SKID PROBLEM.

Auckland Star, Volume LXVI, Issue 250, 22 October 1935, Page 18

SKID PROBLEM.

AMERICAN TESTS

STUDY BY SCIENTIST. THREE DISTINCT FORMS. DETROIT, September 15. In current efforts that are being made to promote highway safety by reducing motor car accidents, the automobile skid is occupying the attention of scientific minds. A study of skidding has been made by Mr. R. A. Moyer, associate professor of highway engineering at lowa State College. Professor Moyer devoted several years to investigation of this phase of tlic motoring hazard.

According to Professor Moyer, there are three distinct forms of skidding on roadway surfaces: (1) Straight skidding in the direction of travel, with the wheels locked. This form is encountered when brakes were applied suddenly, causing the wheels to lock and slide; (2) the impending skid, which is a modified form of straight skidding in the direction of travel, with the wheels just at the point of complete sliding. This form of skid is produced by applying the brakes gradually to the point where the wheels arc still turning, but where skidding is imminent; (3) skidding sideways in a direction to the line of travel. This form is encountered on curves that are not super-elevated enough for the speed at which the vehicle is travelling or when passing other cars on tangents.

Professor Moyer expresses the relationship between the three forms in terms of co-efticient of friction, which is the ratio of force causing the tyres to skid to the load on the tyres. If a large force is necessary to cause the tyres to skid, the co-efficient will be large, but if the force is small the co-eflicient likewise will be small.

Comparative Tests. As part of the investigation, measurement of the co-eflicients were made on road surfaces as nearly as possible under conditions encountered by traffic. The tests were made by means of a twowheel trailer unit attached to a motor car running over wet surfaces, with the wetness produced by sprinkling. Both new tyres with unworn treads and used tyres with the treads worn smooth were used to obtain comparative results. This plan allowed Professor Moyer to take into account the effect of various types of road surfaces in addition to tyre tread condition and characteristics. He also obtained data on the friction of tyres with and without chains running over sleet, ice and snow-covered surfaces.

Some of the conclusions of the investigation were: Co-efficients for smooth tread tyres were consistently lower (that is, more subject to skidding tendency) than for new tread tyres for the three forms of skidding found by Professor Moyer. Water causes a lubricating action in connection with skidding. At lower speeds there is a greater possibility for "squeegee" action of a tyre on road suriace that gives more intimate contact. With increase in speed, tyres "skinned" over the surface. Tyres encased in chains improved traction straight ahead, but offered little or no protection from skidding sideways on ice or sleet.

Mucl on hard pavements is the cause of many accidents. The placing of gravel, shale, cinders or crushed rock 011

the shoulders and at the approaches of all hard roads would not only correct a dangerous skidding condition, but would also provide greater road widths for use in emergency passing 011 shoulders. If the brakes on a car can provide a braking force greater than the skidding resistance of the surface, it is quite likely that in an emergency stop one or more wneels will be locked -and the tyres caused to slide. This is frequently tlievcase on ice, snow, gravel and cinders.

The results of tests 011 the condition of the brakes indicated that the braking effort of an average four-wheel brake car was only 51.5 per cent of the weight of the ear, including driver and passengers. In other words, the average maximum braking force of cars tested was considerably lower than the average resistance to stopping which road surfaces of the higher type could provide, especially in dry condition.