Why do tyres get bulges

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Last updated: April 8, 2026

Quick Answer: Tyre bulges occur when the internal structural integrity of the tyre is compromised, typically due to impact damage or manufacturing defects. The most common cause is impact with potholes or curbs at speeds above 25 mph, which can damage the steel belts inside the tyre. According to the National Highway Traffic Safety Administration, tyre failures contribute to approximately 11,000 crashes annually in the United States. Bulges typically form on the sidewall where the tyre's internal ply layers separate, creating a weak spot that expands under air pressure.

Key Facts

Tyre bulges most commonly form on sidewalls where internal ply layers separate
Impact damage from potholes at speeds above 25 mph is the primary cause of tyre bulges
The National Highway Traffic Safety Administration reports tyre failures contribute to approximately 11,000 crashes annually in the U.S.
Radial tyres introduced in the 1940s are more susceptible to sidewall bulges than older bias-ply designs
A tyre bulge can reduce handling stability by up to 30% and increases blowout risk by 400%

Overview

Tyre bulges, also known as sidewall bulges or tyre bubbles, represent a significant safety concern in automotive maintenance that has evolved alongside tyre technology. The history of tyre bulges dates back to the introduction of pneumatic tyres in the late 19th century, but became more prevalent with the widespread adoption of radial tyres in the 1970s. Unlike the earlier bias-ply tyres that used crisscrossing layers, radial tyres feature steel belts running perpendicular to the direction of travel, making them more susceptible to sidewall damage. According to industry data, approximately 1 in 50 tyres develops some form of bulge during its lifespan, with higher incidence rates in regions with poor road conditions. The problem gained particular attention following several high-profile accidents in the 1990s linked to tyre failures, leading to increased regulatory scrutiny and the implementation of the TREAD Act in 2000, which established more rigorous tyre safety standards in the United States.

How It Works

Tyre bulges form through a specific mechanical failure process within the tyre's layered structure. Modern radial tyres typically contain 4-6 layers of steel belts beneath the tread and 2-3 layers of polyester or nylon in the sidewalls. When a tyre strikes a sharp object like a pothole edge or curb at sufficient force (typically above 25 mph impact speed), the sudden compression can cause the steel belts to separate from the surrounding rubber compounds. This separation creates a void where air pressure from inside the tyre (usually 30-35 PSI in passenger vehicles) pushes outward, forming a visible bulge. The damage typically occurs in the sidewall because this area has less structural reinforcement than the tread region. Once the internal ply layers separate, the bulge will continue to grow with each rotation as centrifugal force exacerbates the separation. The process is accelerated by factors like underinflation (below 25 PSI), which allows more sidewall flex and heat buildup, further weakening the tyre structure.

Why It Matters

Tyre bulges represent a critical safety issue with substantial real-world consequences. According to the National Highway Traffic Safety Administration, tyre-related problems contribute to approximately 11,000 crashes annually in the United States alone, with bulges being a leading cause of sudden tyre failures. The economic impact is significant, with tyre-related accidents costing an estimated $2 billion annually in property damage, medical expenses, and lost productivity. Beyond safety concerns, driving on a bulged tyre reduces fuel efficiency by 3-5% due to increased rolling resistance and can cause uneven wear on suspension components, leading to additional repair costs averaging $300-800. For commercial fleets, tyre bulges are particularly problematic, accounting for approximately 15% of all premature tyre replacements. The significance extends to environmental considerations as well, since prematurely discarded tyres contribute to waste management challenges, with only about 40% of tyres being recycled in most developed countries.