So, now let's give a scientific and systematic explanation.

The essence of replacing the wheel hub is to reduce the quality under the spring (aesthetics are secondary).

What is unsprung mass

The blue line represents the body part, and 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 are also a series of suspension mechanisms on the real car to constrain the jumping range of the wheels, which are 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 hub, tire (including screws), swing arm, spring, shock absorber, pull rod, brake assembly (brake drum or brake disc+abalone), and some models also include the upper axle, integral axle, etc.

In fact, we can think of it this way. The reason why the picture is drawn in this strange way is to make people who have no concept of spring on and spring off think of the car as follows: the car can be divided into two parts, one part is rolling 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 the spring off. The other part is the superstructure where people sit. If the suspension is removed, it will collapse and kiss the ground. It must be supported by springs and shock absorbers. 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 as under spring.).

Understanding this is to understand the root of the problem. This is also where I am not satisfied with the explanations of under spring quality optimization in the current automotive media. After building this simplified model and considering under spring quality optimization, it will become much clearer.

The significance of reducing the quality under the spring

There is a saying that goes: 1 kilogram under the spring, 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 10kg off the spring. This statement is of course only circulating in the community, and the actual situation is more complex, requiring consideration from two aspects: separately considering the benefits brought by the decrease in spring quality; And the impact of the combined spring and spring mass on automobiles.

Firstly, let's take a separate look at the impact of reduced quality under the spring.

Mainly reflected in acceleration and deceleration performance.

This refers to 1 kilogram off the spring and 10 kilograms on the spring.

The principle is easy to understand. As a component directly connected to the half shaft, the rotational inertia of the wheel hub tire has a very direct impact on its performance. Regardless of whether you have 250PS or 280PS, 350Nm or 420Nm, you need to overcome the rotational inertia of the wheel hub tires first in order to transmit 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 impact of rotational inertia on acceleration and deceleration performance is not limited to abalone or swinging arms. Because they do not rotate with the wheels, their ability to drag back legs is not much different from the spring-loaded parts. So what do those who like scales even after changing abalone and dishes think?

This involves another aspect of consideration.

Spring to spring mass ratio

This requires the use of the simplified model from the hand residue above.

For cars, the road surface is definitely not as smooth as a mirror. Not to mention various potholes, speed bumps, manhole covers, and stones that can cause bouncing, let alone carefully observing the asphalt road surface, which can be considered very rough.

But in a comfortable car, what we feel is silky smooth. In addition to using a combination of softer spring shock absorbers, increasing the mass ratio between the spring and the spring is also an effective means.

A truck has a very interesting characteristic: when unloaded, the car shakes in a mess, and it has to be decorated with something to drive like that.

The reason behind this is that the mass ratio between the upper and lower springs has increased.

Let's go back to simplifying the model. All bouncing on the road surface is first applied to the spring-loaded part, and the bouncing on the spring-loaded 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, and when the spring bounces, compresses or relaxes, it will break the balance and generate excess pressure. According to Newton's second law, the pressure applied to the spring below will also be equally applied to the spring. At this point, two options are available (or both):

Firstly, increase the mass on the spring, just like loading things on a truck, by increasing the mass on the spring, the acceleration caused by the spring jumping transmitted to the upper part of the spring can be weakened. Simply put, it relies on the weight of the vehicle to suppress bouncing.

Secondly, reduce the mass under the spring and lower the rebound force required for the spring-loaded part to produce the same bounce, in order to alleviate the impact on the spring-loaded part.

Overall, the ratio of the mass on and off the spring needs to be increased.

Of course, the most ideal situation is for both the on spring and off spring mass to be reduced, but the amount of reduction under the spring is even greater than that on the spring, resulting in an overall increase in the ratio of on spring to off spring mass.

So, some people often say that when a car is heavy, it drives smoothly at high speeds. There is some truth to it, but a more accurate statement is that the mass ratio between the spring and the spring is greater, making it stable to drive.

Springs and shock absorbers that cannot be ignored

As the part that connects two large blocks and is responsible for supporting, transmitting force, and absorbing impact, the selection of springs and shock absorbers has a greater 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, modification is a system engineering, and one-sided modification should be avoided.