Shock Explanation

What’s The Difference in Shocks and How They Work?

What shocks/dampeners do?
A misconception is a spring is a big part of the suspension and does most of the work. In a great shock the only thing a spring does is hold the weight of the vehicle, rider(s), and cargo up at a predetermined measurement between the rear shock mount on the swing arm and on the frame. It does not except in the situation of most inferior suspended motorcycles such as HD. A motorcycle sometimes uses either an Air Bag, Air Spring, Spring, or Coil Over Spring. The actual shock/dampener purpose is to control the suspension movement and not the spring.

HD Air Shocks ~ How it Works
A damper found in these types of suspension is nothing more than a piston, an outer & inner cylinder (twin tube), shaft, and eyes to mount it.
*Look at the bottom of the cut-away damper and you will see an orifice. The diameter of the hole controls the slowing action. Thicker oil = smaller “hole” + more stiction (parasitic drag).
*Push the oil out and it resists due to the size of the hole for the volume being pushed out. Pull it apart and the damper resists the action because the oil is sucking through the orifice.
* A stock HD air shock there are 2 springs, an inner and an outer. There is no spring Pre-Load so the varying weight can not be compensated for by adding pre-load like it can on an externally adjustable spring pre-load gas shock. 
* I provided lines and arrows to detail the order of assembly.  This shock can not be taken apart except for the outer reservoir being separated from the damper.  
* There is little metering to control shock speed. A linear resistance is provided by the damper only and there are crude provisions to control changing shock travel speeds. This is one of the most economical ways to manufacture an adjustable mechanism to control suspension movement.

HD Air Shocks cut away
12" OEM Air Shock

Look for comparison between a 46mm Penske piston found in their shocks and the small OEM HD Air Shock identified with the blue arrow.

  • The movement is dampened in both directions by limiting oil flow through

* Very Cheap to produce.
* Inexpensive way to adjust for different loads

* Poor performance and ride quality.

Air Bag Systems:
The bag is inflated just like a tire and when you push into it the tire resists the action. Just like a tire the more you inflate it the more it expands up to a point. The problem with this system is;
The air bag is not connected to anything; the damper is connected from the swing arm & to the frame but the air bag is not. This type of system will produce more of a loss of control/handling, more so than by a spring. This type of system is the absolute worst way to control suspension movement. The air bag, which replaces the springs in a HD shock function is only to hold the bike in position.

Air Bag

*Most motorcycle air bag systems will allow the motorcycle to move up and down.
* Very Cheap to produce.
* Inexpensive way to adjust for different loads

* Poor performance and ride quality.

Inexpensive Sealed Hydraulic shocks ~ How it Works
The shock oil is forces through small holes in the piston or through reeds much like a 2 stroke weed eater’s carburetor, thus offering resistance and dampening the movement of the piston/shaft. It has springs which usually are an integral part of the suspension. The springs usually do most of the work and the damper slows down the spring.

Deficiencies of a Hydraulic Shock Absorber
*The technology of the piston valving usually is primitive in most cases.
* Usually the cheapest to produce and price is the driving force.
* Sealed unit and usually non-rebuildable, disposable at the end of the lifecycle.
* As the shock “works” harder the energy is dissipated through oil resistance faster. The oil heats up, the hotter the oil becomes the viscosity of the oil becomes thinner, and the shock absorber resists the movement with less efficiency.
* The shock body cylinder is not pressurized and the oil foams like a milk shake, forming small bubbles. The resistance decreases and the shock produces a different designed efficiently that what was intended.

Race Shocks~ How it Works
In a race shock the mechanism is quite a bit more sophisticated. The purpose of these mechanisms is to provide the tire patch to stay in contact with the surface. A loss of traction, no mater how minute means you lose. Great advantage for safety as Bryan TTM pointed out in his post LINK  Ask him what he attributes to not going down in a storm as he pointed out in his posting. This planting of the wheel translates to a smoother ride to the cruisers.  He threw away a set of premium adjustable air shocks ($1,600) prior to investing in less expensive race shocks.

Spring: A great coil over gas shock does not use the spring as a integral part of the suspension in a manor that is widely envisioned. The spring is chosen to be equal to the weight of the motorcycle, rider(s) and gear. Neutral and no heavy duty, just the equal in force (pounds/inch). The spring is adjusted externally only for changing conditions not specified prior to building ie: road conditions, baggage et. Al.

Externally Adjustable Springs

Spring Pre-Load Methods:

Least Expensive to Most Expensive

The Ramp System
Advantages:  Can Work when dirty as there are no threads to strip out as there are in a Threaded Body.
Disadvantages: Spring Pre-Load can only be increased to the length of the internal ramp.  A spring compressor must be used to remove the spring than once removed, R&R the C-Clip to a different groove to change the Ramp System’s starting position.  The spring must than be replaced on the shock.  An exact spring pre-load can almost never be achieved.

Internal Valving:
The behavior of the shock are controlled by many factors to allow for many different shock placement mounting angles, weights of motorcycle, rider(s), expected road conditions, and cargo. Below are some of the adjustable components used to achieve this task:

Piston Behavior
More Piston

External Valving Adjustments:
Can be located either on a standard shock, on the piggyback, or on a remote reservoir (shown)

Compression & Rebound
* Compression Valving
* High Speed and Low Speed Compression

Remote Resy
Rebound Adjustor

Emulsified Shock:
A quality rebuildable Emulsified Shock is one that the oil and nitrogen gas is mixed together in one cylinder.

Emulsion Type

* More development and money is utilized in the valving technology.
* Valving usually is comprised of a better designed piston and a set of shim stacks for both the compression and rebound.
* Great performance in a straight road.
* Nitrogen does not expand or contract as radically as air.
* A gas charged system over an air system is the nitrogen keeps the same density where air (water induced) does not
* More expensive than a hydraulic shock but less expensive than a separated system.
* As the shock “works” harder the energy is dissipated through oil resistance. The oil heats up, the hotter the oil becomes the viscosity of the oil becomes thinner, and the shock absorber resists the movement with less efficiency than designed.
* The shock body cylinder mixture of oil and nitrogen mixture becomes agitated and oil/nitrogen foams like a milk shake, forming small bubbles. The designed resistance decreases thus the shock produce a different dampening efficiently that what unit was designed for.

Separated Systems
A separated gas charged/oil system are available in many configurations.

A) Internal Reservoir: Single chamber with a separator piston

Separator Piston

A: Oil
B: Piston
C: Separator Piston
D: Compressed Gas
*More expensive than all other prior systems but less expensive but minimal increase cost and reduced bulk of a remote or piggyback reservoir system.
* A better quality of ride during aggressive riding as shock fade is kept to a minimum.
* Foaming is non existent.
* Oil and gas is heated in a common shock body under aggressive riding.
* No provision for cooling.

Advantages & Disadvantages:
* Similar as the single chamber with a separator piston.

C) Piggyback


* Shock with a “Dog Bone” connecting the reservoir to the shock body acting as an oil passage between the two. The oil is cooled and the separator piston is located inside of the reservoir.
*More convent reservoir mounting under circumstances where there is ample room.
* Less heat transferred from shock body to dividing piston and nitrogen.
* Bulky size not suited for recessed fiberglass/ABS saddle bag shock cutout.
* Will positively not work well due to interference on the stock FL Classic series saddle bags and we will not use them inverted on a standard fiberglass saddlebag due to application problems and bags shifting.

D) Remotely Mounted Reservoir  (Drawing shown above under “Compression”)
* Then reservoir is mounted further from the shock body and connected by a hydraulic hose rather than a “Dog Bone” as in the case of a Piggyback Shock.
* Oil cools more as the hydraulic hose offers a longer path to cool the oil.
* Better optional valving choices.
* Use of brackets and another step to install.