The spring has two main functions: one is to act as a buffer between the suspension system or chassis and the ground, that is, to maintain comfort; the other is to keep the tire close to the ground when the car passes through uneven roads. To achieve these two conflicting goals, different elastic coefficients are needed. Keeping the tire on the ground has a decisive impact on handling. We need a hard spring setting to keep the tire on the ground. The more bumpy the road, the softer the spring setting we need. To achieve these two purposes at the same time, it is feasible to use the spring with composite elastic coefficient (nonlinear spring), that is, the so-called progressive spring. Progressive spring can increase the elastic coefficient with the compression of spring, which is quite difficult in design and manufacture. When driving on bumpy roads, the elastic coefficient will increase to maintain the stability of the vehicle body. The initial elastic coefficient is soft, which is used to improve the adhesion of tires on bumpy roads. The gradually stiffening spring can avoid the suspension or spring sitting at the bottom. This allows the use of springs with a lower height than the original to reduce the center of gravity of the vehicle body, and maintain the lowest and shortest suspension travel when passing through bumpy roads, so as not to sit at the bottom. To achieve progressive spring is to make nonlinear spring whose elastic coefficient will change with compression. Most progressive springs use unequal pitch spring or coil diameter change spring. When the unequal pitch spring is compressed, it will produce local line to line contact to change the number of effective turns, resulting in the change of elastic coefficient K. The change of the diameter of the upper and lower coils through the spring is the most direct way to change the elastic coefficient