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The Truth About Worn Tires

Most of the information you find when shopping for tires is derived from new tires and tests of new tires. But you’re going to be riding on those tires for the next 30,000, 40,000 or 50,000 miles — and tires change over time and with wear. Where can you get information about how a set of new tires will perform later in their lives? Michelin North America, Inc. set the groundwork to open the discussion about worn tires during a press event at the Laurens Proving Grounds, their on-vehicle testing facility in Laurens, South Carolina.

Technically, a tire becomes worn the instant it hits the road. But in practical terms, tires undergo a transformation as the tread degrades from friction with the road surface over the course of use.

Tires are the only parts of the car that contact the ground, and the grip between tires and ground is the primary determinant of handling and braking.

Tire manufacturers engineer and design tires with three important factors in mind: composition, tread and contact patch.

Composition

Tires are still made of rubber, but every manufacturer has its own formula and additives. In recent years, increasing percentages of silica have been added to the rubber to reduce rolling resistance and help improve fuel economy, a trait that has been forced by the car companies. Adding silica complicates the goal of maintaining good traction, so manufacturers have been forced to tinker with their formulas and with other design aspects to achieve seemingly oppositional goals.

Tread

Tread design is almost as much an art as it is an engineering feat. All of the grip that a tire has comes from the material that touches the road. Tread serves the purpose of channeling away water with grooves and biting edges that aid in cornering and handling. Tread design can help prolong tire life and depends upon the tire’s intended use.

Contact Patch

The contact patch is the part of the tire that is on the ground at any given time. Designers control the shape of the tire to define the contact patch, which can be rectangular or rounded. With limited information about tire composition and tread design, the intuitive choice for contact patch might be rectangular, but Michelin engineers and designers have determined that a rounded contact patch performs better in most circumstances, delivering better traction and even tread wear.

Siping

This highly simplified look at tire design and engineering merely points out how very complex tires are, and how easy it is to get lost in the details when selecting tires for your car. For instance, there’s the subject of siping. Sipes are the thin slits that you can see on new tire tread. These cuts in the rubber are designed to allow the tread block to open and close when the tire contacts the road, creating more surface area for traction. Designers explore the options of sipe depth and shape in search of the ultimate performance. A deep sipe might allow for too much movement; a shallow sipe might disappear with light wear. A complex, wavery sipe might make it too difficult to release the tire from its mold during manufacturing.

Testing and Demonstration

To demonstrate the fact that there are differences between new tires and worn ones, as well as differences between new and worn examples of competitive brands, Michelin set up two demonstrations: an objective demo of straight-line braking and a subjective demo of closed-course track driving. Each demo took place on wet pavement with approximately one millimeter of water, because wet braking and handling represent the primary safety concern for consumers.

Tests were conducted with worn and new tires of two different brands. The differences between brands and states of wear were easily observable during the testing. The revelation from the demos was that the difference between new and worn performance among brands is not consistent — often dramatically so.

The Conversation Continues …

How can a consumer find out how a tire will perform as it wears? The real answer right now is that they can’t. A long warranty period only offers confidence that the tire will remain intact for a number of miles, but does not guarantee that performance will not degrade significantly even if the tire does not fail.

This is the conversation that Michelin wants to start. The question is: Who should be gathering and disseminating the information?

Kudos to Michelin for starting the dialogue. Now it’s up to independent retailers to pick up the string, and to journalists to follow up and hold tire and vehicle manufacturers accountable for providing more information about the performance of worn tires.

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