Tests made on cast-iron car wheels with machined treads, running on steel rails, are reported here. Wheel diameters ranged from 8 to 24 in., load per wheel from about 100 to 1800 lb, speed from 1 to 6 mph. Static or starting tests were also included. The ratio of horizontal resisting force F to the vertical load W is denoted by f and taken as a measure of rolling friction. The friction ratio f was seen to vary almost inversely as the square root of the diameter. It decreased with increasing load, and increased slightly with increasing speed. These results differ considerably from those published in engineering handbooks.

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