So, now let's give a scientific and systematic explanation. The essence of replacing a wheel hub is to reduce the unsprung quality (aesthetics are secondary). What is the unsprung mass? The blue line represents the body part, while the red line represents a series of components such as the wheels. The connection between the two is the spring with yellow lines and the shock absorber with green parts. Of course, there is also a series of suspension mechanisms on real cars to constrain the range of wheel runout, which is omitted here. Structurally speaking, the unsprung mass part includes the red, yellow, and green parts shown in the figure above, namely (but not limited to): wheel hubs, tires (including screws), swing arms, springs, shock absorbers, pull rods, brake assemblies (brake drums or brake discs+abalone), and some models also need to include the upper drive shaft, overall axle, and so on. In fact, one can think of it this way. The reason why the above picture is painted in this peculiar way is to make people who have no idea about the spring up and down think of the car as one thing: the car can be divided into two parts, one part is facing forward against the ground, and this part can still be stably placed on the ground after removing the spring and shock absorber. This part is called the spring down. The other part is the superstructure where people sit. If the suspension is removed, it will collapse and kiss the ground. Springs and shock absorbers must support this part. And this part is on the spring. The two are connected by springs and shock absorbers. Of course, springs and shock absorbers are generally considered under the spring. Understanding this is the fundamental aspect of wanting to understand the problem. This is also where I am not very satisfied with the explanations of underspring quality optimization in the current automotive media. Building this simplified model and then thinking about underspring quality optimization will make it much clearer. There is a saying about the significance of reducing the mass under the spring: 1 kilogram under the spring and 10 kilograms on the spring. The meaning is that the mass under the spring is reduced by 1kg, and the optimization effect achieved is roughly equivalent to cutting off 10kg on the spring. Of course, this statement is only circulating in the market, and the actual situation is more complex. It needs to be considered from two aspects: separately considering the benefits of reduced mass under the spring; And the impact of the combined mass on and off the spring on the car. Firstly, let's take a separate look at the impact of a decrease in mass under the spring. Mainly reflected in the acceleration and deceleration performance. This is what 1 kilogram under the spring and 10 kilograms on the spring refer to. The principle is easy to understand. As a component directly connected to the half shaft, the rotational inertia of the wheel hub and tire has a very direct impact on performance. Regardless of whether you have 250PS or 280PS, 350Nm or 420Nm, you need to overcome the rotational inertia of the wheel hub tires before transmitting torque to the ground through the wheel hub tires. Reducing the weight of the wheels and tires (including the rotating brake discs) can make power transmission more direct. However, the influence of rotational inertia on acceleration and deceleration performance is not related to abalone or swing arm. Because they do not rotate with the wheels, their ability to drag the hind legs is no different from that of the spring part. So what do those who like to exchange abalone and dishes and still like scales think? This involves another aspect of consideration. The mass ratio between the upper and lower springs requires the use of the simplified model left by the upper hand. For cars, the road surface is definitely not as smooth as a mirror. Not to mention the various pit bumps, speed bumps, and manhole covers, various stones can also cause bouncing. Moreover, upon careful observation of the asphalt pavement, the surface can be considered very rough. But in a comfortable car, we feel a silky smoothness. In addition to using a combination of softer spring shock absorbers, increasing the mass ratio between the upper and lower springs is also an effective means. A truck has an interesting characteristic: when it is empty, the car bumps like a mess, and it has to be loaded with something to drive like that. The reason for this is that the mass ratio between the upper and lower springs has increased. Let's return to the simplified model. All bouncing on the road surface is first applied to the underspring part, and the bouncing of the underspring part needs to be influenced by the spring and shock absorber. In a stationary state, the weight borne by the spring is the weight of the upper part of the spring. However, when the lower part of the spring experiences bouncing, the spring compresses or relaxes, which will break balance and generate excess pressure. According to Newton's Second Law, the pressure applied to the spring through the spring will also be equally applied to the spring. At this point, there are two options (or both): first, increasing the sprung mass is like loading a truck. By increasing the sprung mass, the acceleration caused by the sprung jump transmitted to the sprung part through the spring is weakened. Intuitively speaking, it means relying on the weight of the vehicle to suppress the bounce. Secondly, reduce the mass under the spring and reduce the rebound force required to produce equal bounce in the lower part of the spring, in order to reduce the impact on the upper part of the spring. Overall, the ratio of the mass above and below the spring needs to be improved. Of course, the most ideal situation is for the mass on and off the spring to be reduced simultaneously, but the reduced part under the spring is even greater than that on the spring, and the overall mass ratio between the spring and off the spring is still increasing. So, some people often say that when a car is heavy, it is safe to drive on high speed. There is a certain truth, but a more accurate statement is that the mass ratio between the spring and the spring is large, making it safe to drive. The selection of springs and shock absorbers, which cannot be ignored, has a greater impact on the connection between the two large parts and are responsible for supporting, transmitting force, and absorbing impact. The strength of springing under the spring will be fed back to the spring, which is more decided by the spring and the shock absorber the final say. So it can be said that modification is system engineering, and one-sided modification should be avoided.