Drum Brakes

Drum brakes work by hydraulic cylinders forcing curved shoes outwards, onto the inner surface of a cast-iron drum.

Drum Brakes
Drum Brakes

However, this immediately presents the limitation of drum brakes: the friction generation process takes place within what is effectively a sealed environment, with little or no air circulation. There will be a huge amount of heat generated by the friction process. Under normal use, this doesn't pose a problem, as it can generally sink away through the drums themselves, into the wheels and away. However, start to push drum brakes very hard, such as at a track day, and the heat build-up becomes an issue.

The main problem is that as the drums heat up, they expand and the shoes have to travel further to maintain the braking force. A drum setup has compensation for wear in the shoes, so as the drum heats and expands, the adjustment kicks in and the shoes get progressively more and more extended on the adjusters. When the car stops and the brakes cool down, the drum contracts but the shoes don't move back in. as they're on mechanical adjusters. This can lead to seized brakes.