64 



THE INDIA RUBBER WORLD 



[NOVtMBER I, 1913. 



Effects of Overloading Solid Rubber Tires. 



In our last issue we presented some flares which proved conclusively the wisdom and economy of adequate tire equipment. Since publishing that article, 

 The B. F. Goodrich Co. has supplied us with further information regarding the detrimental effects of overloading and overspeeding solid rubber tires, and we 

 present herewith extracts from a paper prepared by Mr. S. V. Norton, of the Goodrich Truck Tire Department, for the beneflt of our readers who are inter- 

 ested in this important subject. 



TIRE manufacturers, after years of observation and experi- 

 ment, have adopted a schedule of carrying capacities, which, 

 with but slight variations, is now considered standard. 

 It seems hardly necessary to repeat the figures at this time 

 for they are given in every tire manufacturer's catalog. How- 

 ever, the factor of safety in the schedule is nil. So many 

 ■other factors enter into the matter, such as excessive strains 

 due to bumps, depressions in the roadway, negotiating 

 grades, overspeeding, and the severe use of brakes, that no 

 provision has been made for overload. 



Just what is meant by the overloading of rubber tires? 

 In the process of manufacture crude gum is mixed with 

 pigments and the whole mass is kneaded together until the 

 compound becomes homogeneous. After various intermedi- 

 ate steps, the rubber is placed in a mold and vulcanized. 

 During vulcanization the tire assumes a more or less perma- 

 nent form, to which it will normally return if it is not 

 stretched or compressed beyond certain definite limits. If 

 the stretching or compression exceeds these limits, however, 

 the strain will invariably cause a rupture or disintegration 

 of the rubber. 



To illustrate: Take a strong, properly vulcanized rubber 

 band. Stretch it moderately, release it, and it will return 

 to its normal shape. The process may be repeated in- 

 definitely, depending upon the quality of the rubber, and 

 the band will resume its shape. But stretch it beyond its 

 limit of elongation and what happens? The rubber breaks 

 and cannot be restored to its previous condition. The 

 damage has been done and the rupture is permanent. 

 Similarly, a piece of rubber properly vulcanized may be com- 

 pressed within certain limits, released, and it will resume its 

 normal shape. The compression may be repeated an in- 

 definite number of times, depending upon the quality of the 

 compound. So long as its limit of compression has not been 

 reached, the rubber will continue to spring back into shape 

 and nothing but abrasion or some other physical or chemi- 

 cal action will destroy its elasticity. Compress it beyond 

 its limit of cohesion, however, or in other words, beyond its 

 power of resistance, and the rubber breaks down or ruptures 

 just as inevitably as in the case of stretching beyond its 

 limit of elongation. The rupture is absolute and permanent. 

 No method yet devised will restore to the unit its former 

 elastic qualities. 



There arc three kinds of overloading: The first is due to 

 undersized original tire equipment. The competition in 

 business makes it necessary to economize wherever possible 

 in the equipment of trucks. Manufacturers build the best 

 machines that money and skill can devise, and ofifer them for 

 sale fully guaranteed — except for tires. They usually equip 

 them with the smallest tires that will come within the tire 

 maker's schedule of carrying capacities. Then, if for any 

 of the reasons mentioned later, the tires fail to deliver the 

 service expected, the responsibility is at once automatically 

 shifted to the tire manufacturers' slioulders, and they are 

 expected to live up to their reputations for liberally "taking 

 care" of their product. Suppose the sales agents order 

 just the chassis and then have special bodies built^ — bodies too 

 heavy in themselves, or so large as to permit the carrying 

 of loads far in excess of those the tires are able to sustain 

 without breaking down. What happens? The customers 



soon have trouble with tlieir tires, and they complain that 

 the cost of maintenance is more than they had been led to 

 believe. 



The second kind of overloading is due to mis-propor- 

 tioned equipment. Sometimes we find that trucks whose 

 tire equipment, regarded as a whole, is ample for the service 

 required, will be lacking in carrying capacity at one end, 

 while at the other end there may be an excess capacity over 

 the actual demands. I have in mind a case in which we 

 were called upon to make an adjustment. The proportion 

 of weight carried by each axle showed that the front tires 

 were considerably overloaded while those on the rear had 

 an excess capacity of nearly a ton. The owner of the truck 

 protested his innocense, saying that he had never been told 

 he was misusing his tires. The difficulty once explained, 

 however, and the proper-sized front tires applied, there was 

 no further trouble. 



The third kind of overloading, which is at once the most 

 prevalent, the most illusive, and the hardest to cope with, 

 is due to the improper loading of the commodity upon the 

 truck. Some times this may be the result of faulty dis- 

 tribution of the load over the front and rear axles, which 

 may cause the failure of the tires wholly without the 

 knowledge of the owner. 



By far the most difficult phase for the tire maker to 

 overcome is the deliberate placing on the truck as a whole 

 of too heavy loads for the tires to carry. In many cases, re- 

 gardless of the original training and explanations given to a. 

 driver, there is always the temptation to fill the truck to the 

 limit of its physical capacity to move the goods. The mer- 

 chandise must be moved, and "a load is a load," whether 

 it exceeds the carrying capacity of the tires or not. Unless 

 the tire equipment has an ample margin of safety, this 

 attitude results in poor economy. 



The drawing of trailers, while more common abroad than 

 in the United States, is another means of improperly loading 

 tires. No doubt this is an economical means of hauling, 

 but trucks so engaged are placing a greater strain upon 

 their tires, and they should be at least the next size larger 

 than those which would safely carry the truck without a 

 trailer. 



Turning now to the effects of overspeeding on solid tires, 

 three things should be considered: They seem so axiomatic 

 that they need only to be mentioned briefly. First, it gene- 

 rates excessive heat in the tires, which tends to shorten the 

 life of the rubber. Even tho not overloaded, they may 

 be driven so fast that they are "burnt up," as it were, in a 

 very short time. .Second, it accentuates shocks and jars 

 due to unevenness in the roadway. The stresses thus set 

 up are really the equivalent of sudden overloads, and they 

 tend just as effectively to cause the premature failure of the 

 tires as tho they were loaded beyond their capacity. Third, 

 overspeeding induces the severe use of brakes, especially in 

 thoroughfares in which there 'S much traffic. This in turn is 

 apt to cause unnecessary friction with the road surface which, 

 of course, wears out the tires promptly. 



Of imports amounting to $68,126,292 from London to 

 the United States for the first half of 1913, crude rubber 

 represented a value of $14,481,358. 



