The brake drum is made of brittle gray cast iron:

The brake drum is a cast iron piece similar to a tambourine, which is fixed with the tire and rotates at the same speed. When braking, oil pressure is used to push the brake shoe to contact the inner edge of the brake drum, and the friction force generated by the contact is used to inhibit the rotation of the tire to achieve the purpose of braking. The drum brake is composed of brake base plate, brake wheel cylinder, brake shoe and other related connecting rod, spring, pin and brake drum. At present, it is only commonly used in the rear wheel. The drum brake has lower cost and higher absolute braking force, and is widely used in the rear wheels of small cars. However, its wear rate is high, so the overall cost is high.