Trailer Tire Sidewall Bulge: What to Do Before Towing

Short answer: do not tow on a bulging tire

If you find a bulge on the sidewall, take the tire out of service before you tow. Plugs and patches cannot repair sidewall damage, so the tire has to be replaced. If you find the bulge at home or at a campground, mount a good spare or a new tire before you leave. If you find it on the road, move the trailer slowly and only as far as you need to reach a safe place off the roadway. Then change to the spare or arrange mobile tire service. Do not try to limp to a shop at highway speed.

The urgency comes from what the bulge is. Air pressure is pushing the outer rubber outward at a spot where the reinforcing cords underneath have broken or separated. That spot can fail suddenly. A trailer blowout can cause sway, tear up fenders, and damage brake or lighting wiring. It can also put a sudden overload on the other tires.

Why a sidewall bulge forms

A tire casing is built from layers of cord bonded in rubber, with an inner liner that holds the air. The rubber you see on the sidewall is mostly a protective cover. When the sidewall hits a curb edge, pothole or debris, it can be pinched between the object and the rim flange. That pinch can cut cords from the inside while the outside rubber stays intact. Without the cords holding it back, the rubber stretches outward and forms a bubble. The bubble may appear right after the impact or days later.

Trailer tires face some extra stresses. They often carry loads close to their limit. Running with low inflation pressure or too much weight makes the sidewall flex more and build heat, which weakens the bond between the cords and the rubber. Trailers also tend to sit for long periods, and sun and ozone age the rubber. On tandem axles, tight turns drag the tires sideways and twist the sidewalls. Any of these can lead to separation that shows up as a bulge.

Bulge, dent or normal sidewall shape

Not every irregular sidewall is damaged. Many radial tires show shallow inward indentations or a slight waviness where the casing plies overlap during manufacture. These usually run straight from bead to tread and sink in rather than stick out. A damaging bulge is the opposite: a localized outward lump that is usually easiest to see with the tire inflated. Sighting along the sidewall or running a hand over it can help, but avoid pressing hard on a suspect spot. A lump in the tread area or near the shoulder points to tread or belt separation, which also means replacement. If you cannot tell which you are looking at, treat the tire as damaged until a tire professional has inspected it.

What changes the answer

Axle layout matters. On a tandem or triple axle trailer, a tire that lost air or failed shifts its share of the load onto the tire next to it on the same side. That neighboring tire may have been overloaded and overheated even if it looks fine. Inspect it closely. If the trailer ran any distance on a deflated or failed tire, have it checked or replace it too.

The age and condition of the rest of the set also matter. If the tires are old by their date code, show sidewall cracking, or have uneven tread depth, replacing them as a matched set is often more sensible than replacing one. Construction should match: do not mix radial and bias-ply tires on the same axle. Keep tire type consistent too, since special trailer tires (ST) and light truck tires (LT) are rated differently. Use only what the trailer manufacturer approves on the placard. Finally, if overloading caused the bulge, a new tire will not fix the cause. Correct the weight first.

Checking the spare or replacement tire

Trailer spares are often forgotten until they are needed, and many have aged in storage. Before you rely on one, read the DOT code on the sidewall. The last four digits give the week and year of manufacture. Look for cracking, flat spots, or damage from the carrier mount. Measure tread depth. Check the inflation pressure cold and set it to the value on the trailer placard or the tire manufacturer's load and inflation table.

Whether you use the spare or buy a new tire, confirm the following: • The tire size matches the placard. • The load range and load index meet or exceed the original. • The speed rating suits how you tow. Many trailer tires carry a lower maximum speed than passenger tires, so read the sidewall rather than assuming. • The wheel matches the bolt pattern, center bore, offset and load rating of the original. A wheel with the right size but a lower load rating is not a safe substitute.

Steps before you tow again

Park on firm, level ground, keep the trailer hitched or securely supported, and chock the wheels on the opposite side. Loosen the lug nuts slightly before lifting. Lift only at the points the trailer or axle manufacturer specifies. Once the damaged tire is off, check the wheel for a bent rim flange, cracks or impact marks. A bulge from a hard hit often comes with wheel damage. Also check the fender, brake wiring and lights near that position.

Mount the replacement and tighten the lug nuts in a star pattern, in stages, to the torque the trailer or axle manufacturer specifies. Lug nuts can settle after the first short drive, so re-check the torque after the distance the manufacturer recommends. Before departing, check cold pressure on every tire, including the other tires on the trailer. Confirm the loaded trailer stays within its rated weights, and plan a towing speed that stays within the lowest speed rating on the trailer. Stop after the first few miles and look at the new tire and its neighbors.

Limitations and when to get professional help

A visual check can confirm that a bulge exists, but it cannot show how far internal damage extends or whether the wheel, bearing or axle was also affected. If the trailer pulls, the tire wears unevenly, or the new tire develops a problem soon after, have a qualified shop inspect the axle alignment, suspension and wheel. Repeated sidewall damage usually points to a cause, such as chronic underinflation, overloading, the wrong tire type or an alignment problem, rather than bad luck.

Specific values for pressure, load, lug nut torque and approved tire sizes come from the trailer's placard and the manufacturers' documentation, not from general guidance. When those sources disagree with what is currently mounted, follow the placard and have a tire professional confirm the setup before the next trip.