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Chemical and Physical Tests of Solid Tires.

5th November 1908
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Page 14, 5th November 1908 — Chemical and Physical Tests of Solid Tires.
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Which of the following most accurately describes the problem?

Keywords : Tires, Tire, Solid

By Clayton Beadle and Henry P. Stevens.

As the result of our investigations, a paper on he above subject recently appeared in " The Chemical News"; it dealt with an examination of four solid cab tires of wellknown makes. In this paper, we endeavoured to trace the connection between the chemical and physical tests and the wearing qualities of the tires. The useful inferences drawn from these tests have been almost entirely borne out by the results of wear and tear on the tires when in use.

In the present communication, we shall record the results of a number of tests vvhirh comprise Our work on a further series of tire samples, and, for purposes of comparison, we shall include some of the figures that were given in it previous paper on the subject. In the original publication the results of the physical tests were not given in terms of gratuities per square millimetre of sectional area, but, in order that the tests of all these tires may be compared with one another, we have recalculated those in our previous paper ; they are given below in the various tables, and numbered ois previously) Nos. 1-4. The results of the determinations are given in Table A.

Table A.—Mineral Matter and Specific Gravity.

The second column gives the percentage of ash obtained by the incineration of a given weight of the original rubber; the third column gives the specific gravity determined by the weighing of a sample of the rubber in air mid water ; the last column (marked " Factor ") gives the ratio of the percentage of ash or mineral matter to specific gravity, or, in other words, represents the figure by which the specific gravity would have to be multiplied in order to give the percentage of ash. The last column is introduced, in order to show the close relationship which exists between the percentage of ash and the specific gravity of the tires under examination, and it will be noted that, with the exception of samples 7 and 9, the factor in all cases approximates to the figure 27; it varies from 26.6 to 27.5.

It is certainly surprising that there should be so near a connection between the specific gravities and the percentages of mineral matter in these tire rubbers, especially having regard to the fact that they are well-known commercial brands, and made by different manufacturers. The relationship between specific gravity and mineral matter will depend, chiefly, on the specific gravity of the mineral matter which is used in the mixing. The nature of the mineral matter in solid tires, even in one class of solid tire, may vary considerably, and likewise the specific gravity. Many mineral ingredients which are found in rubber mixings. such as chalk, magnesia, talc, china clay, etc., have a specific gravity approaching 3. but many of the other minerals employed are considerably hvavivr. Thi45, lithophone—a frequent constituent of tires—has a specific gravity approaching 4, whilst various lead compounds, which are sometimes employed, have specific gravities, varying from 6 to 9. With this considerable variation in specific gravity, the above-noted close relationship between specific gravity and mineral matter is all the more noteworthy.

Making a rough calculation, and putting the specific gravity of the remainder of the tire at unity, it would appear that the specific gravity of the mineral matter employed in most of these tires is about .4. This subject, however, \Ye hope to go into, in a subsequent paper, with more detail. It should be noted, here, that the tires from which the results are given above are all of one class; a much more varying relationship between specific gravity and mineral matter would probably exist, had we compared, say, a solid cat tire with a heavy motorbus tire, but the fact that the percentage of mineral matter ias shown by the figures given increases fairly regularly with the specific gravity would lead us to suppose that the composition of the mineral mattet used in the different tires varies but little.

Percentage of flubber and Tensile Strength.

The next table (IS) gives the figures for percentage al rubber, as compared with tensile strength and elong,atior :n the moment of rupture. These figures do not necessarily show any relationship tc one another. The strength of a tire depends not only upor the amount of rubber present, but upon the kind of rubbet employed ill manufacture, and the extent to which it ii vulcanised, and it would depend, furthermore, upon whethei the rubber present were new rubber or " reclaim " rubber. and so forth • points which cannot generally be distinguishec by chemical analysis. It would appear, from present prac. lice, that the best results consistent with a not too higt cost can be obtained with the addition of something like 35 tc 45 per cent, of rubber, some of which, however, may be it the form of " reclaim " or" crumb." Crumb must be re. garded, more or less, as a loading or bulking material whereas good reclaim has excellent qualities in itself, ever when used alone. Unfortunately, chemical analysis doe not discriminate between these different forms, but physica rests will usually bring out the deficiencies of the interiot tire. The primary object of the manufacturer is to product at a moderate price a tire that will give good wearing an

lasting qualities under ordinary conditions of use. 0: course, the mixing could be cheapened by the introductior of large proportions of substitutes, crumb, and other low. grade material, but this would be impracticable in tin manufacture of such rubber goods as tires which are sub. jected to continuous wear and tear. It is unlikely, therefore, that a satisfactory tire can hi produced at a price which does not admit the use of a fah

proportion of good-grade rubber (either new or reclaim), al though, of course, nothing is gained by increasing the pro portion of new rubber in the mixing beyond a certain figure The mineral matter not only hardens the rubber, bu toughens it, that is, increases its tensile strength. In the particular case of rubber and rubber mixings where rubber is vulcanised with sulphur, and contains littl or no minerals (i.e., what is termed " floating " quality), th tensile strength is lower than with rubber containing, say its own weight of minerals. This inay be illustrated by . comparison of the physical qualities of solid tires with thos of the inner tubes of motor tires. Thus, as the result of number of determinations which we have made from Lim to time, it may be stated that a good sample of inner tubt and this, of course, comes under the category of " floating qualities to which reference has just been made, will silo.% a tensile strength of 200 to 240 grammes per square nun, set tional area, and an elongation (at break) of 896 to 768 pe cent.; we quote, here, from our actual figures. As would b expected, therefore, the rubber of inner tubes has les strength, but far greater elongation, than that of solid tires All the strength tests, in this inyestigal ion, are (hale by the: cutting of sections of the tire in question, and the making of ten tests with each sample upon which the mean " strength " and " elongation " in the above summary is calculated. The details of some of the tests are given in our earlier paper (vide sopra). The sample marked No. 9, which contains very little rubber and gives a low tensile strength, is, in our opinion, useless for solid tires; although much cheaper to illak1s, t WOUld collapse in a %airy short time and, therefore, would hardly pay to put on, even if sold at a low figure.

Physical Qualities of Tire Rubbers, taken in Different Directions.

Tt occurred to us that, in all probability, a difference would be noticeable, in the strength of solid tires, between pieces which were cut in transverse and longitudinal directions. Accordingly, transverse sections were carefully cm, as well as longitudinal sections parallel with the flat base of each of the tires, from each of which ten tests were made. In the case of " 5," as there were tires of two dimensions of a particular composition, it was thought advisable to make the determinations on each of the tires. These are given in table C.

In the last column is given the ratio figure of transverse to longitudinal when the strength of the transverse direc Lion equals too iii each case. It will be noticed, here, in tire " 5," that, in both the large and small sections, the ratio figure is 113. In tires" 6 " and " 7," the strengths are approximately the same in the two directions, whereas, in " 8 " and " 9," the ratio of figures are tog and 118 respectively. We think it highly probable that this variation of strength in the two directions may have some bearing upon the wear of the tire and its resiliency on the road ; at any rate, the subject is worthy of investigation.


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