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Show Study Impact of Motor Bus Over Public Roads When the wheel of a motor vehicle passes over nn obstruction or a rough place in the surface of a highway, there are two Impacts of the wheel upon the road shock Impact when the wheel comes lu contast with the obstruction ob-struction and Impact when it hits the pavement following the bounce. The trend today Is toward higher vehicular speeds, particularly with the heavier wheel loads, such as in motor bus operation and the bureau of public roads. United States Department of Agriculture, has studied the impact forces produced by the rear wheel of a modern bus operating on a concrete pavement at various speeds up to 53 ' miles an hour and over two types of road obstructions. The studies have Included various wheel loads, sizes of tires, Inflation pressures, rim dimensions dimen-sions nnd rim widths. The Rubber Manufacturers association and the Society of Automotive Engineers cooperated co-operated In the Investigations. The operation of heavy vehicles at high speeds has been made practical by the comparatively recent use of high-pressure tires and balloon tires on heavy trucks and busses, says the bureau. High Speeds. The tests revealed that the Impact varies In almost direct proportion with tire Inflation pressure. The variation of Impact with speed proved to be different for the two types of impacts, drop and shock. The drop Impact reached a maximum value at comparatively compara-tively low speeds and decreased at higher speeds. The shock Impact varied va-ried almost directly with speed, but did not attain a value as great as the maximum drop Impact until a very high speed had been reached. The relations of the various factors in de-tali de-tali are described In the April Issue of Public Bonds. These bus impact tests are the latest lat-est of a series of impact investigations which the bureau has been making for ten years or more. The earlier studies dealt with the measurement of impacts im-pacts caused by heavily loaded motor trucks, especially those with poor tire equipment. The recent studios were conducted on a concrete road at Aberdeen (Maryland) proving ground, througn the courtesy of the ordnance department. depart-ment. The vehicle used was a modern high-speed bus chassis witli a special body. The floor of the body was divided di-vided into small compartments for retaining re-taining 100-pound cast-iron weights used to vary the loads. Tire Equipment. The test car tire equipment included includ-ed both high-pressure pneumatic and balloon types, and the range in size was from 7-inch high-pressure to 12-Inch 12-Inch balloon. Inflation varied from fW to 107 pounds per square Inch. Kims were of -0 and 24-inch diameter. The tests involved two types of ob struclion each l'.s inches high ; one an inclined plane :50 inches long, ami the other a rectangular obstruction 12 inches wide. The-nmgnitude ot Impacts was mcas ured by a multiple-element contact ac celeroineter, consisting of ten sensitive elements, which was designed and built by the bureau as a modification of a single element instrument used in earlier impact tests. The aceelero-meter aceelero-meter was mounted on the rear axle of the bus, and was in a position to respond to any vertical accelerations imparted to the axle by the wheel. |