Tires are undoubtedly the most critical safety component on a vehicle. Where the rubber meets the road affects traction, handling, steering, stability and braking. Because of this, a sudden tire failure can have serious consequences especially if it occurs at highway speeds in a vehicle with a high center of gravity.

It's amazing that tires hold up as well as they do considering their vulnerability to road hazards. Many tires today are easily capable of going 60,000 to 80,000 miles or more -- provided they are properly installed, maintained, aligned and inspected regularly. With proper care and "normal" use, most tires will go the distance without a problem. But sometimes tires fail. Maybe it's the installers fault, the manufacturer's fault, the motorist's fault or nobody's fault. Even if the failure rate is only one in a million tires, a trial lawyer will argue it's one failure too many for his client.

The purpose of this column is not to assign blame for tire failures but to examine some of the causes and ways tire dealers can minimize the risk of such failures.

TIRE FAILURES CAUSED BY UNDERINFLATION

Overinflating tires increases ride harshness and may increase a tire's vulnerability to damage caused by potholes and curbs. Overinflation occurs when somebody adds air to a tire until it "looks full" or doesn't use an accurate tire gauge. Or, the person may be attempting to reduce rolling resistance for better fuel economy by adding extra air. Never exceed the maximum pressure rating on the side of the tire. Always follow the OEM recommended inflation pressure for vehicle load and operating conditions.

TIRE FAILURES CAUSED BY OVERLOADING

Overloading a vehicle or driving on tires that do not meet the load rating requirements for the application is asking for trouble. This is more of an issue with pickup trucks, vans and SUVs than it is passenger cars, especially those that may be used as utility vehicles to haul building materials or other unusually heavy loads.

The best way to prevent this kind of failure is to check the load rating of the tires and make sure they match the application. If they do not, recommend upgrading to a tire with a higher load rating.

The load rating is a two digit code that is just ahead of the tire's speed rating on the tire sidewall. If the tire says P205/65R15 86H, the 86 is the tire rating code and the H is the speed rating code.

Load index ratings for passenger car/light truck tires range from 71 up to 110. The higher the number, the more weight the tire can carry:
Sample Load Index Ratings:
71 = 761 lbs. (345 kg)
75 = 653 lbs. (387 kg)
80 = 992 lbs. (450 kg)
85 = 1135 lbs. (515 kg)
90 = 1323 lbs. (600 kg)
95 = 1521 lbs. (690 kg)
100 = 1764 lbs. (800 kg)
105 = 2039 lbs. (925 kg)
110 = 2337 lbs. (1060 kg)

TIRE FAILURES CAUSED BY IMPACT/ROAD HAZARDS

Most motorists will try to avoid really bad potholes, debris on the road and curbs, but sometimes these hazards are unavoidable and cut, puncture or damage a tire. If the damage does not cause the tire to go immediately flat, it may weaken the tire and cause it to fail later or under high speed/load/temperature conditions. Ultra low profile tires are especially vulnerable to sidewall damage when hitting curbs or potholes because of the narrow distance between the tread and rim.

The only way to prevent this kind of failure risk is to avoid driving altogether. Since that isn't practical, the next best thing is to steer around road hazards whenever possible, and to inspect the tires for possible damage if the vehicle ran over something really nasty.

Technicians should make it a point to inspect the tires for possible damage anytime a vehicle is in for service. If a tire has gone flat or punctured, it should be removed from the rim and carefully inspected inside as well for damage. If damaged, the tire is unsafe and must be replaced.

TIRE FAILURES CAUSED BY STRUCTURAL DEFECTS

Here's the category trial lawyers love best because it often means huge liability settlements when defects can be proven. Manufacturing defects that result in poor adhesion between the tread and belts can result in tread separation and blowouts. Fortunately, such defects are rare and are usually covered under warranty.

This risk can be reduced by inspecting new tires when they are first installed and inflated for obvious defects such as bulges, lumps, cracks, excessive runout, etc. Tires should also be inspected for defects or damage anytime a vehicle is in for service. Any tire that is bulging, cracked, has missing chunks of rubber or similar problems is unsafe and should be replaced immediately.

Technicians should be warned that puncture repairs that do not plug the hole may allow moisture to penetrate the tread and reach the steel belts. This can lead to rusting and increase the risk of tread separation and tire failure.

TIRE FAILURES CAUSED BY EXCESSIVE SPEED

Driving at sustained high speed on tires that are not speed rated or are badly worn is just plain stupid, especially during hot weather or with an overloaded vehicle. Speed-rated tires have additional reinforcements and are better able to dissipate heat than ordinary tires and should always be used for these types of applications.

Make sure the speed rating on the tires matches the vehicle requirements and your driving habits. If they do not, you should upgrade to an appropriate speed-rated tire. The speed rating is on the tire sidewall, and is the last letter in the tire size. Example: the speed rating on a tire marked P205/65R15 86H is the letter H, which is 130 mph.

Tire Speed Ratings:
R = 106 mph (170 km/h)
S = 112 mph (180 km/h)
T = 118 mph (190 km/h)
U = 124 mph (200 km/h)
H = 130 mph (210 km/h)
V = 145 mph (240 km/h)
Z = 149 mph (240 km/h)
W = 168 mph (270 km/h)
Y = 186 mph (300 km/h)

TIRE FAILURES CAUSED BY INSTALLATION ERRORS

Whoops. Tires can be damaged if they are not mounted properly. Not using a bead lubricant when mounting a tire on a rim, stretching or tearing the bead because the tire was not correctly positioned on the rim or in the rim drop center can cause bead damage that may allow a tire to leak air or suffer a bead failure later on. Overinflating a tire in an attempt to seat it or failing to fully seat the tire can also lead to problems when the tire is returned to service. We've even heard of instances where people have tried to mount a tire on the wrong sized rim (watch out for 16.5 and 15.5 inch rims).

The best way to avoid these kinds of mistakes is to make sure the technician who is mounting your tires knows how to use his tire mounting equipment. The tire changer machine must also be in good condition.

TIRE FAILURES CAUSED BY WEAR

All tires wear as they accumulate mileage, and eventually they wear out. Tires have wear bars (flat spots)in the tread grooves to visually indicate wear. If the tread is worn down so the wear bars are flush with the surrounding tread, the tire is worn out and needs to be replaced. If you see cords showing through the rubber, the tire is unsafe to drive on and is on the verge of failure. Replace the tire without delay! The same advice goes for any tire that has bulges, deep cracks or the tread is separating from the casing.

Some experts now say the same test should now be done with a quarter. If the top of Washington's head is flush with the tread when you place a quarter upside down in a groove, the tread depth is 4/32-inch (3.2mm). Though the tire still has some tread wear left, braking, traction and handling are significantly reduced compared to a tire with more tread on it. Because of this, many experts now recommend replacing tires when the tread depth is worn down to 4/32-inch or less.

TIRE FAILURES CAUSED BY OLD TIRES

Tires do not last forever, even if the tread shows little visible wear. As rubber ages, it loses elasticity, hardens and can become brittle. The reinforcing cords inside a tire can also deteriorate and lose strength. This increases the risk of a sudden tire failure the older a tire gets. After six years, the risk of failure goes up sharply. Because of this, many safety experts say tires that are more than six years old have expired and should be replaced regardless of how much tread is left on the tires.

All tires have a date code stamped on the sidewall. You will find it in a little recessed rectangle on the side of the tire. The date code reveals the week the tire was manufactured, and the year.

Before 2000, the date code had three digits. Since 2000, it has had four. The first two digits are the week of the year (01 = the first week of January). The third digit (for tires made before 2000) is the year (1 = 1991). For most tires made after 2000, the third and fourth digits are the year (04 = 2004). In the photo below, the date code is 0806. That means this tire was manufactured during the eighth week of 2006.

Click photo to see larger image of date code