116 Proceedings of Indiana Academy of Science 



After mounting on the axle each was carefully balanced on horizontal 

 knife edges, the spoke wheel requiring much more adjustment than the 

 disk wheel. Even after being adjusted with considerable care, two or 

 three accidents occurred. One was due to the fact that after inflating the 

 tire (to 40 pounds in each case) the writers forgot to put on the valve 

 cap. At a speed near the maximum shown in the table the centrifugal 

 force on the valve plunger opened the valve and permitted the air to 

 escape from the tire. The wheel then threw the tire which, after re- 

 bounding from a concrete wall, struck and demolished a twenty-four 

 inch iron pulley on a line shaft. 



Several observations were made at each speed shown in table I, 

 and at numerous speeds not recorded. The results given in the table 

 are sufficient to enable us to plot the curves D and S in figure 1, from 

 which the power required for other speeds can be easily determined. 

 D is the power cuiwe for the disk wheel and S for the spoke wheel. 



Column 2 of the table gives the speed in miles per hour correspond- 

 ing to the r.p.m. of the first column, and the last column gives for each 

 speed the extra horse power required to drive four spoke wheels over that 

 required to drive four disk wheels of the same size. 



The writers realize that the air friction losses in the case of a wheel 

 rotating on a stationary axis can not be assumed equal to the losses 

 where the axis too is moving. In the case of the stationary axis a 

 body of air about the wheel is thrown into rotary motion, with the re- 

 sult that the air friction loss is less than if the air were at rest. Any- 

 thing that tends to prevent the air from acquiring this rotary motion in- 

 creases the air friction losses. When a spoke wheel was rotated in- 

 side a wooden box with the sides of the box within a few inches of 

 the wheel, the driving power required to produce a given rotational 

 speed was greatly increased and the temperature of the air in the box 

 ro.se rapidly. Certainly the air friction losses shown in the last column 

 of table I are not greater than would occur in the case of a spoke wheel 

 automobile moving at the speeds indicated in the second column. It 

 would appear then that the disk wheel has little advantage over the 

 ordinary wood spoke wheel at ordinary automobile speeds. But at the 

 speed at which long distance auto races are usually run — ninety to 

 one hundred miles per hour — air friction losses for spoke wheels are 

 about one horse power more than for disk wheels. This loss, by no 

 means negligible, increases rapidly at higher speeds. 



The results in the table aie for plain spoke wheels such as are used 

 on light cars. The heavier the spokes the greater the air friction losses. 

 For staggered spokes the loss is still greater. It is very much greater 

 for wire wheels. 



If one .stands near a pulley rotating rapidly he readily perceives the 

 air currents produced, chiefly due to the fact that the spokes or arms of 

 the pulley act like the blades of a fan. In some cases these air cur- 

 rents may be desirable, for ventilation, cooling, or something of the kind. 

 But if they are not useful they are wasteful in that power is required 

 to produce them. They can be greatly reduced by using web, instead 

 of spoke pulleys, or by placing a tightly fitting flat circular disk in 

 each end of the pulley so as to enclose the spokes. In the experiment 



