How Does the Damper Work?

The damper is a mechanical device used in automobiles to absorb shock, hence it is also called a shock absorber. The damping done with the help of fluids is viscous damping, it is more effective in providing resistance to vibration. In automobiles mostly we will be using viscous dampers to absorb the energy dissipated by the spring during jounce and rebound. 

To understand the need for a damper in automobiles, firstly we should know the difference between sprung and unsprung mass. The springs and dampers are the components which isolate the sprung mass from the unsprung mass.

Sprung masses are the components that are above springs and damper (Ex: Body, Chassis, passengers, and luggage). Unsprung masses are the components which are below springs, springs and damper will also be accounted for in unsprung mass (Ex: Wheels, suspension links, springs and dampers).

Why Do We Need A Damper?

The objective of the suspension system is to isolate the sprung mass from the unsprung mass, so that when the vehicle undergoes a bump, disturbance/impact load due to road conditions, the noise will be reduced to the passenger. Nearly 2% of the damping action comes from the tire and the remaining damping action is done by the suspension dampers.

During the bump, the spring compresses and during rebound, the spring expands, without a damper, the spring compression and expansion will be violent.

Damper tuning is one of the last tuning parameters to optimize the performance of the car. We can adjust the upper ball joint (strut mounting), which will vary the suspension parameters such as Caster, Kingpin Inclination, etc and the stiffness of the damper by adjusting the valves in the damper.

Types of Dampers 

Dampers basically convert kinetic energy into heat energy. The viscous dampers will have two fluids, high-pressure nitrogen gas(compressible) on one side and oil (in-compressible) on the other side of the piston. During jounce and rebound, the nitrogen gas inside the damper cylinder compresses and expands to give the cushioning effect, and also to absorb vibration.

There are many types of dampers, the following are the most widely used,

• Monotube Damper:

 In the monotube damper, there will be only one valve through which the oil moves. It is highly recommended for performance vehicles and to avoid aeration.                                  

• Twin-Tube Damper:

Here the damper will have two tubes, there will be more than one valve through which the oil moves. Since there is more than one valve there is a high chance of aeration(foaming) but provides good stroke length, highly recommended for off-road vehicles.

 

• Damper Characterisation:

 

The damper curves are used to decide the amount of force that can be applied to the automobile at the wheel input, during pitching, rolling and undergoing a bump. The following figure shows the damper curve of the front suspension, damper curve is plotted between damper velocity and damper force.

The point which separates the low-speed damping and high-speed damping is called ‘Knee speed’. Low-speed damping will occur during rolling and pitching and high-speed damping will occur when the vehicle undergoes a bump at high speed. The slope of the rebound curve should be higher than the compression curve. During compression the spring stores the energy and during rebound, it will release the energy.

Shock Motion Ratio:

The shock motion ratio is similar to the spring motion ratio, if you calculate the amount of spring displacement that has taken place with respect to wheel displacement it is called the spring motion ratio. Similarly, the ratio of the shock absorber displacement (compression and elongation) with respect to the displacement of the wheel centre (up and down movement)  is called the shock motion ratio.

Critical Damping:

Critical damping is the condition in which the system vibration reaches its equilibrium state in the shortest span of time. 

The condition is,                       

Where,

     K ⇒  Stiffness of the system

     m ⇒ Mass of the system

Critical damping is the threshold between the underdamped and overdamped systems.

• If the system is underdamped, there will be more oscillation and the time taken to reach the equilibrium will be very high. 
• If the system is overdamped, the system oscillation amplitude will be less but the time taken to achieve the equilibrium will be more than critical damping.

Damping Ratio:

• The damping ratio is the ratio between the actual damping and critical damping, 
• If the damping ratio < 1, it is an underdamped system
• If the damping ratio = 1, it is a critically damped system
• If the damping ratio > 1, it is an overdamped system

Normally, the sprung mass damping ratio will be between 0.5-0.7 and the unsprung mass damping ratio will be between 0.2-0.3 to get good comfort while driving.

Damping Coefficient:

The damping coefficient is a material property, in the viscous damper. It is the net result of the damper geometries, orifices, gas pressure and property of the oil used inside the damper.

Low-Speed and High-Speed damping:

Low-speed damping takes care of vibrations due to sprung mass actions such as rolling and pitching. High-speed damping takes care of vibrations due to unsprung mass actions, for eg, when the vehicle undergoes a bump at high speed.

Rebound Compression Ratio:

For the passenger car, usually, the rebound to compression ratio will be ‘3’, which means that the rebound damping should be ‘3 times more than the compression damping. The reason is that during compression the spring stores the energy and during rebound it will release the energy. Hence depending on the vehicle load and usage, we will be tuning the damper curve by changing the damper geometries such as cylinder diameter, shaft diameter, number of shims on the piston and gas pressure.