What Are Sidewall Indentations, Undulations & Protrusions?

Sidewall Indentations / Undulations

Fig. A: Vertical indentation on sidewall from rim to tread.

A common byproduct of radial tire construction, sidewall indentations/undulations are more noticeable in tires with taller sidewalls or operate at higher inflation pressures. Fortunately sidewall indentations are a cosmetic condition that does not affect the performance of the tires.

Tire casings are reinforced by multiple fabric cords encased side-by-side in a thin sheet of rubber. Radial ply tires feature one or more layers of cord (depending on the tire's required strength) that run parallel to each other from bead to bead (with each individual cord running up the sidewall, across the tread and down the other sidewall). Because of their "straight across" radial orientation, the overlaps where the sidewall cords are lap-spliced may cause indentations.

When the tire is being cured, it is pressed against its metal mold. However when the tire is mounted on a wheel and inflated to operating air pressures, it is free to expand. These overlapping splices may create slight indentations since the stretching capacity of the lap-splices is slightly less than the rest of the body ply. In reality, the splices are the most reinforced area of the tire's sidewall.

Since radial tires feature steel cord reinforcing belts under their treads to keep them flat, indentations only appears on the sidewalls.

Sidewall Protrusions

Fig. B: Vertical bulge on sidewall from rim to tread.

However, if protruding bulges, as shown in Fig. B (photo provided by a customer), rather than indentations appear on the sidewall of a recently installed tire, it reveals that there is an undesirable gap between some of the body ply cords inside the tire. The tire should be removed from service and replaced under the tire manufacturer's materials and workmanship warranty.

However if a protruding bulge doesn't appear until later in the tire's life, it was usually the result of a road hazard when several adjacent cords were injured as the tire struck a pothole, curb or object in the road.