1. Springs: The oldest and most common type of spring used in motorcycles. The spring stiffness, also known as the spring rate, represents the ratio of force to deformation. Under the principle of not exceeding the load limit, the spring rate of a simple coil spring remains constant under different stress states, unaffected by ambient temperature. After painting or lubrication, its stability and durability are very high. However, the only drawback of coil springs is their weight. Therefore, higher-end forks have begun to use lighter, but more expensive, titanium alloy springs.
2. Air Springs: Because air is light, using air as a spring is arguably the most suitable for bicycles. Furthermore, if different stiffness is needed, there's no need to replace the spring; simply increase or decrease the air pressure. However, using air springs requires sophisticated technology. The high-pressure gas must be completely sealed; once the air pressure is lost, it becomes completely ineffective. Therefore, early air springs had sealing problems, and there were even instances of sudden loss of all air pressure during use. This problem is almost no longer occurring. However, like tires, they don't completely eliminate the need for occasional inflation. Traditional air forks suffer from a stiction problem, where the seals (oil seals) for high-pressure air are very tight, almost causing a jamming sensation. This requires significant external force at the start of compression, making them less responsive to small vibrations. The use of negative springs has largely overcome this issue. While air springs have proven effective in mid-travel suspension forks, they cannot yet replace coil springs in long-travel forks.
