I50 WIDTH OF TIRE CH. X 



nothing was gained by having a greater width than 

 four inches, and on pavement less was sufficient. 

 These figures refer to heavy loads, 2000 to 2500 

 pounds on each wheel, and are applicable only to 

 wheels that are very nearly cylindrical, vertical, and 

 with horizontal axles (see notes on Wheels in Chap- 

 ter III.), in which case there is no grinding action on 

 the road. On a very hard, smooth road, such as 

 good macadam or the best pavement, the draught 

 appears to be independent of the width of the tire. 

 There always has been a belief that a narrow tire 

 affords an easier draught, and it is probable that this 

 opinion was formed when wheels had more dish 

 and axles more drop than is now given in the best 

 practice, in which case, the wheel being a section 

 of a cone, the tire ground on the road. 



Even on a hard, unyielding road like a stone pave- 

 ment, if it is roughs a wide tire is advantageous, 

 inasmuch as it prevents the wheel from dropping into 

 the inequalities between the stones. 



If the wheel is nearly or quite cylindrical, an in- 

 creased width of tire is an advantage to the road 

 itself, but with coned wheels the increased width is 

 a disadvantage ; it increases the grinding action. 



It is obvious, that in stiff mud, sand, or ordinary 

 earth, such as is crossed in hauling' over a field, the 

 wheel, sinking in under the pressure of the load, is 

 constantly surmounting a little hill in front of it, and 

 leaving an open rut behind it, which, owing to the 

 nonelasticity of the material, is not filled up. If, 



