Comprehensive Analysis of Brake Shoes

1. Overview of Brake shoes

The Brake Shoe is a crucial component in the automotive braking system and is usually applied in drum braking systems. Its shape is half-moon or crescent-shaped, mainly composed of a metal shoe body (also called the back plate) and friction materials attached to it. In the rear wheel braking systems of many older models of vehicles, economy cars and some trucks, the braking method of drum brakes combined with brake shoes still plays an important role. ​

2. Working principle of brake shoes

Braking process

When the driver steps on the brake pedal, a series of complex yet precise braking processes are immediately initiated. The pressure inside the master cylinder of the brake increases, prompting the brake fluid to be transmitted through the brake lines to the slave cylinders at each wheel (in a drum brake system, they are the wheel cylinders). After the wheel cylinder is subjected to pressure, the piston pushes outward, thereby pushing the brake shoes apart towards the inner wall of the brake drum. The friction material on the brake shoes is in close contact with the inner wall of the high-speed rotating brake drum, and a powerful friction force is generated instantly. This frictional force can quickly convert the rotational kinetic energy of the wheels into thermal energy, thereby effectively reducing the rotational speed of the wheels and achieving the purpose of decelerating or even stopping the vehicle. ​

(2) Return to position process

When the driver releases the brake pedal, the pressure inside the braking system gradually returns to normal. At this point, the return spring installed between the brake shoes and the back plate begins to function. The return spring, relying on its own elastic potential energy, quickly pulls the brake shoes back to their initial stationary position, causing the brake shoes to separate from the inner wall of the brake drum. This allows the wheels to rotate freely again, and the vehicle returns to its normal driving state. ​

3. Components of brake shoes

Friction materials

The friction material is the part of the brake shoe that directly contacts the brake drum and generates friction. It is a key element determining the braking performance. It is usually made by mixing a variety of materials that are heat-resistant, wear-resistant and have a high coefficient of friction. Common components include graphite, metal powders (such as copper, iron, lead, etc.), ceramic fibers and some organic fibers, etc. Different types of friction materials vary in braking effect, wear resistance, noise generation and dust emissions. For instance, friction plates made of organic materials are relatively quiet when braking, but their wear rate is relatively fast. Semi-metallic friction plates have better heat dissipation performance and a longer service life, but they may generate considerable noise during braking. ​

(2) Shoe body (back plate)

The shoe body, as the supporting structure of the brake shoes, is generally made of metal materials, such as formed by stamping steel plates. It not only provides a stable adhesion foundation for the friction material, but also withholds the huge pressure and shear force between the friction material and the brake drum during the braking process. The design and manufacturing quality of the shoe body are directly related to the overall strength and reliability of the brake shoes, ensuring that no deformation or damage occurs under frequent and high-intensity braking operations. ​

(3) Fixing and connecting components

Anchor Pins: Anchor pins are used to firmly fix one end of the brake shoe plate to the brake backplate, providing a stable rotating fulcrum for it during braking. Anchor pins are usually made of high-strength metal materials, featuring excellent wear resistance and fatigue resistance to ensure they do not loosen or fail during long-term use. ​

Pivot Pins: Pivot pins are installed at the other end of the brake shoes. They allow the brake shoes to swing flexibly outward around the pivot pins when pushed by the wheel cylinder, closely adhering to the inner wall of the brake drum to achieve the braking function. Meanwhile, after the brake is released, the brake shoes can smoothly return to their original position through the pivot pins. The design of the pivot pin needs to ensure its flexible rotation while also being able to withstand a certain lateral force. ​

Return Springs: The return springs are important components that ensure the brake shoes return to their original position quickly after braking is completed. It is usually connected between two brake shoes or between the brake shoes and the brake backplate. By using the elastic force of the spring, it overcomes the residual friction and other resistances between the brake shoes and the brake drum, quickly pulling the brake shoes back to the initial position to prepare for the next braking. The elastic coefficient and durability of the return spring have a significant impact on the response speed and reliability of the braking system. ​

Adjuster Mechanism: With the continuous use of brake shoes, the friction material will gradually wear out, resulting in an increase in the gap between the brake shoes and the brake drum. To ensure the stability of the braking effect, an adjustment mechanism is equipped in the braking system. The adjustment mechanism can automatically or manually adjust the gap between the brake shoes and the brake drum according to the wear condition of the brake shoes, ensuring that the braking system can always maintain good performance throughout the service life of the brake shoes. Common adjustment mechanisms include automatic adjustment arms and manual adjustment screws, etc.

4. Brake shoe type

Organic brake shoes

The friction materials of organic brake shoes are mainly composed of natural or synthetic organic fibers (such as aramid fibers, cellulose fibers, etc.), resin binders and some auxiliary additives. This type of brake shoes offers excellent braking comfort and generates extremely low noise during braking, providing a quiet driving environment for drivers and passengers. In addition, organic brake shoes cause less wear on the brake drum, which is conducive to extending the service life of the brake drum. However, due to the relatively poor heat resistance of organic materials, during frequent or high-intensity braking, the friction materials are prone to performance degradation due to overheating, resulting in a decrease in braking effect. Therefore, organic brake shoes are usually suitable for vehicles that have high requirements for braking comfort and relatively mild driving conditions, such as urban commuter cars, etc.
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(2) Semi-metal brake shoes

The friction material of semi-metallic brake shoes contains a large amount of metal components, such as iron powder, copper powder, steel wire, etc., and also adds a certain proportion of organic fibers and other auxiliary materials. The addition of metal components endows semi-metallic brake shoes with excellent heat dissipation performance and a high coefficient of friction. During braking, they can quickly generate a powerful braking force and effectively shorten the braking distance. In addition, the wear resistance of semi-metallic brake shoes is also superior to that of organic brake shoes, and their service life is relatively longer. However, due to the characteristics of metal materials, semi-metallic brake shoes tend to generate considerable noise during braking and cause relatively greater wear on the brake drum. Therefore, semi-metallic brake shoes are often used in vehicles that have high requirements for braking performance and frequently need to travel under heavy loads or at high speeds, such as trucks and SUVs. ​

(3) Ceramic brake shoes

Ceramic brake shoes are a type of high-performance brake shoes that have gradually emerged in recent years with the development of material technology. Its friction materials are mainly composed of ceramic fibers, ceramic particles and a small amount of metal or organic materials. Ceramic materials possess extremely high heat resistance, wear resistance and stability, which enables ceramic brake shoes to maintain excellent braking performance even in high-temperature environments and is less likely to experience braking fade. Meanwhile, ceramic brake shoes generate less noise during braking and have lower dust emissions, making them more environmentally friendly. In addition, due to the high hardness of ceramic materials, ceramic brake shoes cause minimal wear to the brake drum, which can significantly extend the service life of the brake drum. However, the manufacturing cost of ceramic brake shoes is relatively high and their price is also rather expensive. Currently, they are mainly used in some high-end luxury models or high-performance sports cars. ​

5. Inspection and maintenance of brake shoes

(1) Inspection cycle

Under normal driving conditions, it is recommended to conduct a comprehensive inspection of the brake shoes every 5,000 to 10,000 kilometers. However, if the vehicle frequently travels in bad road conditions (such as urban congested roads with frequent starts and stops, mountain roads, etc.) or if the driving style is rather aggressive, the inspection cycle should be appropriately shortened and the inspection frequency increased to ensure that the brake shoes are always in good working condition. ​

(2) Inspection methods

Visual inspection: First, the tires of the vehicle need to be removed to clearly observe the appearance condition of the brake shoes. Check whether there are any abnormal phenomena such as uneven wear, cracks, peeling or charring on the surface of the friction material of the brake shoes. Under normal circumstances, the friction material of brake shoes should wear evenly and have a smooth surface. If significant differences in the thickness of the friction material are found in different parts, or if deep cracks or large-scale peeling occur, it indicates that the brake shoes have been damaged and need to be replaced in a timely manner. ​

Thickness measurement: Use a dedicated brake shoe thickness measurement tool (such as a caliper) to measure the remaining thickness of the friction material of the brake shoe. The standard thickness for replacing brake shoes may vary among different vehicle models. Generally speaking, when the remaining thickness of the friction material is less than 2-3 millimeters, it is advisable to consider replacing the brake shoes. In addition, it is necessary to pay attention to measuring the thickness on both sides of the brake shoes to ensure uniform wear on both sides. If the thickness difference on both sides is too large, it may be that there is a fault in the braking system, and further inspection and repair are required. ​

Return check: During the inspection of the brake shoes, it is also necessary to check the return condition of the brake shoes. The brake shoes can be manually pushed to observe whether they can be flexibly extended outward and smoothly return to their original position. If the brake shoes feel stuck during the pushing process, or do not return to their original position in time or properly, it may be due to a damaged return spring, rusty pivot pins or other component malfunctions. Timely repair or replacement is required. ​

Brake drum inspection: While inspecting the brake shoes, the inspection of the brake drum should not be overlooked. Observe whether there are problems such as uneven wear, grooves and deformation on the inner wall of the brake drum. If deep grooves or severe deformation appear on the inner wall of the brake drum, it will affect the adhesion between the brake shoes and the brake drum, reducing the braking performance. At this time, the brake drum needs to be repaired or replaced. ​

(3) Key points for maintenance

Keep clean: Regularly use compressed air or brake cleaner to remove dust, oil stains and other impurities from the surface of brake shoes and brake drums. Dust and oil stains can reduce the friction coefficient of brake shoes, affecting braking performance. At the same time, they may also accelerate the wear of brake shoes and brake drums. During the cleaning process, be careful to avoid the cleaning agent coming into contact with other rubber parts of the brake system to prevent aging and damage to the rubber parts. ​

Avoid overloading: Try to avoid driving the vehicle in an overloaded state for a long time, as overloading will increase the burden on the braking system and cause excessive wear of the brake shoes. When loading goods or carrying passengers, operations must be carried out strictly in accordance with the rated load capacity of the vehicle to ensure its driving safety. ​

Correct driving habits: Developing good driving habits is crucial for extending the service life of brake shoes. Avoid frequent sudden braking and prolonged continuous braking. Try to use predictive braking, slow down in advance and brake smoothly. In addition, during driving, gears should be used reasonably to utilize the braking effect of the engine to assist in braking and reduce the frequency of using the braking system. ​

Timely replacement: Once excessive wear, damage or other abnormal conditions are found on the brake shoes, new brake shoes should be replaced in a timely manner. When replacing brake shoes, it is necessary to choose genuine parts that match the vehicle model and specifications, and have them installed and debugged by professional maintenance personnel to ensure the performance and safety of the braking system. At the same time, after replacing the brake shoes, appropriate running-in should be carried out to avoid vigorous driving or heavy-load driving during the running-in period.

6. Impact of brake shoe faults
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(1) The braking performance declines

When the brake shoes are severely worn or malfunction, the friction between them and the brake drum will significantly decrease, resulting in a decline in braking performance. When a vehicle brakes, it may need a longer braking distance to come to a stop, which could cause serious traffic accidents in an emergency. In addition, the decline in braking performance may also be manifested as the vehicle veering off course during braking, that is, the vehicle shifts to one side during braking. This is caused by the imbalance of braking force between the brake shoes on both sides. ​

(2) Abnormal braking noise

Under normal circumstances, the braking system will only produce a slight friction sound when it is working. However, when there are problems with the brake shoes, such as the friction material being completely worn out, surface cracks or hardening, or foreign objects being mixed with the brake drum, it will cause abnormal noises during braking, such as sharp screams, shrill friction sounds or dull impact sounds, etc. These abnormal noises not only affect the comfort of drivers and passengers, but may also be a signal of serious faults in the braking system, which requires timely inspection and repair. ​

(3) Abnormal brake pedal

Faulty brake shoes may also cause abnormal sensations in the brake pedal. For instance, the brake pedal may become soft, and when pressed down, there may be no obvious resistance, and the travel distance may increase. This is due to the excessive gap between the brake shoes and the brake drum, which requires the brake fluid to fill more space in the pipeline. In addition, when the brake pedal is depressed or released, there may be shaking or vibration. This could be due to uneven wear of the brake shoes or faults in other components of the braking system, resulting in unstable braking force. ​

(4) Other components of the braking system are damaged

If the fault of brake shoes is not dealt with in time, it may also cause damage to other components of the braking system. For instance, excessively worn brake shoes may cause grooves and deformations on the inner wall of the brake drum, thereby affecting the service life of the brake drum and requiring its replacement. At the same time, brake shoe failures may also exert additional pressure and wear on components such as wheel cylinders and brake lines, increasing the maintenance cost of the entire braking system.