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The Working Principle of Brakes (Part 2)
In the automotive market, many mid- to low-range models with smaller engine displacements typically feature a braking system that combines "front disc brakes with rear drum brakes"—that is, disc brakes on the front wheels and drum brakes on the rear. Examples include the popular First Auto Works-Volkswagen Jetta, Changan Suzuki Alto and Antelope, BYD Freler, Dongfeng Yueda Kia Qili Ma, and Shanghai General Motors Sail, among others. Let’s first take a brief look at the rear-wheel drum brakes, which are commonly used in these vehicles.
The practical differences are quite clear—disc brakes are superior to drum brakes. Both drum and disc brakes have their own advantages and disadvantages. In terms of braking performance, the gap between disc and drum brakes isn’t significant, since braking relies on converting kinetic energy into heat through friction. If the vehicle is compact with a lightweight body, drum brakes on the rear wheels can actually suffice.
In terms of heat dissipation, disc brakes cool down faster than drum brakes, and ventilated disc brakes offer even better cooling performance. When it comes to responsiveness, disc brakes are generally more sensitive. However, on rainy days or when roads are muddy, if mud and sand stick to the drum brake shoes, the braking efficiency can be significantly reduced—this is actually a drawback of disc brakes. As for cost, drum brakes are typically cheaper than disc brakes and also have a longer service life. That’s why many mid- to low-end vehicles still opt for drum brakes, while most models in the mid-range and above usually come equipped with four-wheel disc brakes.
From an economic and practical standpoint, automotive designers typically opt for a hybrid braking system in sedans: disc brakes on the front wheels and drum brakes on the rear. During braking, the front wheels usually bear 70% to 80% of the vehicle's total load due to inertia, making them carry more weight than the rear wheels. To cut costs, car manufacturers often equip front-wheel disc brakes while reserving drum brakes for the rear wheels. In mid-to-high-end sedans equipped with four-wheel disc brakes, the front wheels are fitted with ventilated disc brakes to enhance heat dissipation. Meanwhile, the decision to use non-ventilated disc brakes on the rear is also driven by cost considerations—after all, manufacturing ventilated discs involves significantly more complex processes and higher expenses. With advancements in materials science and declining production costs, disc brakes are gradually replacing drum brakes in the automotive industry, particularly in passenger cars.
Generally, brakes work by applying a braking torque to a rotating component via fixed elements, thereby reducing the latter's angular velocity. At the same time, the friction between the wheels and the road surface generates a retarding force that slows down the vehicle. Any brake that relies on friction between the working surfaces of its fixed and rotating components to produce braking torque is classified as a friction brake. The friction brakes used in automobiles are broadly divided into two main types: drum brakes and disc brakes.
The rotating component is fixed to the wheel or half-shaft; when the braking torque is applied directly to the brakes on each side of the wheels, it’s referred to as a wheel brake. Alternatively, if the rotating component is mounted on the transmission shaft of the drivetrain and the braking torque is transmitted through the drive axle before being redistributed to the brakes on both wheels, this type of brake is called a central brake.
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① Friction brakes: These rely on the friction force between the braking component and the moving part to achieve braking. ② Non-friction brakes: The main structural types of these brakes include magnetic powder brakes (which use the shear force generated by magnetizing magnetic powder for braking), magnetic eddy current brakes (whose braking torque can be adjusted by regulating the excitation current), and water eddy current brakes, among others.
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