Understanding why tire shoulder punctures can’t be repaired

Tire Shoulder Punctures: Why They Can’t Be Repaired

When a puncture occurs in the tire shoulder area near the sidewall, the repair situation becomes somewhat confrontational. Consumers generally understand that punctures in the sidewall can’t be repaired. I’ll explain the reasons for that below. The same reasons apply to the outer tire should area, yet consumers can’t seem to grasp that concept. This article will delve into the reasons why punctures in the sidewall and shoulder areas can’t be repaired using a plug, patch, or even a combo plug patch.

A quick look at tire construction

If you understand how a tire is built, then the tire repair guidelines will make more sense to you. Here’s a cutaway diagram of a typical tire. It shows the tread, belts, plies, inner liner, shoulder, sidewall, and bead.

Notice how the belts don’t extend into the full shoulder area. That’s why these areas can’t be patched with a plug, patch, or even a combo plug/patch

Tread: The outermost part that makes contact with the road. It’s designed for traction and durability.
Belt: Layers of strong materials like steel that provide rigidity and strength.
Tire Shoulder: The tire should handle the most stress in turns, builds the most heat when the tire is underinflated and is not supported or protected by the belts.
Sidewall Body Plies: The area between the tread and the bead (which seats the tire on the rim). The sidewall absorbs shocks and supports the vehicle’s weight. Plies not only provide the structure of the tire but also act to keep the inner liner from bulging like a balloon when squeezed. Depending on whether the tire is for passenger or truck use it can have two or more body plies. The plies are a woven fabric made from polyester, rayon, or nylon that is embedded into the rubber during the manufacturing process.

Understanding why shoulder and sidewall punctures can’t be repaired safely

Look at the outer edges of the tire shoulder. You’ll see that the belts don’t extend all the way to the sidewall. In other words, the tire shoulder has the same construction as the sidewall, supported only by the body plies and inner liner. Because the shoulder and sidewall aren’t protected by the belts, they’re not designed to accept repair like the tread area. If you apply a plug or patch to these areas, they won’t last due to the high flex, heat, and pressure.

The primary concern with sidewall and shoulder punctures is safety. A compromised sidewall or shoulder can lead to tire failure, such as blowouts, which can be catastrophic at high speeds. The sidewall and shoulder must maintain their integrity to ensure the tire performs reliably under various driving conditions. Repairing a puncture in this area simply can’t restore the tire to its original strength, posing a risk to the driver and passengers.

What types of tire punctures, cuts and scrapes can be repaired?

Now that you’ve seen the cross-section of the tire, it’s important to realize that when you get a flat tire, you’ve punctured a hole through the tread, belts, plies, and inner liner. All three must be sealed for the tire to perform safely. But here’s the really important part: if you drove on a flat tire, even for a short distance, you’ve caused the inner liner of the sidewall of the tire to rub against the inner liner below the plies! That’s like rubbing two erasers together, and it can weaken the inner liner. That’s why every flat tire must be dismounted from the wheel for a complete inner liner inspection.

This is what happens to the inside of the tire when you drive on a flat tire.

This is what happens to the sidewall when you drive on a flat tire

A puncture can be repaired IF:

• The puncture is within the tread area and at least 1-in. away from the shoulder

• The puncture must be ¼-in in diameter or less

• The puncture must not exceed a 45° angle

• The puncture must not overlap a previous tire repair.

• The sidewall and inner liner must be inspected to ensure there’s no damage

Puncture repair guidelines

Punctures up to ¼-in in diameter can be

Safe tire repair area

safely repaired using an inner liner patch and a plug or a combination patch plug AS LONG AS the puncture area is located in the approved repair area shown in this image. In other words, punctures in the tread that are at least 1-in. away from the tire shoulder are repairable. A patch alone or a plug alone is NOT an acceptable repair.

The patch seals the inner liner and prevents air from reaching the belts and plies. A string plug fills the patch of the puncture but does not seal the inner liner. A plug alone can allow air to migrate into the belts and plies and cause early failure.

A plug alone is NOT an acceptable repair. A patch alone is NOT an acceptable repair.

Puncture repair procedure

Tire plug preparation and installation

Because a puncture doesn’t usually provide a clean cut through the tire, the first step in tire repair is to run a carbide drill through the hole to obtain a uniform diameter and remove rubber fibers that might prevent a good bond. The technician then blows out any rubber particles from the newly drilled hole and coats the inside of the hole with an adhesive. Then, the rope plug is installed and set up. The technician then cuts off the excess plug material from the tread and cuts the interior portion of the plug so it is flush with the inner liner.

Drilling the hole with a carbide bit is critical to

Inserting a tire plug

removing treads of ply and belt material from the puncture and helping obtain a smooth bonding surface for the plug to adhere to.

Tire patch preparation and installation

The innerliner must first be prepared using a buffing wheel to slightly scuff the innerliner. Scuffing allows the patch adhesive to “bite” into freshly exposed rubber and that improves adhesion and sealing. Next, the technician applies a special butyl liner adhesive to the innerliner, applies the patch and then rolls the patch using a special tool. Rolling the patch is referred to as “stitching” and is critical to maintaining a good bond.

Buffing tire innerliner

Punctures, bubbles, cuts and scrapes that cannot be repaired

• Sidewall punctures can never be repaired due to the high degree of flex and heat that’s always present in the sidewall

• Punctures in the tire’s shoulder area cannot be repaired

• Slices cannot be repaired

• Scrapes in the sidewall rubber cannot be repaired if the scrape has damaged any of the ply or belt fabric.

©, 2019 Rick Muscoplat