THE CAUSES OF SKIDDING

Press, Volume LXXIV, Issue 22336, 25 February 1938, Page 8

THE CAUSES OF SKIDDING

road board tests IN ENGLAND PRINTS OF HIGHWAY SURFACES New methods used by the English Road Research Board in its study of the causes of skidding are described in the board’s third annual report, which was recently published. Various ways of estimating the texture of road surfaces have been tried, the earliest of them being the use of plaster casts. There were certain disadvantages in all these methods, and the report—which refers to the year ended in March, 1937—shows that something of the mechanism which Scotland Yard uses to take finger prints has now been employed. Surface texture is being recorded by means of prints taken on a portion of road surface which has been inked over. A perfectly smooth tyre is rolled over the inked road and then transferred to a piece of while paper, when an exact record of the texture of the road surface with which the tyre has been in contact is obtained. The skidding characteristics of road surfaces, the report points out, are largely determined by the total area of contact between the tyre on the road and the number of isolated individual points of contact in this area. These small isolated areas must be surrounded by channels of such a size that when the road is wet the liquid on it can escape through them as the tyre passes over. If the channels are too small the individual areas will, in fact, merge into one another and the surface will behave as a smooth one. The requirements for non-skid tyres are similar —namely, as large a bearing area as possible is cut by many small grooves to form a large number of individual contact areas. Changes In Surface “An examination of the texture prints taken from roads known to be very slippery,” the report says, “has shown that on these surfaces the tyre makes contact with the road over almost the entire area of the ellipse of contact, while the number of .individual contacts in this area is small. Prints from roads having high sideway force co-efficients showed that the area of actual contact of tyre and road 1 within the ellipse of contact was appreciably less, while the number oi individual contacts within the ellipse was much higher.” The texture prints have also been found to be valuable in following the wear and other changes which take

place on the surface of a road. More* over, the same method has been used for studying the change In the slipperiness of roads at different seasohs of the year. It Is believed that these depend largely on the changes of the road’s surface texture. During the year, the motor-cycle and sidecar equipment developed at the laboratory for studying the slipperiness of roads was used for skidding tests on a number of special surfaces. Tests on a number of granite sett surfaces appeared to indicate that grouting between the setts with cement mortar instead of bituminous material increases the resistance to skidding. By the provision of a new highpowered engine, it had been possible to make tests of skidding at speeds up to 50 miles an hour. Hitherto, the study of skidding had not been made at speeds greater than 30 miles an hour. In certain Cases the results of these tests have been rather unexpected. With most surfaces the skidding co-efficient tended to reach a steady value between 30 and 50 miles an hour. In others there Were indications that the value of the coefficient was continuing to fall, and therefore the risk of skidding was greater. In view of this plans were made for carrying out skidding tests at even greater speeds. Mud Films in Fog Another direction in which the board has been studying the problem of skidding is by the examination in the laboratory of specimens of mud taken from road surfaces. An attempt was then made to determine what contribution to the slipperiness of the road was made by various types of mud, and the report discusses the question whether the mud films on a road during fog produce a specially slippery surface.

“Any dust falling on the road from vehicles,’’ the report says, “and any material abraded from the road or squeezed out of hollows in the surface by tyre action, is held on the damp road and the solid content, and hence the viscosity, of the film steadily increases throughout the fog. The resulting lubricating film of high Viscosity would be expected, in the light of experiments with viscous liquids on the laboratory skidding apparatus, to ■increase the slipperihess of the road surface. This film will persist until

rain falls or the road surface dries. Any soot particles deposited from the fog will add to the mud already on the road, increasing its amount and probably its viscosity. It is possible, fn addition, that this very finely divided carbon may possess specific lubricating properties which will further increase the slipperiness of the mud film to which it is added. The Road Research Board has also made further progress, it reports, ift the development of a machine for measuring irregularities on load surfaces, and its study of the durability nf different types of road construction has continued. Most Of Ihe work Was done at the Road Research Laboratory on the Colnbrook by-pass road.