Suspension System
The suspension system is a general term for all the force transmission components between the vehicle’s frame and the axles or wheels. Its function is to transmit forces and moments between the wheels and the frame, absorb impacts transmitted from uneven road surfaces, and dampen the resulting vibrations, ensuring a smooth ride. The suspension system should support the vehicle’s weight, improve the ride quality, and provide different driving sensations based on various suspension settings. The suspension system, although seemingly simple, integrates various forces and its characteristics determine the vehicle’s stability, comfort, and safety, making it one of the most critical components in modern automobiles.
Two Major Classifications of Suspension Systems
Generally speaking, automotive suspension systems are divided into non-independent suspension and independent suspension:
Non-independent suspension: The wheels are mounted at both ends of a single rigid axle. When one wheel moves, the other wheel moves accordingly, causing the entire vehicle to vibrate or tilt.
Independent suspension: The axle is split into two sections, and each wheel is independently suspended beneath the frame using a spiral spring. When one wheel moves, the other remains unaffected, allowing the wheels to move independently of each other, which improves vehicle stability and comfort.
As people demand higher ride comfort and handling stability, non-independent suspension systems have gradually been phased out. Since the space beneath the vehicle makes it seem as though the body is floating in mid-air, how can we connect the body with the wheels that contact the ground? This connection device is the suspension system. In addition to supporting the vehicle’s weight, the suspension system is also responsible for reducing vibrations during driving and improving handling performance.
Classification of Suspension System Structures
1. Non-independent Suspension System
The non-independent suspension system is characterized by both wheels being connected by a single rigid axle, with the wheels and axle suspended under the frame or body via an elastic suspension system. The non-independent suspension system is simple, cost-effective, strong, easy to maintain, and causes minimal front wheel alignment changes during driving. However, due to poor comfort and handling stability, it is no longer used in modern passenger cars, but is still common in trucks and large buses.
2. Independent Suspension System
In the independent suspension system, each wheel is independently suspended beneath the frame or body using its own elastic suspension system. The advantages of this system are: light weight, reduced impact on the body, and increased wheel traction; it also allows for softer springs to improve comfort, lowers the engine position and the car’s center of gravity to improve stability, and allows each wheel to move independently, reducing body tilt and vibrations. However, the independent suspension system is more complex, expensive, and harder to maintain. It can be further divided into types like double wishbone, longitudinal arm, multi-link, candle, and MacPherson strut suspensions.
3. Double Wishbone Suspension System
The double wishbone suspension system refers to an independent suspension system where the wheel moves in the transverse plane of the vehicle. It can be further divided into single wishbone and double wishbone systems based on the number of wishbones used.
Single wishbone suspension: This system is simple, with a high roll center and strong anti-roll capability. However, as vehicle speeds increase, the high roll center can cause excessive wheel distance changes during wheel movement, leading to tire wear and increased instability during sharp turns. Single wishbone systems are typically used in rear suspensions.
Double wishbone suspension: This can be further divided into equal-length and unequal-length double wishbone systems. The equal-length version maintains the kingpin inclination angle during wheel travel but causes large changes in wheel distance, resulting in severe tire wear. Unequal-length double wishbone suspensions, when properly optimized, can maintain acceptable changes in wheel distance and front wheel alignment, improving vehicle stability.
4. Double A-arm Suspension
The double A-arm suspension is widely regarded as the best for handling, and most performance cars, including Formula 1 cars, use this suspension design, also known as the double wishbone suspension. The two A-shaped arms form a stable triangular structure, providing excellent resistance to torsion and good lateral force guidance. This design greatly reduces body roll when cornering and can handle extreme off-road impacts when used in SUVs. The suspension parameters in this structure, such as the front and rear camber, toe-in, and kingpin angles, are adjustable, enhancing the vehicle’s overall handling stability. Although more commonly found in performance cars, some regular cars also use a similar structure. However, the double A-arm suspension can be complex and costly, and it takes up more space, making it less suitable for smaller cars.
5. Multi-link Suspension System
The multi-link suspension system consists of 3 to 5 arms controlling wheel position changes. This system allows the wheel to move along an axis that forms a certain angle with the vehicle’s longitudinal axis. It is a compromise between the double wishbone and longitudinal arm systems, offering a mix of both systems’ advantages. The multi-link system provides minimal changes in wheel distance and toe-in, allowing for smooth steering under both driving and braking conditions. However, it may experience axle oscillation at high speeds.
6. Longitudinal Arm Suspension System
The longitudinal arm independent suspension system allows the wheel to move in the longitudinal plane of the vehicle. It is further divided into single-arm and double-arm types.
Single-arm suspension: The main drawback of this system is that the kingpin angle changes significantly during wheel movement, making it unsuitable for steering wheels.
Double-arm suspension: This setup uses two equal-length arms, forming a parallelogram structure, keeping the kingpin angle unchanged during wheel movement. It is mainly used in steering wheels.
7. Candle Suspension System
The candle suspension system is characterized by the wheel moving along a rigidly fixed spindle axis. While the main advantage is that the kingpin angle does not change during suspension deformation, only wheel distance and axle distance slightly vary, providing good steering stability. However, the main disadvantage is that the lateral forces encountered during driving are all absorbed by the spindle, causing frictional resistance and wear. This system is rarely used today.
8. MacPherson Strut Suspension System
The MacPherson strut suspension system is similar to the candle system but with a movable spindle. It combines aspects of the wishbone and candle suspension systems. Its advantages include a compact structure, minimal front wheel alignment changes during wheel travel, and good handling stability. Additionally, by eliminating the upper wishbone, it provides more room for engine and steering system layout. This suspension is commonly used in front suspensions of small and medium-sized cars, including the Porsche 911 and several domestic models.
9. Active Suspension System
The active suspension system, developed in recent years, is a computer-controlled system that integrates mechanics and electronics. It uses sensors to monitor various factors like speed, braking pressure, throttle position, and steering angle, sending this data to a microcomputer. The system adjusts suspension parameters in real-time to maintain vehicle stability and comfort. For example, when a vehicle turns or brakes, the active suspension generates a counteracting force to minimize body tilt and ensure smooth handling.
10. Air Suspension System
Compared to traditional steel spring suspensions, air suspension offers many advantages, particularly the ability to automatically adjust spring stiffness based on driving conditions. For example, at high speeds, the suspension becomes stiffer for better stability, while at low speeds, it softens for comfort. However, the system is still relatively new and prone to issues like seal damage, which can affect performance.