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Performance Tire Technology

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Performance Tire Technology

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).

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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.

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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.

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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.

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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.

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TIRE FOOTPRINT

The tire footprint is the contact patch created when and where the tire meets the road. Circumferential grooves play an important part in controlling the footprint. These grooves provide an efficient method of reducing the hydro-dynamic pressure created when driving on wet surfaces. A series of longitudinal grooves allows the water to move out of the contact patch quickly. This is especially critical in the vulnerable center of the contact patch, since this area of the tread can lift off of the ground sooner than the shoulders of the tire. In as little as 3mm of water, the tires may commonly create a crescent-shaped contact patch as speed increases (changing from the original rectangular contact patch that the tires created when the car was parked). The increasing hydro-dynamic pressure begins to change the contact area, reducing that contact patch area. In some tread designs, circumferential grooves are supplanted by long, sweeping grooves that more effectively split the water evacuation paths.

A short (from front to rear) and wide (from side to side) patch indicates that the tire will respond quicker to driver input, as the steering wheel is turned. A wide footprint also means more rubber on the road during lateral maneuvers. This means (providing the tread design and compound are agreeable) that a tire with a wider footprint provides more lateral support, which means higher cornering force and traction. A short and wide footprint is typical on most performance tires. As the tire aspect ratio decreases on "low profile" tires, the footprint becomes wider and shorter (a 40-series aspect ratio will provide a wider and shorter footprint than a 60-series aspect ratio, etc.).

A narrower and longer footprint, such as that found on typical "high profile" tires, is more inclined to result in less-responsive handling and a smoother ride. A narrow and long footprint also provides superior traction on snow-covered surfaces.

SIDEWALL RIM PROTECTOR RIBS

Many high performance tires now feature thick and narrow "rim protector" ribs on the outer sidewall. This narrow raised rib, located near the bead area, is designed to reinforce and protect the sidewall, especially during mounting and dismounting. This is especially useful on many of today´s low profile tires that feature short, stiff sidewalls. Many of these tires are sometimes more difficult to mount because of the tight fit experienced. These raised ribs provide added reinforcement as the sidewall is flexed and "stretched," and the ribs also serve as "sacrificial" material to help safeguard expensive rims against scuff damage.

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TIRE SIZING

A number of tire sizing systems are currently in place in both passenger car and light truck tire applications.

In terms of passenger car tire sizing, six systems exist. The systems include the P-metric, the alphanumeric, the millimetric, the European metric, the ISO metric and the numeric.

1. P-metric. The P-metric system is the most widely used in North America on the vast majority of passenger performance tires. Developed in the 1970s in an attempt to standardize tire sizing, this system uses a combination of metric and inch designations. The "P" simply indicates that the tire is designed for passenger car use. The first number represents the tire section width in millimeters. This is followed by the aspect ratio percentage figure (the relationship of the tire´s section height to its section width), while the rim diameter is denoted in inches. For example: a P245/50R16 designation means that this passenger tire has a section width of 245mm and an aspect ratio of 50 (the tire section height measures 50% of the section width). The R means that the tire is of radial construction. The 16 indicates that the required rim diameter is 16 inches.

2. European metric. This system is essentially the same, except the aspect ratio may or may not be indicated (if the tire has an 82 aspect ratio, it may not be marked. If the tire has an aspect ratio other than 82, then it would be included in the marking). The label includes the section width in millimeters, the speed rating letter, an "R" if it´s a radial tire, and the rim diameter. Oddly enough, even though this is a so-called "metric" tire, the rim diameter is indicated in inches. For example, a 175SR14 indicates a radial tire with a 175mm section width, an "S" speed rating, and requiring a 14-inch diameter rim. If the aspect ratio is indicated, that number will follow the section width number (such as a 195/70SR14).

3. Millimetric. An offshoot of the European metric system is the millimetric system, in which both the section width and the rim diameter is identified in millimeters instead of inches. For example, a 255/50VR390 indicates a tire with a section width of 255mm, a 50-series aspect ratio, a V speed rating, radial construction, and requiring a rim diameter of 390mm.

4. ISO (International Standards Organization) metric. With ISO metric sizing, the tire´s load rating is added to the main size label, which follows the normal metric sizing system rules. For example, a 185/60R14 82H indicates a tire with a section width of 185mm, a 60-series aspect ratio, radial construction, requires a 14-inch rim, a load index of 82 and a speed rating of H.

5. Numeric. The oldest of the six systems, it is very basic. The label refers to the tire´s section width and rim diameter only. A 8.25-14 indicates a tire with a section width of 8.25 inches and designed to mount to a 14-inch wheel. This system was used back when there were not a lot of aspect ratio choices. Basically, there were two common aspect ratios: 92 and 82. If the section width number ended in zero, then assume that the tire had an aspect ratio of about 92. If the section width number ended in a number other than zero, then assume that the aspect ratio was about 82. For instance, a 7.00-14 had an aspect ratio of about 92; while a 7.75-14 had an aspect ratio of about 82. The actual aspect ratios could vary wildly, as each size/model of tire could have an aspect ratio anywhere in the approximate range of the 92 or 82 index. This system is important when dealing with vintage tires.

6. Alphanumeric. This tire sizing system was used in the 1960s, and remains applicable today for folks who collect and restore many domestic muscle cars (when the car owner wants to make every effort to follow all original specs and details, he/she will locate new replicas or retreaded versions of the original tire brands, models and sizes). This system used a letter to designate the tire load and one aspect of its overall size. The aspect ratio indicated the relationship between section height and width.

For instance, a GR60-15 indicated a tire with a "G" load and height rating, radial construction ("R"), a 60 aspect ratio, and a required rim diameter of 15 inches. The first letter (the load/size rating) ranged from A through N. The higher the letter, the bigger the tire and the more load it could handle. The now classic 1969 Camaro Z28, and similar-vintage cars like the Mustang, GTO, Dart, Barracuda and others, were all equipped with alpha-numeric tire sizes. For example, the 1968-72 Corvettes were equipped with F70-15 nylon belted tires; and in 1973 were outfitted with GR70-15 radials. It wasn´t until 1978 that the ´Vette was shod with P-metric tires (P255/60R15 or P225/70R15 radials). In 1984, the Corvette switched rim size, using a P255/50VR16. In 1989 a P275/40ZR17 was used, and in 1993 the Corvette started using dual-sized systems, with a P255/45ZR17 up front and a P285/40ZR17 in the rear. The 1997 version switched to P275/40ZR18 rear tires.

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SPEED RATINGS

Tire speed ratings are widely misunderstood. This system was initially adopted in Europe in an effort to create a standardization of performance tire ratings that would indicate a speed index at which that tire is certified. The ratings number does not indicate the top speed potential of that tire. Also, a speed rating does not make the car faster or slower... it simply provides an indication of its certified top speed.

In addition to providing a "safety" rating for the tire´s capabilities, a speed rating serves as an indication of the tire´s potential performance. The higher the speed rating, the stronger the implication that the tire maker has designed the tire for higher levels of overall performance.

As tire speed increases, it´s subjected to increased deformation and heat build-up. The higher the speed rating, the more reinforcement is used to control the tire´s tendency to grow and deform, and to control heat. While each tiremaker may use its own construction approach, one maker´s S and T rated tires may feature two steel belts to provide tread reinforcement. For a V rated tire, that same maker may use two steel belts plus nylon plies (either two nylon plies may be placed at zero degrees, running parallel to the circumference of the tread) or one nylon ply that´s folded over the edges of the steel belts.

One tiremaker´s Z rated tires may use a steel belt wrapped by a folded Kevlar belt. The folded Kevlar helps reinforce the shoulder area, while keeping weight down. These are merely examples of different construction and materials among various speed-rated models.

As mentioned earlier, the speed rating alone does not indicate the tire´s performance characteristics. Instead of selecting a tire solely based on its speed rating, the customer must make his/her choice based on the performance they want. Do they want a tire that handles responsively on dry pavement? Do they want a tire that is comfortable and quiet on the highway? Do they want a tire that specializes in wet handling and wet braking?

When replacing one speed-rated tire with a tire having a different speed rating, MTD suggests following the tire manufacturer´s recommendations.

Why do tire makers offer high-speed rated tires if the highest speed limit on public roads ranges from 55 to 70 mph? Because the tiremakers and the carmakers know they can´t control how people drive. After all, if a car is capable of travelling at a speed of 150 mph, the car maker wants to be sure the car is equipped with a tire that´s able to sustain that speed with a reasonable safety margin.

Only Department of Transportation (DOT)-approved tires are speed rated. The speed rating system is determined by the Rubber Manufacturers Association. A look at these organizations´ Web sites is a good idea - www.dot.gov and www.rma.org.

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TREAD PATTERNS/DIRECTIONALITY: Sidewall markings indicate proper installation

There are three basic categories of tread patterns in terms of directionality/functionality: non-directional, unidirectional; and symmetric and asymmetric.

A "unidirectional" tire is designed to roll forward in one direction only, while a non-directional tire can be positioned to roll in either direction of rotation.

A unidirectional (also called directional) tire can only be mounted and positioned in such a way as to rotate forward in its intended direction. A directional tire will be identified accordingly on its sidewall to indicate direction of rotation. This is done because the tire´s tread pattern (and possibly its internal construction) is designed specifically for dynamic movement in one direction. If unidirectional tires are properly installed on a vehicle, to rotate the tires (change vehicle corner location) in order to increase tread life, the tires can be moved from front-to-rear or rear-to-front without dismounting/remounting the tires. However, if the tires are to be swapped side-to-side, the tires will have to be dismounted, flipped over and remounted in order to maintain the tire´s rolling direction when reinstalled on the vehicle.

A tread pattern may be symmetric or asymmetric. A symmetric tread pattern is the same side-to-side (the left side of the tread is a mirror image of the right side of the tread). An asymmetric tread pattern will feature a different tread pattern layout on each side of the tread area. An asymmetric tread pattern is used to increase performance (quite often a tread may feature more grooves for water dissipation on the inboard area, and a broader tread block surface area on the outboard tread surface, for increased contact patch during hard cornering. Most asymmetric treaded tires are also unidirectional.

As far as mounting positions are concerned, always pay attention to the sidewall markings. The tire may be marked with an arrow that notes the direction of rotation, or it may be marked "this side out," also to indicate the proper mounting position on the wheel.

UTQG MARKINGS How tread life, traction properties and temperature resistance are rated

A Uniform Tire Quality Grade rating is required for all passenger car tires (except dedicated snow tires). This rating, required by DOT (Department of Transportation), identifies a tire with regard to expected tread wear, wet traction and temperature resistance. This rating is on the tire sidewall.

The tread wear rating is determined by the tire maker, as a result of wear-testing as compared to a ´control" tire selected by the tire maker. The control tire is assigned a rating of 100. This is used as a base number against which the test tires are rated. A tire with a tread wear rating of 150, for example, is expected to wear 1.5 times longer than the control tire. A tread wear rating of 240 indicates that the tire should wear 2.4 times longer than the control tire, and so on.

Understand that the tread wear rating is not a guaranteed, carved-in-stone rate of wear that will always be achieved with that particular tire. The reason is simple: Since the tread wear will be affected by a number of variables, including driver habits, the specific vehicle being used, road surface conditions, climate, tire inflation and wheel alignment settings and suspension/steering system condition and maintenance, the tiremaker cannot guarantee that a particular tire will last X-number of miles, simply because it´s impossible to make that type of prediction.

The tread wear rating is a good overall indication of the rate of wear that can be expected. In other words, given the vehicle and driving habits, etc., a tire with a tread wear rating of 240 should invariably outlast a tire with a rating of 150. A tire rated at 400 should provide incredibly long tread life, and so on.

The traction rating is indicated with a letter designation of A, B or C. This rating is based on a clearly-defined government test format that measures the tire´s straight-line stopping distance on wet pavement. The rating does not measure a tire´s handling traction during cornering maneuvers. An "A" rating is the highest. This means that a tire rated at "A" traction level will provide better straight-line wet braking performance (shorter stopping distance) than a tire rated B or C. Note: A "C" traction rating does not necessarily mean that the tire stinks during wet braking. It simply means that a B or an A tire will stop in a shorter distance than a C tire.

Temperature resistance (the tire´s resistance to failure and its ability to maintain performance as vehicle speed and tire temperature increases) is also identified with an A, B or C rating symbol. The C-rating represents the minimum for all passenger tires, as set forth under Federal Motor Vehicle Safety Standards. A B-temperature-rated tire will withstand high speed and temperatures better than a C-rated tire, and an A-rated tire will out-perform a B-rated tire in terms of heat resistance.

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PLUS ME!: Just keep an eye on wheelwell and suspension clearance issues

For customers who want to fill up the wheel well with lots of wheel, Plus-sizing is mandatory. Moving to a larger diameter wheel requires a shorter sidewall tire in order to maintain the original rolling diameter.

Keeping the original outer tire diameter is important for a number of reasons:

1. to avoid the need to recalibrate the speedometer

2. to avoid confusing the ABS (if so equipped)

3. to avoid altering final drive ratio (which would change the engine speed and related power band use)

4. to avoid altering ride height (any desired ride height changes can be handled via suspension changes).

Plus-sizing involves increasing wheel diameter while maintaining the original tire OD (outside diameter). There are two reasons to make a Plus move: to enhance appearance and to provide more rubber on the road (since use of lower aspect ratio tires results in a greater section width and tread width).

Plus-sizing is referred to by the growth in wheel diameter, in inches. A Plus-One indicates a move to a one inch larger diameter wheel than stock. Plus-Two indicates a two inch increase in wheel diameter from the stock size, etc. In the early days of Plus-sizing, increases to Plus-One, Plus-Two and Plus-Three were considered the "norm" in the pursuit of utilizing larger wheels and shorter sidewall tires (lower aspect ratios). However, today´s marketplace, based on consumer demand, has resulted in changes up to and including Plus-Nine fitments (for example, moving from a 17-inch wheel to a 26-inch wheel on some SUV applications).

To determine just how large a Plus Size is possible, follow the tiremaker´s fitment guide whenever possible. If the tiremaker does not recommend a certain size, then don´t do it. Wheelwell and suspension clearance issues aside, don´t alter the tire outer diameter beyond what the fitment guide suggests to avoid problems with vehicle gearing and electronic control systems.

CARRYING THE LOAD: The ´strength´ of the tires depends on the Max Load numbers

The maximum load ("Max Load") number, indicated in both kilograms and pounds on a tire´s sidewall, refers to the tire´s maximum load-carrying capabilities. For example: Max Load 990 kg (2183 lbs.) indicates that the maximum load for that tire, when inflated to its maximum recommended pressure, is 2,183 pounds.

Make sure you tell your customers about the load-carrying capacity of their new tires, and advise them never to exceed a tire´s maximum load rating. The tire is constructed to handle a specific load range (per the materials used and the construction design of its belts, beads, carcass and interliner). In other words, if a car weighs 5,000 pounds, don´t install a set of tires that are each rated at a maximum load of 1,000 pounds, since this would only provide a total load rating of 4,000 pounds.

Alpha-numeric-sized tires will provide their maximum load information in letter form. Most passenger car tires will carry a Load Range B designation, while most C, D and E tires are light truck applications.

Also, never exceed the maximum inflation pressure. The "Max Press" designation indicates the maximum operating pressure rating for the tire. This does not indicate its recommended normal inflation pressure. The tire may be designed to normally operate at 32 psi, for instance. The maximum pressure rating simply says this tire can be inflated to its max of, say, 40 psi

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