Speedometer Errors

Timaru Herald, Volume CXXXX, Issue 20165, 20 July 1935, Page 10

Speedometer Errors

CHECKING CARS IN FRONT

The reappearance of traffic control in Britain by means of speed limitation has brought in its train a wealth of inquiry into the various factors affecting methods by which speeds are to be determined. Much agitation was directed to the possibility of “traps” again being operated, but this was not the system mainly adopted; the system mainly adopted is that of dogging, and the dogging is being done by camouflaged police, that is policemen in cars that to all appearances are just ordinary road users going about dinary business of the day. Trapping by such a method necessarily entails something as near perfection in the instrument used as possible—strictly it entails absolute accuracy. There are two sides to the question, however. If a motorist is determined to rely on his own speedometer as his check on speed he must be as certain as any policeman that he is securing a correct reading. Errors, it says, come under three headings:—l. Errors of the instrument 2. Errors arising from the vehicle to which it is fitted. 3. Errors of observation.

The accuracy or inaccuracy of the instrument itself, and of its driving gear, do not vary materially and reveal themselves if the instrument is periodically checked over a measured distance. This method is not infallible, however. The main error due to the vehicle to which the speedometer is fitted arises from variations, themselves due to different causes, of the effective diameter of the back wheels of the car. All speedometers give, in effect, the reading of the revolutions per minute (converted into miles per hour) of the back wheels of the car. A variation of the diameter of the road wheels will, therefore, affect the reading given by the instrument, reduction in diameter erroneously increasing the apparent miles per hour. Consequently, during a periodical check made by timing the car over a measured distance, if the tyres of the ear are wellinflated and subsequently, either by adding further passengers to the load, loss of air, or even by reduction in temperature, the effective diamete* «! j

the wheel is reduced, the speed record will be in excess of the real speed. The reasonable possible error due to variation in wheel diameter through these causes is in the region of 2 per cent. When considering the possible causes of altered effective wheel diameter, the possibility of a wheel having been changed owing to a puncture or other cause must not be ignored, since the error due to the difference between a new tyre and an old one, of the same make and nominal size, may also be in the region of 2 per cent.

The errors of observation may be caused by :(a) Inaccurate reading of the instrument, due to parallax. This is the error due to the eye of the observer, the needle of the speedometer, and the essential mark on the dial not being in one straight line. Both the observer and the driver must be able to read the instrument accurately; the one to observe the speed recorded and the other to drive at set speeds, either when driving at 30 m.p.h., normally, or when checking the speedometer over the measured distance. No single instrument can be positioned to suit both persons, and the reasonable possible error due to this cause is about 3 per cent, (b) The difficulty in driving by constant speedometer speed, or accurately "keeping station,” i.e., keeping a set distance behind a vehicle being checked. It is unlikely that any driver, driving behind a car about 30-40 yards ahead, can judge his position so accurately that a fluctuation of 1 m.p.h. (i.e. 3 per cent, at 30 m.p.h.) can be avoided. This errdr can be increased considerably when using a car having rapid acceleration, or if the distance between the cars is greater. It should be noted that this fluctuation of speed also introduces a further error due to lag. Where the method of checking a car is by driving another at a constant speed of 30 m.p.h. (by speedometer) it is also unlikely that a driver will be able to drive steadily enough to avoid an error of less than a halfmile per hour, i.e., 1.5 per cent, (c) A variable inaccuracy of the speedometer, due to lag, i.e., delay on the part of the needle at once to indicate

the true speed. This is due to the fact that a necessary damping effect is introduced .to steady the needle which results, when recording an increasing speed, in it showing a speed slightly under the real speed, and when recording a decreasing speed, showing one slightly over the real speed. The possible error due to this cause is 1.5 per cent. The. errors dealt with above are those which may arise in the act of checking the speed of one car by the observation of the speedometer in another car. It has been stated that the error of the instrument and its gear become evident in the periodical checks of the

car over a measured distance. It must be realised, however, that errors are unavoidably present in this checking. This checking is usually carried out by running the car at a speedometer speed, as constant as possible over a measured distance, the time being taken by a stop-watch. The distance is frequently as short as one furlong, and while the observer may have obtained the necessary skill (not easily acquired) to operate the watch correctly, the possible error of operation at 30 m.p.h., over a furlong, is certainly 1.25 per cent. The magnitude of the error due to timing is exactly proportional to the length over which the test is carried out, i.e., if the test is carried out over a mile the timing error will be only one-eighth of that introduced if the test is over a furlong. As the speedometer has to be read constantly by both driver and observer the error due to parallax (3 per cent.) also comes into this checking, as do the errors due to the inability to drive at a constant speed—--1.5 per cent., and the lag error—l.s per cent., making a possible total of 7.25 per cent. It has further to be remembered that only the most expensive stop-watches are completely free from the serious error of the secondhand flying forward or back a fraction of a second on stopping or starting the watch. it is impossible to indicate the likely error due to this cause. It may be nil, but it may be very considerable.

It will be realised from the foregoing that whilst a speedometer itself may be accurate within the limits of its operation, it cannot, for the reasons given, be regarded as an instrument of precision for exactly recording the speed of another vehicle, and it is not a reliable instrument when used for such a purpose.