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Shock Absorber Primer

Shock absorbers (SA) control motion. They help control the rate at which the suspension moves up or down. Shock absorbers work in conjunction with springs (or torsion bars) to isolate, or reduce, the impact of surface irregularities upon a vehicle.

Without a SA the suspension bottoms easier and the smallest bump can cause it to oscillate wildly. That is the SA only task, to control compression and rebound.

When the suspension moves up (hits a bump), this is called compression.
When the suspension extends (after the bump or drop into a hole), this is called rebound.
These parameters are controlled by a piston on the end of the main shock shaft.
Different methodologies exist to accomplish this but the goal is to control the shock shaft speed for compression and rebound. This is what is referred to as valving.

There is no right or wrong when it comes to shock valving, only incremental differences. Typically a valve “stack” is used to control the oil flow from one side of the piston to the other. A valve stack consists of what are essentially small precision washers that flex once a fluid force is applied to them. Many different combinations can yield essentially the same action. This means your shock guy needs some experience or a shock dyno when you start to change the dampening characteristics of the shock.

Shock absorbers, like most things in life, come in different qualities with performance parameters or features that can vary widely. Typically, inexpensive sealed shocks are set-up for average conditions, or the engineers “best guess” on how it will be used. Take-apart shocks can be “tuned” for individual preference. Another advantage of take-apart shocks is that they can be serviced. Individual parts can be replaced and the oil can be changed to renew the function after hard use.

Take-apart shocks can be identified by (typically) a small brass fitting on the shock body or reservoir. This is where the shock is charged with nitrogen, a special tool with a needle fitting is required.

You cannot judge a book by it covers and neither can you judge a shock by viewing its exterior. A perfectly fine looking shock may have the oil contents of some prehistoric tar pit. They’re sealed so how do they get like that? Moisture, dirt, and air can get past seals and combined with wear on the internal bore will turn once high quality oil into goo. Cheap new oil is better than old expensive oil.

Service intervals go by time, use, mileage, or a combination of any of the three.

A high pressure shock is typically deemed superior to a low pressure shock as these shocks incorporate a floating piston or gas bag to separate the oil from the air. This eliminates foaming of the oil and increases the boiling point of the oil. Nitrogen is used instead of compressed air due to the low moisture content of the nitrogen. You cannot use a tire-type pressure gage to check the shock pressure (typically 200 -300 psi.) due to the low volume. Never recharge a shock without rebuilding first.

The shocks are usually referred to as having a high-speed dampening circuit and a low speed dampening circuit. This means shaft speed – not MPH. You can hit a square edge bump at 10 mph and use the high speed dampening circuit.

A dialed-in inexpensive shock (a shock that is tuned for your weight and riding style) is better than the trickest multi adjustable, triple rate, reservoir equipped shock that is not set up for your riding style. One mans soft is another mans firm so it is hard to generalize when describing shock action.

Keep spring preload to 1" or less. If you bottom or think you need to dial up the preload you probably need stiffer springs.

Servicing shocks can be done by the Do-It-Yourselfer with the right tools, a good tech manual, a few spare parts, and some mechanical aptitude – sometimes a little cursing helps also.

Happy Trails, JK
 
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