Cracks in Tread Grooves: Serious or Cosmetic?

What groove cracks are

The grooves are the channels that run around the tire between the tread ribs. Federal safety standards define a groove as the space between two adjacent tread ribs, and a tread rib as a tread section running circumferentially around a tire. Cracks in the tread grooves are thin splits in the rubber at the base of the groove or along its walls.

The same standards define cracking as any parting within the tread, sidewall, or innerliner that extends to cord material. That definition separates a cosmetic surface flaw from structural damage. A crack that reaches the cords is, by definition, a structural condition that takes the tire out of service.

Why groove cracks appear

Three causes dominate: age, ozone, and load. Rubber compounds slowly lose flexibility as they age, and ozone in the air attacks the rubber surface, creating fine cracks. The tread grooves bend and flex as the tire rolls, so they are a natural place for surface cracks to start.

Load matters because a tire carrying heavy weight flexes more, and the groove base is a high-strain area. Federal standards for tire selection require that the vehicle maximum load on the tire not exceed the maximum load rating marked on the sidewall, and that the normal load not exceed 94 percent of the rating at the recommended cold inflation pressure. Running a tire near its limit increases flexing, which can accelerate groove cracking. Keeping tires at the vehicle manufacturer's recommended pressure, not the maximum molded on the sidewall, helps keep flexing within design limits. See How Much Air Do Your Tires Need? A PSI Guide and Towing and Hauling for more.

How to inspect groove cracks

You can inspect groove cracks yourself with a bright light and a small probe, such as a plastic trim tool. Look at the base of each groove, especially the outer ribs and the areas near the sidewall. Wipe the groove clean with a cloth first so you can see the rubber clearly.

The critical question is whether the crack reaches cord material. If you can see fabric or steel cords at the bottom of a crack, the tire is unsafe and must be replaced. A crack that is wide enough to admit the tip of a pick also deserves close attention, because it may extend into the belt package.

Federal standards for new tires require that a tire show no visual evidence of cracking before it is subjected to endurance or high-speed tests. That same visual standard is a good field check: if you can see a crack that appears to enter the carcass, treat it as serious.

Age and ozone: the silent drivers

Age is the most common cause of groove cracks. The DOT date code on the sidewall tells you the week and year of manufacture; a tire that is six or more years old has likely lost some of its original flexibility, even if the tread looks deep. USTMA notes that today's tires are engineered to last longer, but no tire is immune to aging.

Ozone cracking is recognizable by a pattern of fine, closely spaced cracks that run across the rubber surface, often in the grooves and the sidewall. It is a surface condition unless it penetrates. If the cracks are only on the surface and the tire is within its age range, they may be cosmetic. But if the tire is older than six years, or if the cracks are deep, replacement is the safe choice. See Tire Age and Dry Rot and Tire DOT Code for a fuller guide.

When groove cracks mean replacement

Replace the tire if any crack exposes cord material, if the crack is wider than 1/16 inch, or if the crack is accompanied by a bulge or separation in the tread area. Also replace if the treadwear indicators are flush with the tread surface, because a tire at the end of its tread life is more vulnerable to groove cracking.

Federal standards set the minimum tread depth for passenger tires at 1/16 inch, and the treadwear indicators are designed to show when the tire has worn to that point. A tire that is worn to the indicators and also shows groove cracks is doubly unsafe. See Tread Wear Indicators and When to Replace Tires for full guidance.

If the tire is still well within its age and tread limits, and the cracks are only surface-deep with no cord visible, you may continue to drive on them, but check them at every tire rotation. A tire that develops deep cracks in more than one groove should be replaced.

Preventing groove cracks

The best prevention is to keep tires within their design limits. Maintain the vehicle manufacturer's recommended cold inflation pressure, avoid overloading the vehicle, and rotate tires on schedule to distribute wear. Check the pressure at least once a month, and before long trips.

Storage matters for spare tires and off-season tires. Store tires in a cool, dry place away from direct sunlight and ozone sources such as electric motors. The federal standards do not set a storage temperature, but a cool, dark location slows rubber aging. See How to Store Tires Properly for Long Life for a checklist, and How to Check Tire Pressure for the right way to set pressures.

What to pick for your use

Questions

Are small cracks in tire tread grooves normal?

Fine surface cracks in the grooves are common on tires that are a few years old, caused by age and ozone. They are cosmetic as long as they do not reach the cord material. Check the DOT date code and use a probe to test depth.

Can I drive on tires with cracked grooves?

You can drive on them if the cracks are only surface-deep and no cord is visible, provided the tire is within its age limits and has adequate tread. But inspect them often, and replace the tire if the cracks deepen or multiply.

What causes cracks in the tread grooves?

Age, ozone, and load are the three main causes. Rubber compounds lose flexibility over time, ozone attacks the rubber surface, and heavy loads increase flexing at the groove base, which can open up cracks.

How do I know if a groove crack is serious?

A crack is serious if it exposes cord material, if it is wider than 1/16 inch, or if the tire is worn to the treadwear indicators. Use a bright light to look for fabric or steel at the bottom of the crack.

Do groove cracks affect tire safety?

They can, if they extend into the belt package. The federal standard defines cracking as a parting that reaches cord material, so a crack that reaches the cords is a safety issue. Surface cracks that do not reach cords are less concerning.