It is not safe to drive a vehicle that has brake drums with cracks that are longer than:

Brake drums are vital components of a vehicle's braking system, and their integrity is essential for safety. When evaluating cracks in brake drums, the length of these cracks relative to the width of the friction area is critical. If a crack exceeds half of the width of the friction area, it poses a significant risk because it compromises the structural integrity of the drum. Such extensive cracking can result in failure during operation, leading to a loss of braking effectiveness or even catastrophic failure.

Therefore, the guideline that cracks longer than half of the width of the friction area are not safe to drive with is based on maintaining optimal safety and functionality of the braking system. It ensures that the drum can withstand the heat and pressure generated during braking without risking fracture or complete failure, which could increase the chances of an accident.