Q: What is a directional tire? A: Directional, also called unidirectional tires, are designed to offer optimum straight-line acceleration...

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Q: What is a directional tire?

A: Directional, also called unidirectional tires, are designed to offer optimum straight-line acceleration, dry-weather cornering performance, and wet-weather traction by utilizing large tread blocks and angled grooves and sipes (narrow slits).

Tread blocks are the portions of tire between the grooves. During dry weather, widely spaced grooves (larger tread blocks between) allow a greater contact patch for better cornering. In wet weather, grooves channel water away to avoid hydroplaning.

By optimizing tread design for a particular direction of rotation, directional tires provide the best compromise between dry and wet performance.

A directional tire’s diagonal grooves need to be oriented correctly (when viewed from behind the tire, they should point upward). So, a directional tire has arrows molded into the sidewall indicating the proper direction of rotation. When installing and rotating tires, right-side and left-side tires are not interchangeable.

Should a directional tire be installed backward, it will not hurt the tire, but performance — particularly wet-weather traction — will be reduced.

Periodic tire rotation is a good idea. But in cases where front and rear wheels have differing offset or diameter, tire rotation can be achieved only by dismounting/reversing/remounting each tire to the opposite side identical wheel.

Performance tires may also have one tread pattern on the inside of the tire and another on the outside. These need to be mounted correctly (out side versus in) on each wheel to provide the intended performance benefits. Look for a notation on the sidewall, indicating front versus back.

E-mail Brad Bergholdt at under-the-hood@juno.com