SKIDDING.

Auckland Star, Volume LXVII, Issue 71, 24 March 1936, Page 18

SKIDDING.

SMOOTH ROADS. $ EFFECT OF OIL. BANKING IMPRACTICABLE. All methods for preventing slipping and skidding by automobiles aim at increasing tlie friction between the road and the wheels. Some try to solve this problem by seeking to roughen the surface of the road, while others change the structure of the tyres. It is interesting to note that the roughness of a road is determined by the ratio of surface irregularities to the size of the wheel; to an extremely small wheel a billiards table would prove irregular, whilst if the wheels were large enough it would be possible to cross the stony desert in comfort. The comparative and unnecessary smoothness of some main roads is demonstrated by the- fact that with the normal size of wheel using the road it - would be possible to have ribs an inch wide and a quarter of an inch deep in the surface of the road without causing perceptible riding discomfort.

It has been shown that the conditions which give rise to skidding arc a smooth wet road surface coated with a lubricant composed of oil, grease, or a film of mud, though on certain types of surface which develop a slippery skin through chemical change it may be possible to skid on the water film alone. It has also been stated that the action of oil is accentuated by the rays of the sun and that a limited amount of water produces the most slippery surface. From these premises is seems a legitimate deduction that the worst period of the year for skidding is in the late summer and autumn when a shower is often followed by a spell of brilliant sunshine. Tyres Polish Road Surfaces. It is ironical that tlie motorist should help to bring about his own downfall, sincc this smoothness is achieved by the polishing action of rubber-tyred vehicles, which wear smooth the original roughness of the road and obliterate any indentations with which the original surface may have • been branded. The smaller the size of the particles in the superficial coat the easier and quicker become the consolidation and polishing by traffic. The heat and rays of the sun also have a considerable effect on the rapidity of this process; under its rays a hard, slippery skin is formed on certain types of asphalt through chemical change, whilst any excess of binder in tar and bituminous surfacings readily melts and wells up to the surface to assist. However, mere smoothness in a road surface is not enough, since few roads are sufficiently smooth to give rise to a skid under normal running conditions when merely wet. The presence not only of water but also of a lubricant is essential. The lubricant may consist of oil and grease droppings from cars, or of a film of mud, which sometimes also contains a soapy, slippery residue produced by the disintegration of certain types of stone used in road surfacings. All cars tend to drop oil as they grow older, and whilst it would be irrelevant to embark on a criticism of the design of vehicles in a discussion on slippery roads, its effect is serious. A little consideration of the mechanics of skidding and of the forces and resistances acting on a car under conditions which give rise to it, will help tlie motorist to understand why and when he may skid, and how he may, in some degree, avoid doing so. Tho principal external forces acting on any vehicle in the course of its normal running are those due to camber on the straight road and to cornering. The amount of camber applied to modern smooth-surfaced roads is slight, and its effect is not serious, but the action of centrifugal force on corners produces particularly vicious conditions for skidding. r , , Banking on Corners. The amount of centrifugal force exerted on a vehicle rounding a corner depends on - its own weight and speed and-on the radius of . the curve. Simo the centrifugal force, and the amount of /banking (technically known as superelevation) necessary to neutralise its effect, will alter with the variation of any of these factors, it is clear that a curve cannot be super-elevated for all the weights and speeds of vehicles using the curve, but. only for a particular combination of these factors. The extent of the possible- fluctuation for different combinations of these three factors may be seen by considering a variation, for example, of the factor of speed; the banking for a normal yehicle on a moderately sharp bend, to equalise the pressure on the inner and outer wheels, might vary from a crossfail of 1 in 300 at five milcfl per hour to 1 in 33 at 15 miles per hour, and to 1 in 8 at 30 miles per hour. At 40 miles per : hour the crossfail would be approxi- | mately 1 in 4J. Now, 40 miles per hour is not an exceptional nor an excessive I speed for a motor vehicle to-day in the

country, yet no one would suggest banking a curve to a cross-slope as steep as 1 in 41.

Thus it is clearly impracticable to apply banking to suit the. speeds of modern traffic except on very gentle curves; and, even where this may be possible, the odds are very'much against any vehicle taking the corner at the precise speed for which it has been super-elevated. In this case the effect of the centrifugal force will be mitigated, but not neutralised. At all

speeds other than that for which the curve was designed the weight and pressure will be unevenly distributed; if the vehicle's speed is below the designed speed the greater weight will fall on the inner wheel; if above the designed speed the greater weight will fall on the outer wheel.

The probable sum of all these influences on a vehicle rounding a corner is that the wheels are unequally loaded, the vehicle is changing direction and probably speed, it is subjected to a sideways thrust due, to centrifugal force, the rear wheels experience a torque due to driving or braking, and the lightlyloaded wheel is tending to spin under the action of the differential. Therefore since a vehicle travels on curves for a high percentage of its journey, it is during these periods iii a state of instability.