From construction elements to tread design, the goal is producing the perfect performer.
To say there´s more than meets the eye is a gross understatement. A tire is not created by a one-shot injection-molding process. Rather, a tire is constructed piece by piece to create a layered "carcass," which is subsequently "skinned" by a rubber compound in a mold to create the outer sidewall/tread outer body.
CONSTRUCTION ELEMENTS OVERVIEW
The parts, or elements, of a tire´s construction include the bead assembly (this may include a "bead bundle" of steel wires, which are encased in a "bead filler" and entombed inside the bead wrap, or new technology rubber that complement this needed stiffness and bead stability.
The body plies wrap around the bead and run across the body of the tire to the opposing bead. An inner liner of rubber coats the inside of the tire´s air chamber, serving as a seal (sealing against air and moisture to ensure air pressure containment and to prevent moisture from attacking the carcass).
Belts and cap plies are positioned under the tread to provide tread reinforcement and to help provide the particular handling and ride characteristics intended by the design engineers. This bead, body ply, belt and cap ply assembly is generically referred to as the carcass.
The rubber compound material that encases the carcass and provides sidewall and tread material is placed onto the carcass and is formed to a specific shape and contour in a tire mold. This is followed by finishing and curing procedures that are closely guarded by tire companies (each has its own procedures and tricks of the trade, and are treated with a tremendous amount of security).
A SYSTEM UNTO ITSELF
Designing today´s ultra-high performance tires is very similar to designing aircraft shapes. While aeronautical engineers concern themselves with the displacement and directional and load management of air and concerns with temperatures, performance tire engineers must consider even more factors such as friction, temperature, mechanical flex and distortion, weight, water management and more. Most consumers don´t have an understanding of what is involved in ultra-high performance tire technology. If they did, they´d realize that these tires are a bargain at any price.
It´s completely inaccurate to consider a performance tire by only one aspect (tread width, aspect ratio, tread design, etc.). Each element of a tire´s construction must work together in a complex system approach, with each component affecting the other.
Ultra-high performance tread patterns have noticeably evolved over the years. One aspect that has changed deals with wet weather driving and the tire´s ability to evacuate water while maintaining high dry traction properties. Where a very blunt "V" pattern may have previously worked well with the tread compounds of the time, in the 1990s the trend was to use a steep V pattern to move more water. It was found that while a blunt V may have moved water quickly, the movement may have been too abrupt, creating unwanted turbulence. By moving water off to the sides more gradually, it was found that the water actually moved out of the tread faster, because water turbulence was greatly reduced.
From a compounding standpoint, today´s ultra-high performance tires are able to take advantage of proprietary polymer blends that in turn allow the use of wide channeling grooves, due to advances in compounding materials. The objective is to provide maximum dry grip and water evacuation without the need of one benefit sacrificing another.
Another change as part of the evolution of ultra-high performance tires involves the bead. The bead must provide stiffness in order to create adequate stability in severe lateral loads and at high speeds. Likewise, the tread area must provide stiffness for the same reason. The "hinge" area between the bead and tread is the area that flexes (sidewall). With the low aspect ratio of today´s ultra-high performance tires, this hinge area is kept at a minimum. This is the mechanical aspect of a tire that provides its "handling" capabilities.
Consumers tend to place too much emphasis on tire dimensions, assuming that tread width automatically translates into handling performance. The tire must actually be viewed as a complex system, where all of the design elements act in unison to create the tire´s performance characteristics. In other words, while size may be important, it isn´t the all-inclusive answer.
A small alteration in compounding can affect required tread design, which can affect bead stiffness, etc. One of the reasons that technology is changing so rapidly in ultra-high performance tires is the tire designers´ ability to utilize complex computer modeling programs to test specific construction and design elements without the need to build prototypes and perform exhaustive testing. Through computer analysis, they´re able to create tire system scenarios that narrow the potential parameters much more quickly. Once a tire system has been optimized, the tiremaker can then build prototypes and perform track testing in a much faster timeframe. They can finalize new tire technology much faster than dreamed possible only five to 10 years ago.
TIRE COMPOUNDS
The tread compound represents the material of the tire that makes contact with the road. In order to gain a basic understanding of tire tread compounding, we must first appreciate and understand the complexity involved in overall tire design. Any tire represents a balanced compromise among a variety of performance factors. If one design or manufacturing variable is changed, others are affected as a result. Since any single element of tire design, by itself, does not define a tire´s characteristics, tire design is anything but simple.
The realization of a tire´s performance characteristics is achieved through a concerted effort among a variety of elements, including tread compound, tread design, undertread design, sidewall and bead design, reinforcement, etc. The variables and possibilities are staggering, since each basic design element may also involve a variety of choices in terms of material, and the way in which those materials are connected and cured. Tread compounding is but one facet of a tire´s overall potential and character.
If a customer wants a tire to "handle" better, in terms of response and lateral grip, tread compounding will certainly be one of the areas to consider, but the end result involves a working inter-relationship among all design elements operating in harmony. Increased grip won´t be limited to a "simple" change to a "softer" compound, but may involve a rethinking of the sidewall construction, undertread, bead reinforcement, tread design, materials used, and the curing process to name but a few ingredients. By the same token, if one of those areas is changed, the compound may need to be altered to compensate.
As performance is gained in one area, some performance may invariably be lost in another area. For instance, if a change is made to increase traction, then rolling resistance and tread wear may be adversely affected as a result. Tire designers must remain fully aware of the chain reaction that will be caused by any given single change in the overall formula. The tire design as a whole is always considered, as opposed to only one or two elements. The rule-of-thumb is that one aspect always affects another.
The tread compound is formulated to play a part in achieving a targeted balance of rolling resistance, wear and traction. In that regard, three basic material categories are considered: elastomers, fillers and processing oils.
The elastomers include the rubbers and polymers that are used to provide the compound with its elastic properties. There are hundreds of polymers that may be used in an incredibly wide range of combinations. While the majority of lay people and even performance enthusiasts are in the habit of referring to "rubber," the fact is, generic use of that term isn´t really correct. In reality, an incredibly sophisticated process is involved in polymer selection.
The fillers used in compound creation involve various types of carbon black, styrene and silicas to name but a few. The filler´s job is to provide a reinforcement for the elastomers, from a dynamic standpoint. For example, while styrene may be advantageous for dry grip on pavement, it doesn´t work well for winter tires subjected to cold temperatures. In this case, more silica may be used to keep the compound pliable under extreme cold conditions. To use an engineering term, the filler helps to provide the "hysteresis level" of the tread.
Hysteresis is the measure of the tread´s energy absorption. A "high hysteretic" compound indicates that the tread absorbs more energy (in simple terms, easier to deform), resulting in increased rolling resistance... it takes more energy to roll down the road. A "low hysteric" compound is more resilient, absorbs less dynamic energy, and is therefore lower in rolling resistance. The result is that the vehicle exerts less energy, which translates into more effective use of the engine´s power and, at least in theory, reduced fuel consumption.
For purposes of illustration, a tire that was made of Silly-Putty would have a very high hysteretic level; while a solid steel railroad car wheel would have a low (near zero) hysteretic level. That´s a very simplistic explanation, but it really illustrates the point. The softer of the two deforms more under load, and thus requires more energy to roll. It´s easy to relate to this by comparing an underinflated tire to a properly inflated tire on a "dead" car that must be pushed from behind. If the car´s tires have very low air pressure, the car is more difficult to push. If the tires are properly, or even over-inflated, the car is easier to push. It´s a crude analogy, but again, it makes the point.
A wide variety of oils may be used during the manufacture of the compound, which serve to adjust both the resiliency of the tread in the finished product, as well as the ease of extrusion in the mold. Often, these two directives are opposed: The easiest material to extrude may not give the best in terms of performance, while the better-performing tread may be the most difficult to manufacture. The selection of processing oils helps designers to achieve optimum results in both tread performance and manufacturing ease and quality.
While a tread compound is created through a balancing act of polymers, fillers and processing oils, the job doesn´t stop there. Other variables enter the picture during the manufacturing process itself. The curing system (involving sulfers, activators, etc.) and curing temperatures and cycles are adjusted to further define the tread´s performance character.
While not a dynamic performance variable, the appearance and life of the materials in terms of aging is nonetheless important. Part of a compounding formula will also include an anti-degradent system. This involves both anti-oxidants and anti-ozonates. Various specialty chemicals are added to the compound for long-term protection against environmental oxidation and ozone damage. When a tire begins to look a bit "brown," it´s a normal condition, as it´s actually bringing some of the anti-degradents to the surface. The anti-degradents are doing their job, sacrificing themselves to protect the tire materials. So, if a customer notes that a set of new tires look a bit "brownish" when they´re pulled out of the warehouse, don´t jump to conclusions. Granted, it´s evidence that the tires have been stored for a while, but it does not mean that they´re no good. If the brownish tint easily disappears when the tire is wiped clean, then don´t worry about it.
The compounding variables that are available (type of elastomers, type and concentration of fillers, type of oils and curing processes) make it possible to tailor the compound to meet desired requirements. Some attributes are mutually exclusive, while others are complementary.
As formulations in materials change to improve wet adhesion, it may be expected for the tire to lose something in wear resistance; or as a change is made to gain dry handling, there may very well be a sacrifice in terms of snow grip. By the same token, changes made to improve ride comfort may also help to improve wet adhesion. This is one of the challenges being met by creative uses of synthetic materials, in trying to minimize the performance compromise.
That compromise (of improving in one area while losing ground in another) has always been a primary challenge to designers. While a compromise in performance parameters will likely always exist, it is being lessened through the ongoing developmental efforts.
As an example, from 1980 to the present day, the tire industry has achieved more than a 50% reduction in rolling resistance, while performance has been improved. A 5% reduction in rolling resistance generally equates to a 1% improvement in overall fuel economy. It´s this industry-wide push for increased fuel economy that has placed additional focus on rolling resistance. While it´s traditionally been difficult to achieve a balance between rolling resistance and traction, the use of silicas and proprietary compound production methods have enabled designers to create less-compromising tires that are better at "doing all things well."
Probably the most important aspect to consider when discussing the subject of compounding is the enormity and complexity of the issue.
TREAD DESIGNS
The creation of a tread design involves a number of criteria. Aesthetically, the tread must be pleasing to the eye. The tread noise factor must be as low as possible. It must be able to provide adhesion on dry road and to shed water in wet conditions, and it must provide acceptable steering response and predictable control. Depending on the target use of the tire, one or more of these parameters will take priority.
In terms of an ultra-high performance tire, one prime direction generally takes precedence over everything else: It must handle responsively and it must provide maximum road grip in dry conditions. Other benefits may be obtained, but never to the detriment of the prime objective.
If the only directive was to produce a tire that provided maximum traction on dry roads, to the absolute exclusion of everything else, then a sans-design "slick" would provide the biggest footprint. However, it would provide no ability to evacuate water.
Aside from the willingness of most high performance tire buyers to make slight compromises in order to achieve a high level of handling and traction, an excessive noise level, or a noise frequency that is truly noticeable and irritating isn´t likely to be tolerated.
If the goal of a tire addressed one issue only (dry grip or wet grip, for instance), the designer´s job would be much easier. However, since a multitude of issues must be addressed, a tread designer´s job becomes very complex.
So, even though the tiremakers are fully capable of producing mega-sticky smooth faced treads, the reality of street applications demand that engineers take advantage of grooves, sipes, channels and passageways in order to extract the intended performance within a package that also meets acceptable standards in a variety of modes, including comfort, noise, braking, response, and dry and wet traction.
Therein lies the challenge. While some folks may think that tire companies change their tread designs simply to create a new "look," nothing could be further from the truth. Teams of skilled and highly knowledgeable engineers fight a never-ending struggle to create a superior performance tire.
The theoretical goal is to produce the perfect performer: one that provides outstanding wear resistance as well as compliance and comfort, offers maximum grip on dry roads, wet roads, snow and ice, provides complete road hazard protection, is absolutely quiet at any speed, provides stability, agility and progressive response in any situation, and allows the vehicle to brake surely, quickly and confidently.
Since it´s probably impossible to create a single tire that provides ultimate performance in every category, tire engineers establish a priority target and strive to create the best compromise when creating a specific tire model.
TREAD ELEMENTS
While the reason for a block, groove, sipe, radiused shoulder or other design aspect of a tread may have varied intentions (depending on the overall design package), certain aspects of a tread makeup can be generalized for the sake of basic understanding.
The shoulder of the tread (where the tread face rolls over toward the sidewall) provides a continuous contact with the road surface during turning maneuvers. This is what´s most responsible for overall "handling" and grip during lateral moves.
The tread blocks (the raised elements of the tread face that contact the road) provide traction. They will vary in size and shape depending on a number of design parameters.
Grooves provide an avenue for water escape, essential for wet weather performance. Their job is to eliminate water from the tread at the contact patch. Longitudinal grooves (also called circumferential grooves) route along the circumference of the tread, while lateral grooves run "sideways" at a design angle relative to the circumference of the tread. Large grooves move water quickly, while narrower grooves are used to "fine tune" water movement, tread noise and traction.
Sipes are very narrow slits that provide an extra bit of water removal, and provide an extra biting edge that helps to move water and to grip in snow, ice and loose dirt.
The shape, number and size of a tread´s grooves are not chosen at random. Rather, they are carefully planned through a combination of engineering experience, computer plotting and on-car testing.
Placement of the tire´s grooves, and the number of those grooves, changes depending on the tire´s intended design. Is it a summer tire? Is it an all-season tire? Is this tire designed to be great in the dry or excel in the wet? If wear is a concern, lateral grooves must be added. The entire subject is addressed based on the intended target. The desired goal dictates the shape, width and number of grooves.
Also, tread block shape and orientation can be changed to manipulate dynamic forces. Tread block walls can lean forward or rearward, and this orientation can change from the outside of the shoulder to the tread center. Choosing the angles of the tread block walls (the lateral groove walls) create additional opportunities to distribute forces.
By positioning grooves to channel water in a desired direction, tread blocks can be kept large, and bigger tread blocks result in a more beneficial void ratio (more rubber meeting the road surface, and fewer voids between tread blocks). A popular trend today involves the use of more angular grooves, geared to maintaining wet traction while keeping tread blocks as large as possible.