4o 



WHEEL 



CH. Ill 



S> 



_, 



Fig. 18. 



parallel arm the box must be fitted to it with accu- 

 racy, requiring very good workmanship and perfect 

 tools, and no adjustment of the fit is afterward pos- 

 sible. The greatest strain on the arm being at the 

 back end, a tapered arm can be made lighter for a 

 given strength. 



With a straight, vertical wheel on a 

 taper arm, the centre line of which is 

 horizontal, the tendency of the wheel will 

 be to run off, or against the nut, or linch- 

 pin, while to be safe it should tend to run 

 on, or against the collar (Fig. 18). 



To obviate this defect, the taper axle- 

 arms are so set that their under sides 

 are horizontal (Fig. 19). 



This arrangement places the wheels 

 farther apart at the top than at the 

 bottom, and evidently brings a strain 

 upon the lower part of the wheel, 

 tending to push it inward. To provide 

 against this, the wheel is dished, so that 

 the lower spoke is always vertical or 

 plumb (Fig. 20). 



This turning down of the point of the 



axle is called variously : hang, dip, swing, 



and pitch. 



The amount of dish, is the distance of the front 



side of the root of the spoke from the face of the 



rim, measured in the direction of the axle. 



This construction of the wheel also strengthens 



Fig. 19. 



Fig. 20. 



