Welcome to the NEW Flatout Suspension!

Technical Page

How Shocks Work: Compression and Rebound

What are shock absorbers?

Shocks are a really important suspension component, which are, unfortunately, very misunderstood. Some of the confusion can be attributed to the misleading name of the part.

Shocks, are not actually a load bearing part, the majority of a vehicles weight is supported by the springs. Shocks do absorb though, they absorb the energy of the spring as it compresses and decompresses. It’s what basically keeps your car from bouncing over and over again after you hit bumps.

In the most basic sense, Shock absorbers regulate how the springs operate. We need shocks because springs, if used alone, will continue to oscillate for an extended period of time as energy dissipates. This is why cars with blown shocks tend to bounce up and down for a while after they hit bumps.

How do shocks work (compression and rebound)?

Inside the of a shock, there is a piston which moves up and down through an oil, this piston has holes in it. As the piston moves oil is forced through these holes. The movement through the holes is regulated with shims. The way the shims are arranged combined with the type of holes/slots in the piston is what determines the valving. The Resistance is created as the fluid or oil is forced through these holes during every up/down every cycle, which is what causes the damping. These resistances are called Compression and Rebound.


Compression is what controls how the shock is compressed, and regulates the spring’s movement as the wheel moves toward the car. Compression resists things like:

-Nose dip on a car as heavy braking is applied.
-The bump movement of a car as the driver hits bumps or elevation changes in the track.
-The downward movement of the rear caused by weight transfer when a vehicle is accelerating.
-Body roll as lateral weight transfers to the outside of the vehicle when the driver deviates from a straight line to turn.


Rebound is the opposite of compression, referring to the movement of the shock and spring back to the original position. Rebound can be misleading because it also controls how the shock extends beyond the spring’s original position. Rebound does things like:

-Resists the rear of the vehicle rising as heavy braking is applied.
-Resists the front of the vehicle lifting during acceleration.
-Resists the lifting of the inside wheel as a driver deviates from a straight line that is caused by weight transfer.

Shock Valving:

Shock Absorbers dampen the movement of suspension by forcing specially formulated oils through holes in the shock piston, this converts the kinetic energy into thermal energy, this transition allows the shock to control the movement of the suspension.

High performance shocks feature a piston on the end of the shock shaft, the shaft moves up and down inside a cylinder filled with shock oil. Precision machined shims are placed on the top and bottom of the shock piston, the configuration of these shims is what controls the movement of oil through the ports in the piston. As the piston moves at different speeds the shims flex and allow the shock oils to pass through the holes. Through the use of different shims the stiffness of the rebound and compression are controlled. Additionally different piston designs can also change the dynamics of the oil flow, such as Digressive Pistons used by Flatout Suspension and a few others.

What are shims?

Shims are precision machined metal washers which are heat treated to handle temperatures upwards of 500 Degrees without affecting their strength. Shim thickness is usually referred to in thousands of an inch(.001″) and is referred to as “compression over rebound” like 12/15 being .012″ compression and .015″ rebound.

There are two shim stacks, a compression stack on one side of the piston and a rebound stack on the opposite side. It’s important to keep in mind that the rebound and compression stacks are located on the opposite side of the piston from their associated direction of travel.

Shims aren’t always plain washers, they can be different sizes and shapes, with slots or holes to allow more dynamic control of the damping forces.

The most common arrangement for the shims (standard valving) is in a pyramid shape,which features the discs getting progressively smaller for well controlled damping at all speeds. This is used as the standard configuration because it works very well, soft damping at low speeds, hard damping at high speeds.

As the shock moves faster/harder and more oil is forced through the hole and the pressure against the shims increases causing more to come into play.

Compression and rebound are tuned independently of one another and each stack is usually made up of between 5 and 10 shims.

An important thing to remember is that all shocks have their limit, all the damping is achieved by converting the kinetic energy into heat, and the heat must to be dissipated through the shock oil and body. If the shock oil reaches a point, at which the oil begins to break down or expand to the point of cavitation, the piston can hydro-lock. This is a problem that is much more likely to occur in a poorly valved shock and is one of the reasons you should only have your shocks re-valved by a qualified individual.

Shock valving types:

There are 3 types of shocks (not talking about mono-tube/twin-tube):

Progressive Shocks: These shocks are an old design and are the least useful for very good reasons. The valving in these is simply the holes in the piston that control the flow of shock oil, so as shaft speed increase, damping increases at an exponential rate. These shocks are unable to absorb sharp, high energy forces like holes or ruts in roads/tracks. At the same time slow constant forces like body roll during turns are poorly damped. This shock design is unacceptable for modern performance applications.

Linear Shocks: These were the next big advance in shock technology. As shaft speeds increase, damping increases at a predictable, linear rate. Linear valving allowed more control at lower shaft speeds, controlling body roll during turns, providing far more control than progressive valving ever had.

Digressive Shocks: These are the latest development in shocks and are the opposite of the progressive type. As the shaft speed increases the damping forces increase at a decreasing rate. (Yes, that’s a confusing way to say it.) Digressive shocks still provide low speed damping without being excessively harsh on rough roads and still increases damping at higher speeds, but at a more reasonable and predictable rate. Digressive shock designs are what allow rally cars tackle intense terrain at high speeds.
Source : www.crawlpedia.com

 Spring Rate Conversions:

kg/mm to lbs/in
16 = 896 15 = 840 14 = 784 13 = 728 12 = 672 11 = 616
10 = 560 9.0 = 504 8.5 = 476 8.0 = 448 7.5 = 420 7.0 = 392
6.5 = 364 6.0 = 336 5.5 = 308 5.0 = 280 
4.5 = 252 4.0 = 224 3.0 = 168