Edge Wear

Both-shoulder wear — both edges wearing faster than the center — commonly points to underinflation or overloading. The tire spreads wider than designed under load, putting extra stress on both shoulders. Single-edge wear — one shoulder only — more often indicates alignment, camber, or scrub. Identifying which pattern is present helps narrow the cause before deciding on a repair or correction.

Severe edge wear that exposes belt or cord material significantly increases the risk of casing failure. Even before reaching that point, a tire that has been run significantly underinflated — which causes edge wear — may have accumulated internal heat damage that is not visible from outside. Do not judge the structural safety of an underinflated tire only from the visible wear; get professional inspection when the history is uncertain.

Incorrect toe, camber, or axle alignment can cause one shoulder to wear faster than the other. Toe problems — where the front of the tire points inward or outward more than specified — often produce feathering and single-shoulder wear. Camber problems typically wear the inside or outside shoulder depending on the direction. Trailer axle misalignment produces a scrub pattern characteristic of the trailer running at a slight angle to the direction of travel.

Both-shoulder edge wear — where both the inner and outer shoulder ribs are lower than the center — typically points to underinflation or overloading. The tire spreads under load, causing both shoulders to carry more contact than designed. One-side edge wear — where only the inner or outer shoulder is lower — points to a mechanical cause: alignment, camber, or scrub. Trailer axle misalignment commonly produces inside-shoulder wear on both tires of an axle because the trailer tends to run slightly crabbed relative to the direction of travel. Measuring all three ribs separates the patterns clearly.

The controlling number is the lowest tread depth reading, not the average. If the shoulder rib has reached the fleet pull depth (commonly 6/32 inch steer, 4/32 inch other positions) or the federal minimum (4/32 inch steer, 2/32 inch other positions), the tire should be removed — regardless of what the center rib reads. As a practical rule for investigation, a difference of 4/32 inch or more between the shoulder rib and center rib depth warrants identifying and correcting the cause, even if all readings are still above minimums. Edge wear that appears suddenly or progresses faster than expected is a more urgent investigation trigger.