Friction. The brake pads clamping on the rotor creates friction which creates heat.
They get hot because of friction between the rotor (the disk part or the brake system), and the brake pads. (the pads that clamp on the rotor to create friction to slow the vehicle.
Because fire is very hot.
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The minimum front brake rotor thickness is three aids of an inch. When the rotor does not have the minimum thickness the auto mechanic replacing the brakes can not use the rotor.
Front Rotor Minimum Thickness after machining the rotor is .996
The front brake rotor, on a 2004 Chevrolet Colorado, can be removed by first removing the wheel. Next, remove the brakes. The rotor will slide off
The minimum thickness is cast into the rotor, either on the edge of the rotor or into the rotor hub.
There should be 2 screws on the front of the rotor, remove those and the rotor should come right off....you also have to take off the caliper bracket.
Replace rotor / cut rotor & replace brakes.
You have to heat up the contact points the rotor around the studs with MAP gas then try hit it but if that doesent work then you need a torch (MAP and oxgen) to get it red hot.