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Tires, Page 1 of 4

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Tires are among the most important and least understood parts of the vehicle. Everything concerned with driving -- starting, moving and stopping -- involves the tires. Because of their importance to driving ease and safety, learning the basics of tires will pay off in dollar savings and safe driving.

Common sense and good driving habits will afford maximum tire life. Fast starts, sudden stops and hard cornering are hard on tires and will shorten their useful life span. Make sure that you don't overload the vehicle or run with incorrect pressure in the tires. Both of these practices will increase tread wear.

For optimum tire life, keep the tires properly inflated, rotate them every six months or 6,000 miles (9,600 km), and have the wheel alignment checked periodically.

Inspect your tires frequently. Be especially careful to watch for bubbles in the tread or sidewall, deep cuts or under-inflation. Replace any tires with bubbles in the sidewall. If cuts are so deep that they penetrate to the cords, discard the tire. Any cut in the sidewall of a radial tire renders it unsafe. Also, look for uneven tread wear patterns that may indicate the front end is out of alignment, the tires are out of balance or are improperly inflated.

Tire construction
See Figure 1

Modern tires use a combination of materials to contain pressurized air. The foundation of the tire is the plies (layers of nylon, polyester, fiberglass or steel) just beneath the tread that provides flexibility and strength. Regardless of size, cost or brand, there are only three types of tires-bias, bias-belted, and radial, now the standard on all passenger vehicles.

Bias tires, the old stand-by, are constructed with cords running across the tread (from bead-to-bead) at an angle about 35° to the tread centerline; alternate plies reverse direction. Crisscrossing adds strength to the tire sidewalls and tread. When properly inflated, these tires give a relatively soft, comfortable ride.

Bias-belted tires are similar, but additional belts of fiberglass or rayon encircle the tire under the tread. The belts stabilize the tread, holding it flatter against the road with less squirm (side movement). Belted tires offer a firmer ride, better traction, improved puncture resistance and longer life than bias ply tires. Bias tires are now found mainly on antique vehicles to preserve their original appearance, and in some commercial applications.

Radial tires now rule the road as they are original equipment on virtually every passenger car and light truck. Radials are constructed with steel or fabric carcass plies crossing the tread at approximately a 90° angle, and two or more belts circle the tire under the tread. The sidewalls flex while the tread remains rigid, accounting for the characteristic sidewall bulge of a radial. The tread runs flatter on the road with a better grip and the inherently harsher ride is offset by superior handling and mileage.

Figure 1 Various components of a radial tire are shown in this cutaway view.
Various components of a radial tire shown in this cutaway view.

Tire technology
See Figure 2

Since 1985, when the 27.5 mpg CAFE (Corporate Average Fuel Economy) standards took effect, there were a lot of changes in tires. Tires are very important in meeting the CAFE standards because they are responsible for 20% of a vehicle's total drag. The reduced rolling resistance of radial tires has made them the standard design. However, even with all the recent tire developments, it has to be a compromise. A tire that handles well sacrifices tread wear; a soft-riding tire sacrifices traction; a tire that reduces rolling resistance and delivers improved fuel economy sacrifices braking stability.

Figure 2 Of the three basic tire types, the radial design has now become the standard.
Three basic tire types, the radial design has now become the standard.

Elliptic tires
See Figure 3

A variation of the radial tire is the elliptic tire-a polyester cord body within steel belts. It resembles a conventional radial, except that it has a slightly more squatty appearance. The elliptically shaped sidewall forms a curve to the point where the tire meets the wheel rim, allowing up to 50% higher inflation pressures without causing an uncomfortable ride. The higher inflation pressure reduces rolling resistance and can increase fuel economy up to 3 or 4% at highway speeds.

A conventional radial inflated to 35-40 pounds per square inch delivers better gas mileage, but also transmits more road shocks from the tread to the wheel because the sidewall is almost vertical. Lower inflation pressures tend to lessen road shocks because the curved sidewall absorbs much of the vibration. The elliptic tire has extremely thick sidewalls that maintain the curved shape even at high inflation pressures.

The problem with elliptic tires is that they require an entirely new wheel to hold the tire on the rim. A special "low flange rim" required for the elliptic tire will not support a conventional radial, nor will a conventional rim support an elliptic tire.

Figure 3 The elliptic tire uses inflation pressures as much as 50% higher than conventional radials.
The elliptic tire uses high inflation pressures.

Spare tires

A conventional wheel and spare tire weigh about 38 pounds, and the jack another 10 pounds, so the auto industry has turned a good deal of attention here toward saving weight by eliminating the conventional spare. There are three alternatives to the spare tire problem.

Temporary spare

The most prevalent alternative today is the Temporary Spare used on many new vehicles. The Temporary Spare is a special, emergency-only tire that is more compact than the original tire yet retains the outside diameter dimensions. These tires usually run at a much higher inflation pressure than the conventional tires. Check the sidewall of the spare or your owners manual for correct inflation pressures. The Temporary Spare is designed to be used only as a device to get the vehicle off the road or to a local service center to have the original tire repaired or replaced.

Space-saver® spares
See Figure 4

Now obsolete, the Space-saver was a special, emergency-only tire that was stored deflated and folded around the wheel when not in use. It was designed to be mounted on the vehicle and inflated only with a special inflation canister that is supplied with the tire. Once used, the canister was discarded and a new one purchased from the auto dealership, tire manufacturer or an auto supply store.

Figure 4 Space-saver spares required special handling to preserve their usefulness.
Click on the picture to enlarge the view

Self-supporting tires
See Figure 5

Self-supporting tires look like conventional radials with super-thick sidewalls. They retain the shape of an under-inflated radial even with no pressure inside. When deflated they can be driven as far as 50 miles with no handling problems. They can also be re-inflated without damage. The main roadblock to their use is the development of a workable low-pressure warning system to alert the driver that the tire is flat.

Figure 5 A conventional and a self-supporting tire. Note the extremely thick sidewalls on the self-supporting tire.
A conventional and a self-supporting tire. Note the extremely thick sidewalls used on a self-supporting tire.

Run-flat tires
See Figure 6

For years, tire engineers have been trying to develop a "run flat" tire-one that would allow the vehicle to run a given distance even if the tire loses air pressure due to a puncture. Drawbacks on prototypes developed by major manufacturers have been the need for expensive "tire-within-a-tire" designs, lubricants to cool the tire sidewall due to high temperatures generated when the tire operates at low air pressure, and the need for special wheels to support unconventional tires.

However, tire engineers developed a run-flat tire that uses a simple design, requires a conventional air valve and can be mounted on the wheel with conventional tire changing equipment. It is based on a steel-belted radial tire, but uses reinforced sidewalls for support in a run-flat condition. The tire weighs about 25% more than a conventional tire, but about 15 pounds per vehicle can be saved because the spare and the jack are unnecessary. The deflated tire can run about 100 miles (160 km) at a maximum speed of about 50 mph (80 km/h).

Figure 6 Cross-sectional view of a N-type "run-flat" tire.
Cross-sectional view of a N-type run-flat tire.

Tire selection

Tire design
See Figure 7

For maximum satisfaction, tires should be used in sets of four. Mixing of different types (radial, bias-belted, fiberglass belted) must be avoided. In most cases, the vehicle manufacturer has designated a type of tire on which the vehicle will perform best. Your first choice when replacing tires should be to use the same type of tire that the manufacturer recommends.

When radial tires are used, tire sizes and wheel diameters should be selected to maintain ground clearance and tire load capacity equivalent to the original specified tire. Radial tires should always be used in sets of four.

Figure 7 P-Metric tire coding .
P-Metric tire coding

Radial tires should never be used on only the front axle.

When selecting tires, pay attention to the original size as marked on the tire. Most tires are described using an industry size code sometimes referred to as P-Metric. This allows the exact identification of the tire specifications, regardless of the manufacturer. If selecting a different tire size or brand, remember to check the installed tire for any sign of interference with the body or suspension while the vehicle is stopping, turning sharply or heavily loaded.

Understanding tire sizes

The P-Metric system is now used as the measurement standard for passenger car tires. The first number in the P-Metric system is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28 inches.

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28 inches wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28 inches x 0.6 = 4.36 inches.

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by two,(remember that the diameter is made up of two sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example... 185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14 inches)= 577.6mm or 22.74 inches

Plus sizing

A popular trend with high performance wheels and tires is "Plus Sizing". This is done to improve both the performance and appearance of the vehicle. By using a larger diameter wheel with a lower profile tire it's possible to properly maintain the overall diameter of the tire, keeping odometer and speedometer changes negligible. By using a tire with a shorter sidewall, you gain quickness in steering response and better lateral stability. The visual appeal is obvious, most wheels look better than the sidewall of the tire, so the more wheel and less sidewall there is, the better it looks.

Tire performance rating
See Figures 8

Late model vehicles, whether the family sedan or the sleek sports coupe are lighter with considerably better suspensions and handling characteristics than their predecessors. It is only natural that drivers want tires that match the vehicle's capabilities.

The original push for high performance tires started in Europe, where tire manufacturers worked together with auto manufacturers to perfect the handling characteristics of high performance vehicles and where tires are speed-rated for performance.

Many drivers of high performance vehicles are familiar with the European speed ratings, where the speed-rated tire is matched with the speed of the automobile. U.S. tire manufacturers have adopted the European speed rating system. The most common speed ratings are:

  • M-for cars speed rated up to 81 mph, (130km/h).
  • N-for cars speed rated up to 87 mph, (140km/h).
  • P-for cars speed rated up to 93 mph, (150km/h).
  • Q-for cars speed rated up to 99 mph, (160km/h).
  • R-for cars speed rated up to 106 mph, (170km/h).
  • S-for cars speed rated up to 112 mph, (180km/h).
  • T-for cars speed rated up to 118 mph, (190km/h).
  • U-for cars speed rated up to 124 mph, (200km/h).
  • H-for cars speed rated up to 130 mph, (210km/h).
  • V (VR)-for cars speed rated up to 149 mph, (240km/h).
  • W (ZR)-for cars speed rated up to 168 mph, (270km/h).
  • Y (ZR)-for cars speed rated up to 186 mph, (300km/h).
  • Z -for cars speed rated 149 mph and over, (240km/h).

Current tire speed rating markings include the use of the service description to identify the tire's speed capability (P215/65R15 95V-maximum speed 149 mph). Previous customs included the speed symbol in the size designation only (P215/65VR15) and the speed capability was listed as "above 130 mph."

Any tire with a speed capability above 149 mph (240 km/h) can, at the tire manufacturers option, include a "ZR" in the size designation (P275/40ZR17). If a service description IS NOT included, the tire manufacturer must be consulted for the maximum speed capability (P275/40ZR17-speed capability is >149 mph). If a service description IS included with the size description, the speed capability is limited by the speed symbol in the service description.

Example...(P275/40ZR17 93W- maximum speed 168 mph).

Figure 8 Old vs. new speed ratings.

Old Speed

New Speed


















Over 130

The contact patch

The shape of a tire's contact patch or "footprint" greatly influences its performance and is dependent on its profile or "aspect ratio". Low profile tires (most performance tires) have a short and wide contact patch that is effective in converting the driver's input into very responsive handling, cornering stability and traction, especially on dry roads.

High profile tires (light truck and most passenger tires) have a long and narrow contact patch which helps to provide predictable handling, a smooth ride and especially good traction in snow.

Tire types

Original Equipment (OE) tires are usually a compromise. The vehicle designer needs to blend handling, noise, ride, and wear to create a "perfect tire" for the average driver. Most people find that their needs may be biased more toward handling or ride than the average driver. When replacing OE tires it is recommended, and in some municipalities required, that the replacement tires are of OE specifications or better. Tires are known by many names and styles. Each type has a slightly different function, as well as different capabilities:

  • Max/Exotic performance tires are usually technically advanced, combining expensive materials and precision lightweight construction techniques to provide superior handling in wet or dry conditions and have extremely high speed capabilities (Z rated or higher).
  • Ultra high performance tires are the top level of tires that are available in a wide range of sizes. They are low profile tires designed to give high speed capabilities and quick steering response with outstanding cornering in wet or dry conditions (V or Z speed rating).
  • High performance tires are a slight upgrade from OE tires in terms of handling. They offer nearly the handling of the ultra high performance tires, but at a significantly lower price (H or lower speed rating).

Ultra high performance and high performance all season tires are derived and usually based on the ultra high/high performance non-all season tire. They are altered in tread pattern to gain in snow traction and the rubber compound is changed to be pliable over a wider temperature range. The snow capabilities of this type of tire is minimal, but is usually enough to get through an inch or two of snow (H to Z speed ratings).

  • Touring tires combine the appearance and responsive handling of a performance tire with more of a smooth ride quality and lower noise levels. this type of tire will, typically wear longer than a performance tire (S to V speed ratings).
  • All season (or mud+snow) tires are aimed mainly at domestic sedans, these tires offer very long tread wear, plush ride, and predictable handling.
  • Snow tires are specially constructed to grip snowy surfaces. The rubber is specially formulated to stay pliable in the cold and provide better traction on ice. The interlocking tread pattern of the snow tire allows it to bite the snow for additional stop-and-go traction.
  • Competition tires are the highest performing street legal tire you can purchase. These are special purpose tires that feature a very shallow tread pattern, stiff construction, and high grip tread compounds. Usually these tires are very prone to hydroplaning, and offer practically no tread wear.

A few words on snow tires

If the weather in your area includes strong possibilities for snow or ice, we highly recommend that you consider mounting 4 snow tires on your car, truck or sport utility vehicle every winter. Most snow and ice tires are made of specialized rubber compounds which will maximize the available traction on the cold, slippery surfaces found in winter driving. Most snow tires include specialized tread patterns to channel snow away from the tire (allowing it to find traction on the pavement underneath). Some snow tires use siping (slits cut into the rubber allowing tread to flex outward, increasing the available surface area) to help increase the amount of rubber in contact with the road.

Although snow tires cannot work miracles (and must obey the same laws of physics as other tires) they will provide the best combination of driveability and increased traction that you will find in a foul weather tire. Even if you have a four wheel drive vehicle, snow tires will increase the ability of the vehicle to deal with foul weather. Remember that a four wheel drive may offer certain advantages in foul weather, but it doesn't do you any good if the tires do not provide sufficient traction.

Note: that snow tires, whether 2 or 4, will affect vehicle handling in all non-snow situations. The stiffer, heavier snow tires will noticeably change the turning and braking characteristics of the vehicle. Once the snow tires are installed, you must re-learn the behavior of the vehicle and drive accordingly.

Most manufacturers strongly recommend the use of 4 snow tires on their vehicles for reasons of stability. If snow tires are fitted only to the drive wheels, the opposite end of the vehicle may become very unstable when braking or turning on slippery surfaces. This instability can lead to unpleasant endings if the driver can't counteract the slide in time.

Consider buying extra wheels on which to mount the snow tires. Once done, the "snow wheels'' can be installed and removed as needed. This eliminates the potential damage to tires or wheels from seasonal removal and installation. Even if your vehicle has styled wheels, see if inexpensive steel wheels are available. Although the look of the vehicle will change, the expensive wheels will be protected from salt, curb hits and pothole damage.

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