Shock Tutorial 101
By Michael Randolph
Hello, and thank you for taking the time to learn a little more about shocks and suspension. As most of you know, shock absorbers can be a very expensive purchase. And everyone wants to get their moneys worth. The purpose in me writing this is to make sure that everyone is well informed as to the function, design, servicing, and performance of the many types of shock absorbers (dampers) available today. There are many people who have varying opinions about the best methods for suspension design and setup. I believe there are many ways to properly setup a suspension, but would like to layout a basic foundation of understanding for everyone.
Purpose of dampers:
Shock absorbers (dampers) are designed to control and resist motion, specifically in our case, for suspension. In doing so, they produce friction between the oil and the working components of the damper. So basically they turn mechanical energy into heat. When we talk about duty we will be referring to the amount of friction the damper is capable of withstanding .
This duty will help a customer determine what shock will work for their application. For example: A 2.0 non reservoir shock provides less duty than a 2.0 reservoir shock. Neither of the two are able to produce more friction than one another, but the reservoir shock can sustain the same level of performance without fear of being overheated with it’s additional oil capacity and surface area (to radiate heat).
Why is overheating a shock problematic? Well, as oil increases in temperature it looses its ability to produce a consistent amount of friction. In fact if it gets too hot, oil can loose its friction coefficient permanently and become “burnt oil” just like used engine oil.
When your damper overheats, there are two main effects. Your suspension becomes less predictable as your shocks do not dampen consistently at varying temperatures. And, the internal working components, which are protected from wear by the oil, will be more susceptible to premature wear.
What does that mean for you? Your suspension handling will not be consistent over time, and you will spend more money and time servicing and replacing parts on your dampers than if you provided your suspension with dampers of adequate duty.
Design of a damper:
Almost all dampers today consist of a shaft with the working components bolted to one end. Of the working components, the most important piece of the system is the piston head as flow around the head is crucial to the ability of a damper to perform. Valve shims, generally in the range of .008”-.020” are stacked in decreasing diameter on both the top and the bottom of the piston head as the shims are farther away from the piston. As the piston head is forced thru the oil in the shock the valve shims deflect, allowing oil to pass thru ports of the piston. As the valve shims increase in thickness, so does the resistance to deflection, hence stiffer valving.
As the piston head travels downward, the shims on the top are deflected upward and in doing so resisting extension. These are called the rebound shims. Generally the rebound shims required are a smaller diameter as well as the ports in the piston head. Why? Because the pressure inside the shock is exerting a force on the shaft and working components as a whole, trying to force the shaft outside of the body. So having ports smaller than the compression ports on the rebound side allows for more friction with less shim deflection. Actually, if the ports were the same size on the compression and rebound side of the piston, the rebound side would require much stiffer valving to achieve the same results.
Generally the compression side of the piston head has larger diameter shims, and more of them. Not only to accommodate for more precision tuning, but to help compensate for the resistance the shock has to the shaft displacing the volume of the high pressure nitrogen inside the shock. This resistance to compression is exactly what makes an air shock function as a spring. The shaft temporarily exerts a force on the nitrogen inside the shock, compressing it, and causing the pressure to resist compression thus creating a spring force. On the piston head is the wear band. This band wraps around the circumference of the piston and makes contact with the inner diameter of the shock body. It does this with two main purposes. One is to locate the piston without wearing either the body or the piston. And the other is to seal the outer diameter of the piston, forcing nearly all of the oil in the shock thru the piston head’s ports. When the wear band is worn excessively it will allow the shock shaft to wobble slightly in the shock, and oil will bypass the piston without any resistance from the shims. Some shock companies have used soft metal wear bands such as copper, however, many of the benefits of added wear life are negated by wear on the body and high density particulate suspended in the oil(small pieces of copper).
The seal assembly of a high pressure shock is a little more complicated than most care to understand. So we will breeze thru it only touching on the highlights. One of most notable parts of the seal assembly is the guide bushing. The guide bushing has a low friction surface allowing the shock shaft to slide in and out with very little drag, while keeping the shaft located and straight. Even with very little play in the guide bushing, there is a significant increase in wear on many of the working components. Many shocks with steel shafts that have been hard-chrome plated will eventually rust and pit. When these pits and burrs are forced thru the seal assembly they tear away at all the seals and heavily damage the guide bushing. Most notable in sealing are the first two seals the shaft comes into contact with. The outermost seal can be seen on the shock in the end cap. And is called the scraper or wiper seal. Its primary responsibility is to clean the shaft from any debris as it enters the shock. A good visual indication of the condition of the shock’s internals is the condition of the wiper seal. A tight seal will leave a small ring of oil at the seals lowest point after compression.
The shaft plays a very important role in the function of a damper for obvious reasons. But choosing a shock with the best shaft for the application is essential. Most shock suppliers use either a 14mm or .625” shaft on standard dampers. Still others only offer .875” shafts. While the .875” shaft offers more strength and resistance to bending, it also created more resistance to compression. This can be a problem for light vehicles that need softer damping. Another common shaft size is 1.25”, which are used in air shocks as mentioned earlier. Whatever you choose for your shaft size, be certain it is made of stainless steel, as hard-chrome plating can flake off from dings and corrosion.
We provide much more in-depth information in our other informational write-ups. Please take the opportunity to read some of our HOW TO guides as they are made available.
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