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Sidewall Indentations, Undulations and Protrusions


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It is not unusual to find subtle indentations in the sidewalls of radial ply tires. Fortunately they are a visual condition that will not affect the performance of the tires.

Sidewall indentations (also referred to as sidewall "undulations") are a common characteristic of tire construction on almost all radial tires. These indentations are more noticeable in larger tires with taller sidewalls, as well as tires that operate at higher inflation pressures.

Tires are reinforced by encasing individual fabric cords (typically polyester or rayon) in rubber. Radial ply tires feature one or more layers of cord (depending on the tire's required strength) that basically 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" direction, the overlap where the sidewall cords are lap-spliced may sometimes cause a slight indentation.


Fig. A: Vertical bulge on sidewall from rim to tread.
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 typical air pressures, it is free to naturally 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 material because 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.


However, if protruding bulges, as shown in Fig. A (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 defects in materials and workmanship warranty.

If a protruding bulge doesn't appear until later in the tire's life, it was usually caused by a road hazard when the cord was injured as the tire struck a pothole, curb or object in the road.

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