Tires look simple and they look much the same to the untrained eye. They are not. Tires are hiding very advanced engineering secrets inside their black rubber casings, and the variations in tire builds are endless. Even the black rubber itself is full of high tech and carefully guarded secrets. For each kind of tire, Pirelli, Michelin, Nitto and other manufacturers choose a specific blend of different rubbers for different parts of the tire. The rubbers are mixed in carefully determined proportions with process oils, carbon black, pigments, antioxidants, accelerators and various additives. Each component in the mix contributes a specific set of properties that the manufacturer wants in the finished tire. The physical build of the core of the tire is just as complicated and just as important. Today just about every tire for passenger cars is of the radial type. This type of tire was developed in the late 1940s, and was a great leap forward in tire technology, providing great improvements in road-holding properties and tire tread wear.
The body of a tire is the so-called "casing". It is made up of one or two layers of plies. In radial tires, the strands of the plies are positioned at a right angle to the edge of the rim, each strand of the ply material on one radius of the wheel (thus the term "radial" ply). In belted and belted bias tires, the strands of the plies are crisscrossed at angles to the rim edge, thus traversing several radiuses This older construction method, belted or diagonal ply construction, is still used for many heavy-duty tires. It makes the sides of the tires stiffer, and tires of this design are more likely to follow ruts in the road, thus making it more difficult to control the vehicle. The ply is made of a synthetic fabric impregnated with rubber. Most modern passenger car tires feature plies made in polyester.
This is a material that adheres well to rubber and is lightweight and strong. This is important, as heavy tires get warm, which can cause separation between the rubber and the fabric, with disastrous consequences. Nylon and rayon cords are also used, as they have properties similar to polyester. Next comes the belt system. The belts are designed to provide stability to the tread of the tire, to make it stay flat on the road. Most modern tires use two plies of steel belting for this purpose. They provide a lot of strength and stability without adding much weight.
The sidewalls are also carefully engineered, different for different applications. The sidewalls have to be designed to a specific rigidity. They must be flexible enough to not bend the tire tread up from the road surface on hard cornering, yet stiff enough to provide quick steering response, and strong enough to carry the load of the vehicle. To achieve this, yet other specialized rubber compounds are used for the sidewalls, which also have to be UV- and weather resistant. Some sports tires feature steel and/or nylon inserts for extra stiffness and quick steering response.
In the old days, tires had an inner tube to make sure that they were airtight. Inner tubes are still used, but in most cases their function is taken over by an inner lining in the tire. This lining is not reinforced with any kind of fabric, and is there only to make the tire airtight. Securing the tire to the rim was a big problem for a long time. In fact, it still is a problem in off-road and drag racing, where racers use specially designed bead locks to make sure that the tires don't come off during the races.
On passenger cars, the tire bead is sufficient to keep the tire on the wheel. The core of the tire bead is made of steel filaments that are wound together to a cable or ribbon. The steel cable is shaped into loops that are placed around the inner edges of the tire, and the cord of the casing plies is folded around the cables to hold them in place. Another specially designed rubber compound is used to fill the space around the steel cables, and to impregnate the casing fabric that is looped around the steel. This bead filling is a hard and wear resistant rubber compound that can take the strains of tire mounting and chafing. It fills the bead and goes up on the sidewalls.
The tread is the last part to go onto the tire. It is made in two layers, with the base layer made in a rubber that adheres well to the belt system when the tire is cured. The rubber compound of the outer layer is chosen carefully to suit the purpose and use of the finished tire. There are endless variations, with special rubber compounds for dry weather, wet weather, snow, sports car driving, energy efficiency, etc.
The tread is pressed onto the tire with rollers, that at the same time press all the components of the tire hard together. This makes the tire look almost like a finished tire. The tires in this stage of production are called "green tires" (which has nothing to do with the energy efficient, finished tires that are marketed as "green tires"). These tires are green in the sense that they are complete but not completed they are about to become the ripe finished product. The green tires are carefully inspected for flaws and then cured.The curing is done in a curing press, with a mold that is engraved with the tire tread, the lettering and the markings required by law. The curing takes place at about 300°F for 12-25 minutes, depending on tire size. When cured, the tires are popped from the molds and run through both manual and automated quality checks before being sent out to dealers.
The curing of the rubber is done to vulcanize the rubber. The different rubber compounds contain sulfur in certain proportions. When a mixture of rubber and sulfur is heated, the molecular structure of the rubber changes and new chemical bonds are created. The process was invented by Charles Goodyear in 1839 and has been refined over the years, now making it possible for tire engineers to give exactly the desired properties to the rubber that goes into different parts of the tires.
The basic structure of tires can be varied in innumerable ways. The stiffness relationship between the sidewalls and the tread varies, the design, the reinforcements and the rubber compounds of the tire shoulders are subject to a lot of experimentation when tire manufacturers are struggling to provide more grip on the very edge of a tires performance limits. The number of plies and the shape of the tires vary with load ratings and comfort levels. In addition to all the secrets hidden inside the tires, there is also the very complicated science of optimizing the tread patterns for different vehicles and driving conditions. The complications of tire manufacturing also make it complicated to choose the tire that is absolutely the best one for you and your vehicle. Choosing a good tire is not that difficult as the tough competition in the tire market makes sure that bad tires disappear very quickly. Qualified dealers and quality and price conscious consumers have driven manufacturers to produce tires of a quality that only could be imagined only a few years back.Today's tires are almost unbelievably strong. A few decades ago, punctures were commonplace. Now punctures are as rare as they were common before. If tires are changed according to manufacturer recommendations, a tire puncture is extremely unlikely.
The hidden secrets of modern tires are very difficult to uncover even when the tires are worn out. The tires are in fact so strong that they have become an environmental problem. They are virtually indestructible, and the mountains of worn-out tires are growing all over the world.