HIGH-ROAD & BY-ROAD

Waikato Times, Volume 125, Issue 20995, 23 December 1939, Page 24 (Supplement)

HIGH-ROAD & BY-ROAD

WHEEL LOCK WHEN BRAKING LOSS OF EFFICIENCY It has long been realised by experienced motorists that braking efficiency is reduced when the brakes are applied so forcibly in an emergency as to lock the wheels, causing them to slide forward along the road instead of rotating at a speed proportionate to that of the car. Similarly, it has been known that wheels locked by heavy braking are far more prone to skid sideways than when they continue to rotate while braked. To what extent braking efficiency is reduced by locked wheels is a point that has not, however, been generally known; but recently a report was published in America giving the results and conclusions of some twenty thousand separate tests conducted by the experimental section of the Ohio State University. As regards braking with locked wheels, it is shown in the report that when a car is brought to a standstill from thirty miles an hour with its wheels locked by the brakes, the stopping distance is increased by 40 per cent, as compared with the distance in which it can be stopped under the same conditions, on wet or dry roads, with the wheels continuing to roll. At thirty-five miles an hour the stopping distance is increased by 57 per cent, if the wheels are locked, and at higher speeds the loss of braking efficiency becomes increasingly greater. Aggravating Position Another point brought out by the tests is that the faster a car is travelling the more readily will the wheels be locked by heavy braking. This is because coefficient of friction between tyres and road—often termed tyre “adhesion”—is reduced as speed increases, which accounts, also, for the fact that the faster a car is running the more prone it is to skid sideways if its steering should be suddenly or extensively diverted, either on a straight road or at a comer. Further tests showed that the risk of side skidding, with or without braking and the tendency of the wheels to become locked by braking are increased by from 25 to 35 per cent, on roads moistened by a “light sprinkle” of rain as compared with the same roads made thoroughly wet by a “downpour” of rain. The results of these tests have received confirmation in a report since published by the (British) Road Research Board, though in this case the greater risk of skidding was determined by tests made on roads merely, damp by moisture deposited from fog. Matter of Treads One other result of the American tests may be mentioned. It was proved that tyres with their nonskid tread worn smooth will lock and start skidding with 60 to 70 per cent, less braking force than tyres with treads in good condition. From these tests results the following conclusions may be drawn:— (1) Emergency braking must not be carried to excess; the faster the car is travelling the more a driver must resist his natural inclination to “stand on” the brakes. (2) “Just damp” roads should be viewed as far more treacherous than thoroughly wet ones. (3) On no account should tyres be kept in use when their non-skid tread has worn away. ECONOMY IN PETROL KEEP AN EYE ON PLUGS

The usual routine in preparing for a long journey is first to check over the more obvious safety factors, such as steering, brake adjustment and tyres and then the ignition and carbur ati on systems. The steps taken to ensure efficiency here also serve the ends of petrol economy, for a carburettor in need of adjustment or a set of worn or dirty plugs, increase petrol consumption to a greater degree than is generally recognised. It is worth considering whether the plugs should not be renewed before a long tour commences, unless this has been done recently. Good quality sparking plugs have a long effective life, but there comes a time when they begin to deteriorate. The reduction in efficiency is so gradual that they are frequently retained long after they should be pensioned off.

ULTIMATE SPEED LIMITS *

Lecturing before the Royal Aeronautical Society in London recently, Dr. W. F. Hilton, of the aerodynamical research department of the National Physical Laboratory, had some interesting remarks to make about motor cars in a talk which naturally dealt mostly with aircraft. Speaking of. ultra-high speeds in the region of the velocity of sound, Dr. Hilton said that other things being equal, the limit of speed for an aeroplane would be somewhere between 550 and 600 miles an hour, owing to the advent of compressibility stall—which appears to be the state when the wing drag of the machine becomes insuperable, no matter how much horse-power is used. In the case of a properly-designed car with a power output of 4000 or 5000 horse-power (which is by no means impossible in view of existing aero engines), a car could be made to travel at 700 miles an hour—tyres, driver and space permitting. Another point he made was that the ideal proportions of such a car would be: half as great as width, and length 3.4 times the width, and that the ideal frontal shape would be a complete semi-circle. The lecturer indicated that if anyone wanted an aeroplane to travel at 700 miles an hour he would have to use folded wings, the machine travelling through the air owing to the lift on the fuselage alone—like a shell.