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In a car's braking system, brake shoes are the core component of drum brakes. While not as well-known as disc brake pads, they still play a crucial role in certain models. For car owners, understanding the structure, working principles, and maintenance of brake shoes can help better diagnose brake system malfunctions and ensure driving safety. This article will provide a comprehensive explanation of brake shoes, from basic concepts to practical applications.
Brake shoes are the components in drum brake systems that directly generate friction, working with the brake drum to achieve braking. Their definition and core characteristics are as follows:
Main Material: Consists of a steel plate (base plate), friction pad (brake lining), and rivets/adhesive. The steel plate provides structural support, while the friction pad (mostly made of asbestos, semi-metallic, or ceramic) is fixed to the steel plate by rivets, generating friction directly with the brake drum.
Shape: Curved (matching the inner curvature of the brake drum), the overall shape resembles a horseshoe, hence the name "brake shoe." Most drum brake systems include two symmetrical brake shoes (left and right), mounted on the inside of the brake drum.
Additional components: Some brake shoes are equipped with return springs (to reset the shoe after braking), adjustment mechanisms (to compensate for wear clearance), and wear warning plates (to produce a noise when wear reaches its limit, prompting repalcement).
Different applicable systems: Brake shoes are only used in drum brakes (such as the rear wheels of some models and truck/bus brake systems); brake pads are used in disc brakes (where the brake disc is mated to the caliper).
Different operating methods: Brake shoes rub against the inside of the brake drum by expanding outward; brake pads generate friction by clamping the brake disc.
Different friction areas: Brake shoes have full-circle contact with the brake drum, resulting in a larger friction area; brake pads only have partial contact with the brake disc, resulting in a smaller friction area.
The braking process of brake shoes relies on mechanical or hydraulic transmission, using frictional resistance to slow the wheels. The specific process is as follows:
When the driver depresses the brake pedal, a hydraulic pump (or mechanical lever) generates pressure, which is transmitted through the brake lines to the wheel cylinder (the power actuator of drum brakes).
The piston in the wheel cylinder, under pressure, pushes the end (support point) of the brake shoe outward, causing the shoe to rotate about a fixed axis.
The brake shoe expands outward under the thrust, and the friction pad on its surface comes into close contact with the inner side of the brake drum, which rotates with the wheel, generating friction.
The frictional force resists the rotation of the brake drum and is transmitted to the wheel through the wheel hub, forcing the wheel speed to decrease, thereby slowing the vehicle.
When the brake pedal is released, the wheel cylinder pressure disappears, and the brake shoe retracts under the tension of the return spring, separating the friction pad from the brake drum, and braking ends.
If the return spring fails, the brake shoe cannot fully return to its original position, resulting in "drag" (driving with the brakes on), which requires prompt repair.
Based on differences in design and material, brake shoes can be divided into different types, each with its own application focus:
Leading and Following Shoes: One of the two brake shoes is a leading shoe (which rotates under braking, driven by the brake drum, increasing the braking effect through friction) and the other is a following shoe (which is pushed away from the brake drum, reducing the braking effect through friction). This has a simple structure and low cost, and is mostly used on the rear wheels of small vehicles.
Double Leading Shoes: Both brake shoes are leading, offering stronger and symmetrical braking force. This is suitable for front wheels (which bear the primary braking force), but requires a dual-piston wheel cylinder and is more expensive.
Self-Amplifying: This type uses a lever mechanism to amplify braking force, resulting in high braking efficiency. It is suitable for heavy-duty vehicles (such as trucks), but its structure is complex and maintenance is difficult.
Asbestos: This material has a low cost and a stable friction coefficient, but asbestos fibers are harmful to health and are gradually being phased out (although some older models still use it).
Semi-metallic material: A blend of metal fibers (steel, copper) and organic materials offers brake shoes strong wear and high-temperature resistance, making it suitable for frequent braking scenarios (such as urban trucks).
Ceramic material: Based on ceramic fibers and resin, it produces low noise and dust, but is more expensive and is primarily used in drum brakes for passenger cars that prioritize comfort.
Although brake shoes are less commonly used in passenger cars, they remain irreplaceable in certain scenarios due to their unique advantages:
Providing Braking Force: Through friction with the brake drum, they directly generate resistance to wheel rotation and serve as the "braking actuator" of the drum brake system.
Adaptability to Harsh Environments: The enclosed structure of the brake drum protects the brake shoes from mud, water, sand, and gravel, making them more durable on unpaved roads or in dusty environments.
Assisted Parking Brake: On some models, the parking brake (handbrake) directly applies a cable to the rear wheel brake shoes, ensuring stable parking during parking.
Rear Wheels of Small Cars: Entry-level sedans (e.g., models under 100,000 yuan) often use drum brakes (with brake shoes) for the rear wheels to reduce costs, and disc brakes for the front wheels (to ensure braking efficiency).
Trucks/Buses: When carrying heavy loads, a large friction area is required to provide strong braking. The combination of brake shoes and brake drums provides more stable braking force and reduces maintenance costs.
Off-road Vehicles/Specialty Vehicles: In muddy and flooded conditions, the enclosed structure of the brake shoes prevents water from entering, thus preventing brake failure (e.g., military off-road vehicles and engineering vehicles).
Brake shoe failures can directly affect braking safety and must be identified and addressed promptly:
Excessive Wear: When the friction pad thickness is less than 2mm (new pad thickness is approximately 8-12mm), a sharp "iron grinding iron" noise is heard during braking, requiring immediate replacement.
Poor return: A broken or stuck return spring prevents the brake shoe from fully disengaging from the brake drum, resulting in increased driving resistance, heated wheels, and increased fuel consumption.
Friction pad detachment: Loose rivets or ineffective adhesive may cause a "clicking" sound during braking and a sudden drop in braking force, making this a high-risk fault.
Deformation or cracking: The steel base material may be deformed by high temperatures or impact, resulting in an uneven fit between the brake shoe and the brake drum, causing the vehicle to veer during braking.
Regular Inspection: Disassemble and inspect every 20,000 kilometers, measure the friction pad thickness, and check the return spring elasticity (for looseness or rust).
Cleaning and Maintenance: Remove dust and oil from the brake drum (oil reduces the friction coefficient and can cause brake failure). Apply high-temperature grease to the movable shaft to ensure smooth rotation.
Avoid Overuse: Avoid prolonged braking on long downhill slopes (this can cause the brake shoes to overheat and carbonize). Use engine braking to alternate deceleration.
Replacement Guidelines: Replace in pairs (both wheels simultaneously) to avoid uneven braking force. During installation, ensure the proper clearance between the friction pad and the brake drum (typically 0.2-0.5mm). Too loose will cause abnormal squeaking, while too tight will cause drag.
Although disc brakes have become mainstream, brake shoes still play a vital role in certain vehicle models due to their simple structure, low cost, and resistance to harsh environments. Understanding their structure, principles, and maintenance can help drivers detect brake problems (such as squeaking and deviation) promptly and avoid safety hazards.
Regular inspection and standardized replacement of both brake shoes and pads are crucial to ensuring braking safety. After all, brake system reliability is always the first line of defense for driving safety.If you want to know more information about brake parts,welcome to consult us.Our EV is a reliable brake parts supplier.Our website is https://www.evfriction.com. And our phone number is +86-13363216781.Or you can email us by jessicabrakes12@gmail.com.
