ON THE RESISTANCE OF ROAD VEHICLES TO TRACTION. 



369 



Runs were m.ide at speeds of from 12 miles to 35 miles per hour, 

 over both good macadam, frozen hard, and good dry setts. The springs 

 .supporting the trailer were 3 feet 2 inches long, each having four 

 plates 2\ inches by {\. inch, and the tyres were in all cases pumped to 

 a pressui-e of GO lbs. per square inch, the total load on the wheel being 

 896 lbs. The results obtained and plotted in fig. 4 show that the tractive 

 effort under similar conditions as to road surface and speed is less for the 

 tyre of smaller cross-section than it is for that having the larger section. 

 This may be due to the fact that the tread of the larger tyre was much 

 thicker than the smallei-, rendering it in consequence more after the 

 nature of a solid tyre, it being well understood that a perfect pneumatic 

 tyre should have as little inelastic, or comparatively inelastic, material 

 about it as possible ; or, the greater tractive effort may have been due to 

 the greater cross-sectioii. Repeated experiments alone can definitely settle 

 this question. 



Fig. 3. — Curve showing Tractive Effort per ton for Pneumatic Tyre 24" x 2|" on 



Macadam. 





Velocity— Miles per hour. 



Fig. .5 shows these curves plotted as tractive effort per ton. On the 

 same axes the tractive effort per ton has been plotted for the previous 

 wheel (pneumatic tyre 34 inches by 2| inches), and it is very nmch greater 

 than that for the 34-inch wheels. 



This Committee is not yet in a position to state the exact relation 

 between tractive effort and diameter of wheel ; but, taking the results of 

 General Morin, that the draught is inversely proportional to the diameter 

 of the wheel, a curve has been plotted (tig. 5) which reduces the tractive 

 effort of the 24-inch wheel to that of an equivalent 34-inch wheel. Con- 

 sidering the variations that may have existed in the roads on which the 

 wheels were tried, as it was at different times of the year, these results 

 harmonise fairly well. 



II. Suggestions hy Mr, B. J. Dijilock. 



The following suggestions with regard to trials of wheels for heavy 

 traffic werfe submitted to the Committee by Mr. B. J. Diplobk (member) 

 1903. B B 



