This metal-base tire is made in two types, the pressed-on and demountable. In the larger cities, the former is quite popular, whereas, in the smaller outlying cities and towns the demountable type has the following. The reason for this is: To remove a pressed-on tire from a truck wheel or put one on a truck wheel requires a powerful press, which means a considerable outlay to the dealer in proportion to the amount of work he may get.
The demountable tire can be removed from the wheel and a new one fitted, without the truck owner having to take it to a garage. Unfortunately it is more expensive than the pressed-on tire, due to the forged and rolled steel parts used with it.
Crude rubber is a vegetable product gathered from certain species of tropical trees, shrubs, vines and roots. It was first used for pencil erasers and in waterproof cloth and finally in solution in cements. Vulcanizing or curing rubber was discovered in 1844; thereafter the development of the industry was rapid, though it was but an infant in size, compared with now, up to the development of the automobile industry.
There are many kinds and grades of rubber, and these may be divided into two classes, wild and cultivated.
This is collected from trees that have grown wild and where there has been no cultivation process. Such trees and shrubs are found mostly in Northern South America, Central America and Central Africa. Fine Para comes from the Amazon region of South America. For over a century this rubber has been gathered in practically the same way. The native goes into the forest, selects a tree, cuts V-shaped grooves in herring-bone fashion around the tree, with one main groove down the center like the main vein in a leaf. The latex of the tree (not the sap) flows from the smaller veins and down the center vein into a little cup placed to receive it.
When full these cups are gathered and brought into the rubber camp, and there the latex is coagulated by means of smoke. This is done by the use of a paddle, which is alternately dipped into a bowl of latex and then revolved in the smoke which seems to have a preservative effect on the rubber as well as drying it out and causing it to harden on the paddle, each successive layer of latex causing the size of the rubber ball or biscuit to increase. When a biscuit of sufficient size has been coagulated it is removed from the paddle and is ready for shipment. There are other grades of rubber which are coagulated by adding some alkaline solution and allowing it to dry out. Central America produces a grade of rubber which is cured by being mixed with juices which are obtained by grinding up a certain plant which grows in that district.
In Central Africa some of the rubber is gathered from trees, but most of it comes from vines and roots, and the methods of coagulation are varied. Practically all of them are dried out in the sun.
Cultivated rubbers are obtained from East India, Ceylon, Malayan Peninsula and southern Mexico. The claim is made that the best of these is the Ceylon rubber, which has been grown from sprouts taken from the wild Para trees of South America. These cultivated trees have been very carefully reared and scientific methods used in tapping them, so as not to in any way hurt the bearing qualities of the tree. This product is very uniform, as very scientific methods are used, coagulating, drying and otherwise treating the rubber before it leaves the plantation, so that there is a minimum deterioration due to oxidation and other actions during the time the rubber is en route from the plantation to the manufacturer. Of late, far east rubber is being given the preference, because it is cleaner and contains less foreign matter than the wild para.
This rubber as it comes into the market, contains a lot of impurities, and before it can be used it has to be washed. This washing is done between rolls which are grooved to tear the rubber apart, water being fed on the rolls to wash off all foreign matter. In this process the rubber loses considerable of its weight.
When the rubber is washed and dried it is mixed with chemicals and into a compound. These chemicals and particles of rubber are placed in metal boxes a couple of feet square. The formula for these compounds are, of course, kept secret by the rubber factories, because they represent the outcome of very long and tedious experiments which are looked upon as one of the chief assets of any rubber mill. These masses of compounds are chewed and rechewed, ground and reground, extenuated and re-extenuated, between the giant steel rolls of the calendering machines that are needed just to thoroughly mix the ingredients. Each compound requires its separate mixing, its special treatment.
The rims to which the hard rubber is vulcanized are in most cases copper plated, as rubber compounds do not take kindly to steel. The bard rubber base is applied to the metal rim and then the tire is completed as previously mentioned.
The compound rubber is in graduated consistencies from the bard rubber base, which forms the inside circumference next to steel base, to the resilient rubber forming the wearing part of the tire.
When the tire is completed it is pressed into moulds which are securely bolted and placed in large cylindrical vulcanizers which vulcanize or cure the rubber. When this process has been completed and the Urea have cooled, slight edges remain which are huffed off.