To accurately measure the diameter of a brake disc, use a caliper or a measuring tape to measure across the center of the disc from one edge to the opposite edge. This measurement will give you the diameter of the brake disc.
You measure it. Any millimeter graded ruler or measuring tape or yardstick would tell you the diameter, but the thickness is most easily and accurately measured with a vernier caliper.
Take the wheels off and measure the diameter of the brake rotor (disc) or drum with a ruler or tape measure.
There is a engraved number on the outer rim of the rotor. This will be where the rotor is the largest in diameter. It will be hard to read if the rotor is rusty. This number is the minimum recommended thickness. You may have to use a dial caliper to accurately measure the rotor.
use a pad wear indicator :)
disc brake components
The rotor is the disc in the disc brake system.
worn thin
In case of cost..drum brake is better than disc brake.
Radius or diameter in inches or cm. thickness in thousandths or hundredths of an inchor mm.
Converting a rim brake wheel to a disc brake system involves replacing the wheel hub with a disc brake-compatible hub, installing a disc brake rotor, and attaching a disc brake caliper to the frame or fork. This process may require specific tools and expertise, so it is recommended to consult a professional bike mechanic for assistance.
There are many purposes to using a disc brake. Disc brakes are mainly used in industrial drills because the disc brake stops the drill gently so it doesn't overheat.
Disc brake pads and drum brake shoes