The patches used on a radial tire are very different from the ones used on a bias tire which means that the dimensions of the injury needing repair is different too.
Step 2: Is the injury in a non-repairable zone?
Just like the Bermuda triangle, if the injury is too close to the bead, the flex zone of the tire is too large. As shown in the picture below, it is more convenient to measure from the inside of the tire in question. For example, the “no repair” zone on a 20,5R25 is 3 and ¾ inches.
Step 3: What is the tolerance for the type of tire and its injury?
In a conventional tire, the zone of injury is not important since the “X” structure made of nylon distributes the force evenly. Therefore, the ply rating will determine the tolerance. The higher the ply rating, the higher the tolerance will be, and the patch will follow accordingly.
In a radial tire, the tolerance will be different in the sidewall and the tread. Since the cables are closer at the sidewall, the tolerance will be greater. Also, a tire can’t be repaired if a high number of cables are broken. For example, a 20.5R25 tire has a tolerance at the sidewall 4” x 9” and the 9” is measured in the radial direction.