Overview

　　Brake pads are also called brake shoes. In a bicycle's braking system, brake pads are one of the most critical safety components. The quality of braking depends heavily on the brake pads, so good brake pads are the heart of a mountain bike's braking system.

　　The working principle of brakes mainly comes from friction and engagement, using the resistance from the friction between the brake pads and brake discs, as well as the tires and the ground, to stop the vehicle. A good and efficient braking system needs to provide stable, sensitive, controllable force, and have good hydraulic transmission and heat dissipation capabilities.

　　 Development of Brake Pads

　　At the 1907 New York International Auto Show, brake pads made from camel hair, cotton fiber, asphalt, and rubber were considered the latest technology. However, because this material could not withstand high temperatures, intense use could cause fires. From then on, the search for suitable wear-resistant materials had only just begun.

　　With the endless progress of technology, according to the current situation, relevant technical experts have simply classified the development history of brake pads. In addition to the formative period, it is divided into three stages: the first stage is mainly asbestos, with resin as an auxiliary material; the second stage is mainly iron fiber, with traditional fibers as auxiliary materials; and the third stage is mainly reinforced fiber, with special metals and ceramics as auxiliary materials.

　　Brake Pads We Use Today

　　 Brake Pads

　　In 1935, asbestos was discovered and used in automobiles. Since asbestos itself is a very wear-resistant material, compared to other traditional fibers, asbestos is cheaper and can provide longer wear without damaging the disc. The tensile strength of asbestos fiber is similar to that of high-grade iron fiber, it can withstand hundreds of degrees Fahrenheit, and it has a certain inhibitory effect on noise.

　　Later, asbestos materials were widely used until the 1980s. As front-wheel-drive cars gradually became the mainstream on the roads, the demand for brake pads that could withstand higher temperatures began to increase significantly. At the same time, asbestos materials are very harmful to the human respiratory system and may even cause cancer. Considering both human health and safety and braking performance, people have once again begun to seek other alternative materials. Today, asbestos brake pads are rarely seen. Due to the low cost of asbestos materials, this type of asbestos brake pad still exists in third-world and underdeveloped countries.

　　Asbestos Pad Material

　　All-metal brake pads were initially used in competitions and high-load military vehicles (such as trucks or tanks). The manufacturing method is to sinter metal powder at high temperature and high pressure, so there are no resin or other adhesives. Although all-metal materials will not cause vaporization and overheating failure at high temperatures, these brake pads have poor initial braking effect at low temperatures and only gradually generate braking ability after reaching the working temperature. Therefore, semi-metallic brake pads, which combine the high-temperature characteristics of metal and the low-temperature characteristics of traditional fibers, were developed.

　　Semi-metallic brake pads—as the name suggests, they "contain about half metal and half synthetic fiber". They combine the advantages of both materials, are more wear-resistant than traditional fiber-based pads, and have better resistance to temperature rise failure. The best use environment is medium and high temperatures. This type of brake pad was widely used in front-wheel-drive cars in the 1970s and 1980s. These cars have most of their weight and braking load on the front wheels, which means that the front brakes will have higher operating temperatures. However, semi-metallic materials also have their maximum operating temperature limit—approximately 538℃ (1000℉), at which point the iron fibers and friction materials will begin to melt and eventually bond to the disc surface.

　　Both metallic and semi-metallic materials contain a considerable proportion of iron. The simplest way to distinguish them is to use a magnet—metallic brake pads will have higher magnetism. Of course, the backing plate of the brake pad is made of steel or iron, and it will inevitably have magnetism. If a magnet with weaker suction power is used to adsorb from the front of the brake pad (that is, the friction surface), it can be clearly distinguished. The higher the iron content, the higher the hardness, and the more obvious the characteristics of noise generation and disc wear compared to other material brake pads. In addition, metallic materials have better thermal conductivity, which helps to dissipate heat, but it will also transfer high temperature to the wheel cylinder piston, causing the brake fluid to boil (the boiling point of general brake fluid is about two hundred degrees Celsius), thus leading to brake failure. Therefore, metallic brake pads need to be equipped with an insulation layer between the metal friction material and the backing plate to prevent high temperature from being transmitted back to the brake fluid.

　　The higher the temperature performance requirement, the thicker the insulation layer needs to be, and the effective wear thickness will be relatively reduced. Therefore, the minimum wear thickness specifications of brake pads from different manufacturers vary, which is related to their temperature resistance performance and insulation layer thickness. Especially for metallic brake pads, performance and heat insulation are often sacrificed for the sake of service life and effective thickness.

　　Common Metallic Brake Pads We See Today

　　After continuous improvement and breakthroughs, semi-metallic materials can also approach or even surpass traditional fiber brake pads in terms of noise and disc wear characteristics. However, in humid and corrosive environments, such as cold countries where salt water is sprinkled after snowfall to prevent ground icing (lowering the freezing point), or when vehicles are idle for a long time, brake pads and discs may rust, causing the brake pedal to jump after starting the car until the rust and dust are worn off. In serious cases, the friction material may be torn off from the backing plate of the brake pad, in which case a new brake pad must be replaced. The author once used brake pads that seemed to have been soaked in water, and as a result, in a short period of time, the brake pads expanded and peeled off like a steamed cake, and the safety factor was almost zero at this time. If the production date can be known, this is definitely an extra layer of protection for consumers, especially for brake pads with high iron content.

　　In fact, non-metallic materials are constantly evolving, and friction materials are constantly seeking a perfect balance between performance, durability, and noise. For safety and performance, manufacturers often make cost a secondary consideration. Fiber-based brake pads, from the use of leather and cotton materials to the discovery of asbestos, have not stopped developing. With the advancement of technology, they have evolved from the initial natural hydrogen-carbon organic compounds to fully synthetic fibers.

　　This type of brake pad, which uses synthetic fibers to replace asbestos fibers, is called Non-Asbestos Organic (NAO). The heat resistance and wear resistance of early traditional organic synthetic fibers were two major bottlenecks in their development - especially in terms of heat resistance, traditional organic fibers were very close to asbestos materials, meaning that they could not dissipate heat in time, leading to whitening and cracking on their surface.

　　The resin brake pads we commonly see

　　The latest NAO uses fiberglass and DuPont Kevlar fiber, which has 2-6 times the wear resistance of ordinary traditional synthetic fibers, and also has excellent noise control performance. This kind of fiber is also added with non-ferrous metals such as brass and copper powder to improve its heat transfer capacity, and even magnesium alloy, titanium alloy, and ceramic components are added to increase its thermal stability, making its performance comparable to metallic materials.

　　This type of brake pad contains relatively less metal components, using special metals to replace ferrous metals, and has excellent wear resistance and noise performance. It has gradually been used by major automakers as OEM standard parts for new cars. Because the technology of ferrous fiber is in the hands of European and American countries, Japanese manufacturers attach great importance to the research and development of the latest generation of NAO and strive to make it a mainstream product. In addition, perhaps due to the extensive use of iron fiber materials, the noise generated by some brake pads is still considered acceptable in Europe, which surprised and amazed me. In addition, in terms of rust and other safety and environmental protection aspects, NAO is far superior to other brake pad materials. However, due to its high cost and the difficulty in breaking through mass production technology, the popularization of the after-sales service market still requires some time.