NACA-TN-4406

Abstract

An exploratory investigation was made by the National Advisory Committee for Aeronautics to study typical tire behavior on wet runways, to determine the mechanisms by which water on runways reduced tire forces, and to determine the extent of this tire-force reduction. A specially constructed the treadmill served as a tire test vehicle which allowed easy control of such pertinent, parameters as water depth on the treadway or belt which served as the runway, tire inflation pressure, belt velocity, braking load, yaw angle, and tire-tread pattern. A strain-gage balance mounted on the wheel chassis of the treadmill measured both the braking friction forces and cornering forces while tachometers recorded the wheel and belt velocities. Measurements of these parameters were made in testing a smooth-treaded and a diamond-treaded 3.00 x 7 tire. <br>Observations during the tests and evaluation of data have indicated that, under certain conditions of tire pressure, velocity, and water depth, the smooth-treaded tire stops rotating and begins to plane even without the application of brakes. For example, for a tire inflation pressure of 13¹/₂ lb/sq in. gage and a 0.09-inch water depth, this planing condition occurred for the smooth tire at a velocity of 76 feet per second. The tire with the diamond tread behaved in a similar manner except that, instead of stopping completely, the wheel rotated at about 50 percent of the belt or treadmill speed. With both tires, the tire maximum and full-skid (locked wheel) braking friction coefficients decreased rapidly with increase in belt velocity, and for certain operating conditions of the smooth tire the full-skid braking-coefficient values fell below that of the rolling friction. In some of the cases for smooth tires the values of tire maximum braking friction coefficient dropped so low that the free-rolling friction became the maximum friction force. Under these conditions any slight and momentary braking forced the tire into a stable full-skid condition. <br>Cornering-force coefficients of both the smooth- and diamond-tread tires also decreased with belt velocity and reached nearly zero values at planing velocities. Braking had the effect of greatly decreasing the cornering-force coefficients even at relatively slow velocities.