ON THE RESISTANCE OF ROAD VEHICLES TO TRACTION. 



371 



at a meeting held at the Society of Arts, London, on Friday, May 15, 

 1903 :— 



Assuming that : 



1. All wheels are coned or straight in cross-section ; 



2. All roads are arched or flat in cross-section ; 



3. It seems evident that coned wheels on flat roads, thus — 



or flat wheels on arched roads, thus — 



cause increased road resistance in proportion as the wheels are coned 

 or the roads arched in relation to each other, and that the same result 

 is caused by inequalities in the road surface — viz., that wide tyres do 

 not obtain an even bearing throughout their entire width, except on very 

 soft and yielding ground. 



Starting from the above statements, and as the results of long ex- 

 perience and observation, he had come to the conclusion that for heavy 

 traffic wheel tyres of more than, say, 9 inches in width have, in practice 

 little or no material value as tending to reduce road resistance or damage 

 to the road surface, and he suggested that experiments might be carried 

 out with a view to testing the accuracy of this conclusion. 



Assuming that 9 inches were adopted as a useful maximum width 

 of tyre for heavy haulage on average roads, he submitted the following 

 theory : — 



1. That the supporting power of a road is limited by the cohesive 

 friction of the road molecules or particles against each other. 



2. That this supporting power limit varies very considerably according 

 to the material used in constructing the road and the moisture absorbed 

 in the road. 



He would be prepared to find that road resistance up to certain limits 

 of weight on each wheel (for each class of road) varies approximately in 

 direct proportion to the increased weight on each wheel. 



He was of opinion, however, that if this limit of weight per wheel is 

 exceeded so as to overcome the frictional cohesion of the road molecules 

 against each other, then an entirely new set of conditions arises ; and he 

 would be prepared to find that I'oad resistance would, under such con- 

 ditions, increase altogetlier out of pi'oportion to increased weight on the 

 wheels; 



Sb 2 



