Some Notes on Skidding.
Page 9
If you've noticed an error in this article please click here to report it so we can fix it.
By Francis Wood, M.Inst.C.E., F.G.S., Barough Surveyor of Fulham.
The skidding of motor vehicles on roads is a characteristic which it will be difficult to overcome. No matter how perfect the road surface may be in ordinary weather and during the greater portion of the year, there are certain periods when it becomes very difficult for the selfpropelled vehicle to maintain its position on account of the greasy condition of the roads.
This greasiness is not due to a defect in the mode of road constructicn, but it, is rather owing to the fact that a slight drizzling rain brings down from the atmosphere a considerable quantity of carbon, which acts as a lubricant to the surface and so prevents adhesion.
Fine material in the shape of sand is thrown on to the surface by the road authorities to act as a counter irritant., but this is soon churned up, and its effect is nullified after a time. A better result would probably he secured if water carts were turned out and the roads freely sprinkled with water when it began to drizzle, but this course will not be adopted until it. is an accepted method and is no longer considered as against the interests of the users of the roads and footpaths. There is, of course, the difficulty to be considered of obtaining both water and carts at a moment's notice. On the face of it, it would appear to be absurd to water a, road while it was raining.
It is generally acknowledged that skidding is confined to motor vehicles. It is not due to the higher speed of such conveyances, because it usually occurs when the vehicle is travelling at a low speed and rarely when the vehicle is moving at high speed, except, perhaps, when a sharp corner is being turned. It does not occur when the vehicle is travelling in the centre of a roadway, and it is noticeable that drivers are well aware of this fact and make for this position of the road as quickly as possible.
Perhaps a brief examination of the methods of construction of the road and of the vehicle will be of service in order to show that the present type of design of a motor vehicle is not adapted to the surface on which it has to travel; an alteration may assist in preventing in some measure the skidding which is so prevalent.
A roadway is constructed with a contour or curve which is barrel-shaped, i.e., it is flatter in the centre third and falls more quickly as it approaches the channel; the average grade is about 1 in 30. In all horse-drawn-vehicles, the wheel is set to run on a greater grade than this. An examination of worn tires will show that the inner edge of the tire is worn less or more than the outer edge, according to the setting of the wheel and to the average contour of the road over which it travels. But in all motor vehicles, for no apparent reason except for ease in construction, the wheels are set vertically or at right angles to the axle bed, as if the vehicle had to travel on a perfectly-horizontal plane.
It must therefore be evident that, even in the centre of the road the greater portion of the load is transferred to the inner edge of the tire. If the tire and road surface be non-resilient, then, when the vehicle is running in the centre of the road, for each wheel there must be a force acting at right angles to the tangent of the contour of the road and one acting along the tangent away from the centre of the vehicle. As the curve in the centre of the roadway is regular, the forces along the tangents of the pair of wheels arc balanced and the vehicle keeps a straight course. But, when the vehicle is running out of the centre of the road and nearer to the kerb, the greater portion of the weight of the vehicle is on the lower wheel, and the force acting along the tangent of the curve is greater than that at the higher-placed wheel, therefore there is as distinct tendency to skid towards the curve, if the wheel tire is not in full contact, and if the effect of friction is insufficient to counteract this sliding motion. It must be evident that the greater the area of the tire that is in absolute contact with the surface of the road the better is the chance of non-skidding. With rubber tires that are worn and have adapted themselves to the contour of the road, the tendency to skid should be less.than would be the ease with a vehicle fitted with new tires. Apart from the tendency to skid, there must also be greater wear on the axle under such conditions, and it would develop a wobbling motion. The effect on the road material is, of course, greater than is intended by the provisions of the Motor Car Act, ‘vhich implies full contact with the road surface, although no definite statement is made to that effect.