Shock Hydraulics

This month I will describe how a shock works hydraulically. The hydraulic shock is just a pump.  However, the fluid does not get pumped out like it does in a normal pump. Rather, the fluid is only pumped from one chamber to another.  Since fluid does not compress, the shock or fork must have someplace to pump the fluid (oil). Because the oil has body, or viscosity, it will cause a resistance to the pump, slowing it down.  By knowing these few facts we can start to understand how a hydraulic shock works.

Early shocks had a shock body, a separate cylinder, and a shaft with a piston that traveled in the separate cylinder.  The cylinder was inside the body, and the shaft and piston were inside the cylinder – with some oil and air filling the shock body and a cap covering the top of the body to contain all the parts.  As the piston moved up and down in the cylinder it would pump the oil out of the cylinder into the space between the outside of the cylinder and the inside of the shock body.  Since the shock was not completely filled with the oil - some of the space inside the shock was left with only air - the oil could be pumped in and out of the cylinder.  The problem with this type of shock design was that the air would mix with the oil causing the oil to become thinner, therefore losing dampening control.  Also, air contains impurities (just look at an engine air filter sometime) which will cause the air to heat up. This would cause a change in internal pressures in the shock and cause dampening forces to change. 

Today’s shocks have a shock body, a shaft and piston, and a chamber attached to the body – either directly or with a hose.  The purpose of this chamber (reservoir) is to give the oil a place to be pumped as the shock goes through its travel.  In the reservoir there is a separator – either a bladder or a free piston – that separates the oil from the air space so the oil and air do not mix.  With no air to mix with the oil, the oil viscosity does not thin out, hence a more consistent dampening.  On the other side of the separator is pressurized nitrogen instead of air.  Nitrogen is cleaner and heat does not affect nitrogen as much as it affects air, so the pressure stays more consistent, creating a more consistent dampening force. 

As the piston goes down in the shock body (on the compression stroke) some of the oil is forced into the reservoir, while most of the oil passes through a series of holes in the piston and ends up on the top side of the piston.  The act of the oil passing through these holes slows down the speed of the compression stroke. 

As the load is taken off the shock, the spring then tries to return the shock (on its rebound stroke) to its fully extended length.  When the shock starts rebounding, the oil in the reservoir is sucked back into the shock body.  The oil that passed through the piston is now compressed between the top of the shock body and the piston.  The oil then passes through another series of holes, slowing the rebound.  This oil and the oil from the reservoir mix together so the process can be repeated again and again.  The nitrogen pressure helps push the oil back into the shock body, so the shock body always remains full of oil. 

Basically, the front forks (cartridge variety) work just like the old style shocks with the separate cylinder (cartridge) inside the fork tubes.  The only difference is that the modern fork is much larger and therefore holds a lot more oil.