What is Break Disc in Vehicles? - Functions & Depreciation
The brake disc is a powerful module of the brake system. For the brake system to slow down the vehicle safely and comfortably whenever necessary – to stop it entirely if necessary – the brake disc must combine with the brake pads to produce braking torque (a braking force). This torque is transmitted to the wheel hub and from there to the rim. During braking, the vehicle's kinetic energy is converted into heat energy by friction between the brake pads and the brake disc, reducing speed.
Brake discs were initially developed for motorsport. After considerable success in this area, it was fitted to the front wheels of passenger cars in the 1960s. Drum brakes used before this date had weak points in design, heat problems, deterioration and fading, oscillating friction values, inadequate dosing, high wear, and noise (squeak). Many years later, the brake disc was rarely used on rear wheels with a low thermal load.
90% of the heat generated during braking penetrates the brake disc, where it was initially deposited. It is then transferred to ambient air. Therefore, the function of the brake disc is like that of a heat exchanger. However, its capacity to absorb heat is limited. Thus, the heat must be rapidly dissipated into the ambient air to avoid damage due to overheating.
During downhill driving, the temperature of the friction ring can reach 700 °C (red heat). For this reason, ventilated brake discs are often used for better cooling – primarily on the front axle. When the surface area is larger, it is more suitable for heat exchange. Compared to ventilated brake discs, solid brake discs dissipate heat much more slowly to the environment.
In ventilated brake discs, the friction rings are connected using ribs or a dome-shaped mesh. The rotation of the brake disc creates the air intake function, which draws air into the brake disc through the ventilation duct. Small particles in the air that meet the surface of the brake disc absorb thermal energy and transmit it outward.
More effective cooling can be attained with perforated or grooved brake discs. Such discs are less sensitive to wetness. However, they are more expensive and, in some cases, produce more noise during braking.
Friction rings on brake discs are generally prone to deformation when overheated. This causes unpleasant noise and vibration during braking (brake vibration). For this reason, well-known brake manufacturers are working to find ways to adapt brake discs to prevent deformation.
In some cases, brake vibration is unavoidable, even when there is no active braking, as vibration or play in the brake bearings causes the brake pads to come into repetitive contact with the brake discs. The localized flattening of the brake disc, which leads to impact braking after a specific time, becomes apparent to the driver in the form of vibration.
Brake Disc Material
Brake disc material must meet stringent requirements. At high wheel speed and thermal load, the centrifugal forces must be such as to withstand mechanical stress because of the pressure and tensile forces applied during braking.
Most brake discs are made of a special gray cast iron (pearlitic gray cast iron). Chromium and molybdenum alloys increase wear resistance and improve the material's hot crack properties. Also, high carbon content increases heat absorption rates.
Ceramic materials (carbon fiber ceramic or carbon-ceramic) are also increasingly used to manufacture brake discs. The benefits of these brake discs are high dimensional stability in all temperature ranges, low weight, good braking response, extremely long service life, and excellent damping properties. The disadvantages are poor thermal conductivity (which requires unique materials for the brake pads) and very expensive. The last reason clarifies why ceramic brake discs are only used as special equipment in high-power premium class vehicles.
Brake discs are subjected to very high mechanical and thermal loads. Environmental conditions such as water sprayed from the road, road dirt, and dispersed materials are added. Therefore, it is subject to natural wear. The condition of the brake discs is tested during each vehicle inspection. If the wear limit has been reached, the brake system must be replaced to maintain its reliable functioning.
Many manufacturers have started offering coated brake discs for brake repair. Such brake discs are coated with special paint that protects the brake discs (not the friction surface) against corrosion. Coated brake discs are a good choice for open rims, as they retain their attractive appearance over time.