0!Sr f^t UlSStSTAlSTE 0^ ftOAto VEfildLES fo »fRAC'riO>f. 3o0 



Morin concluded that the resistance varies inversely as the first 



power of the diameter of the wheel. 

 Dujmit that it varies Inversely as the sqtiare root. 

 Clark that it varies inN'ersely as the cube root. 

 Baker that it varies very nearly inversely as the square root of 



the mean diameter. 



'(b) Width of tyres. — If the wheel cuts into the road, the traction is 

 thereby increased ; but for surfaces where there is little or no indentation 

 the width of tyre has practically no effect upon the traction (see Tables 

 II. and III.) 



(c) Effect of speed. — The rolling resistance increases with the velocity 

 owing to the effect of the shocks or concussions produced by the irregu- 

 larities of the road surface. It i-equires from two to six or eight times as 

 much force to start a vehicle as to keep it in motion at two to three miles 

 an hour. The extra force required is due to: — (1) During the stop the 

 vehicle has partly sunk into the road ; (2) Axle friction is greater at 

 starting than after motion has begun ; (3) Energy is consumed in 

 accelerating the load (see Table IV.) 



{d) Effect of springs. — Springs decrease the traction by decreasing the 

 concussions due to the irregularities of the ground, and are therefore more 

 effective at high speeds than low, and on rough roads than on smooth. 



(fi) Diff^erent road sxirfaces. — The tractive force was obtained by a 

 Baldwin dynagraph. The instrument consists of two long flat springs 

 fastened together at their ends, and having their centres farther apart than 

 their ends. One end of the apparatus is attached to the waggon, and 

 the other to the team. The pull of the team causes the centres of the flat 

 springs to approach each other. One spring supports a graduated disc, 

 and the other is connected to an index arm which is pivoted at the centre 

 of the graduated disc. From one end of this index arm the pull can be 

 read directly from the graduation. There are two extra index arms, one 

 to indicate the maximum power developed, and the other to indicate a 

 rough average. The end of the index arm, opposite the graduated arc, 

 records the amount of traction upon a strip of paper which is wound from 

 one cylinder to another by clockwork (see Tables V. and VI.) 



Table I. — Effect of the Size of Wheels on Traction. 



Description of the Eoad Surface 



Tractive force, lb. per ton 



Blean Diameter of Front and 

 Rear "VMieels 



Macadam, slightly worn, clenr fair condition 

 Gravel road, dry, sand 1" deep, loose stones 



„ ,, up grade 22 per cent., i" wet sand, 

 frozen below. 

 Earth road, dry and hard 



„ „ V" sticky Diud, frozen hard below, rough 



Timothy and bluegrass sod, dry, grass cut . 

 „ wet, spongy . 

 Cornfield : flat culture ; across rows, dry . 

 Ploughed ground ; not narrowed, dry, cloddy . 



Average value of the tractive power 



Z 2 



