Several factors can make a motorcycle tire illegal:

Tread Depth: If the tire's tread depth falls below the legal minimum specified by the regulations in your country or region, it would make the tire illegal.

Damage or Wear: Tires with excessive damage, such as cuts, bulges, or exposed cords, may be considered illegal. Similarly, if the tire has excessive wear, such as worn-out tread patterns that no longer meet safety standards, it could be deemed illegal.

Age: Some countries or regions have regulations regarding the maximum age of motorcycle tires. Even if the tread depth is still within legal limits, the tire may be considered illegal if it exceeds the specified age limit. This is because tire compounds can degrade over time, affecting their performance and safety.

Non-compliance with Standards: Motorcycle tires must meet certain manufacturing and safety standards set by regulatory bodies. If a tire doesn't comply with these standards, such as lacking appropriate labeling or certification markings, it may be deemed illegal.

Modifications: Altering a tire by adding substances, such as tire softeners or chemicals, to enhance performance or grip, can render it illegal. Modifying the tire's structure or dimensions beyond the manufacturer's specifications may also result in illegality.