How Shock Absorbers and Struts Work

[speedkar9]

Here’s how a strut or shock absorber works in your car to dampen the ride. I took apart a strut and grinded it open to see the mechanics inside and made a short video on it:


https://www.youtube.com/watch?v=ncyKSDq2FDs


The spring is responsible for supporting the weight of the vehicle. It absorbs the majority of the impact when the wheel travels over a bump in the road. However, the spring stores that energy and needs to release it. Left uncontrolled, this would lead to a bouncy ride.
To counteract this, we have the damper, also known as the shock absorber. In a strut assembly, the coil spring sits on top of the shock absorber. I’m use these terms interchangeably throughout.
A car acts like a spring-mass-damper system, with a free body diagram roughly as such:

Disassembling the strut:


I grinded open the lamination at the top:

 
And I pulled out the cylinder from the housing; inside which contains a piston. Some oil dripped out.

The cylinder has a valve on the bottom:

This valve is an orifice containing a few spring washers on either side. This works to control the flow of oil in a particular direction:

The piston itself has another valve, pretensioned with this spring on the end:

Here’s a rough diagram of what goes on inside a shock absorber:

As the piston moves up and down with the suspension, the oil inside the pressure tube is forced to go through the piston valve and the base valve to move into the adjacent chambers. The orifice in the valve controls the rate of oil flow, and provides resistance to motion, giving it that damping effect.
The idea behind a shock absorber or strut is to ease the natural bouncing motion of a spring. If the shocks are worn, and the system becomes underdamped, then that wheel is going to be bouncing down the road (red line).
If the shocks are too aggressive, then it can create a situation where it delays the time it takes for the tire to rebound to its position before the bump (green line).
At critical dampening, the tire will rebound as quickly as it can to the road, without overshoot (blue line). In reality, critical dampening isn’t used, rather its slightly tuned underdamped for a more comfortable ride.


And that’s pretty much all the guts inside of a strut or shock absorber.

Enjoy
-speedkar9