most cars are fitted with gear-driven speedometers, nnd an instrument in good working order should give unvarying l esultsVhen standard wheels .and tyres are used. If 'tills gear drive its taken off the rear of the gear-box, 'alteration of the ratio is rather a. complicated process, often lass preferable to the fitting of U' new speedometer. Tjjjte simple front wheel ring tooth drives are changed with, less trouble. The fast disappearing belt and pulley drives are naturally best from the point of view of simplicity. Whether spring or' leather drives are fitted, there is always «. tendency tot ®!iin. Cl'riled spring belts working in grooved pulleys are more satisfactory than flat leather or fabric belts. If a, speedometer driven by a belt dis incorerct through its full range it is merely necessary to change the diameter of one, pulley, either, by replacement or by reducing its diameter in a. lathe. Small errors inlay even be corrected on flat pulleys by binding them with adhesive tape. It- :is not generally known that the 1 fitting of an oversize tyro may cause a 'speedometer to read about three per cent, low, while different makes of nominally the same -size of tyre may affect the instrument as much as two per cent, in either direction.