Shock Tech 101

For the most part, the inner workings of motorcycle suspension seem to be shrouded in a great deal of mystery. Although the spring element of the up and down motion is readily obvious, the magic that happens within and which truly defines the difference between good and not-so-good suspension tends to elude most riders. Having a basic understanding of these critical components is not only important, but is ultimately the path to your becoming a better and certainly more comfortable rider.

The Basics of Motorcycle Suspension

Motorcycle shocks (as well as the suspension element of forks) can be divided into two basic components. The spring whose simplified function is to carry the load as well as absorb the impact of bumps and other road imperfections and a damper that dissipates the energy created by the spring’s compression and resulting extension (usually referred to as rebound).

Springs


The spring is (for the most part) wound steel and is defined by its rate. Rate is a measurement of the force required to compress the spring and is expressed in lbs per inch. For instance a spring with a 100 lbs/inch rate will require 100 lbs to compress it one inch.

Each subsequent inch of compression would require an additional 100lbs of force. This is referred to as a straight or linear rate spring. The alternative, is a progressive rate spring which allows a single spring to essentially exhibit multiple rates. By utilizing varied spacing spring coils, the initial rate may be 100lbs/inch, but each subsequent inch of movement would require more than an additional 100 pounds, and the third inch would require still more. Progressive-rate springs become stiffer as they are compressed. 
The advantage is a spring that is supple enough to soak up small bumps, yet firm enough to handle a big hit.


Dampers


Now that your motorcycle has “absorbed” a bump, the compressed spring has stored the energy of the hit and without a damper it would be released in a fury of rebound might, extending the spring beyond its original static length and repeating the cycle until the energy had dissipated. The resultant store and release sequence would cause your bike to pogo down the road. Imagine a basketball bounced hard onto the asphalt, not only does it bounce back, but it would bounce higher than your hands where it had started.

Now, imagine if you bounced it under water. This is the effect that a damper has on the stored energy of a spring. In fact, a damper, like the water analogy, uses a liquid (in this case oil) and forces it through a series of small holes. The ensuing resistance controls the return of the spring energy. The kinetic energy (motion) of the spring is transferred to the oil and dissipated as heat. Oil based damping is a clever solution, yet comes with its own set of challenges. For instance, rapid damper movement reduces the space within the shock for the fluid, thus a compressible medium must come into play or the shock would likely stop moving. The obvious solution would be to leave a bit of air space for the oil to move into, but air rapidly expands when heated (remember the shock has transferred the energy of motion to heat) and the damping characteristics of the shock would be ever changing. The solution is a separate chamber of a heat tolerant gas (in this case nitrogen) that allows the oil to expand without impacting the damping.

Shock Preload Adjusters


All of Progressive Suspension’s shocks are Preload Adjustable. This allows the user to easily tune the shocks for their specific combination of bike and rider weight. Although we design a shock to fit and perform on a specific motorcycle, we have no way of knowing if the rider will be 140lb or 240lb and the addition of cargo and passenger further complicates the issue. An obvious solution is an adjustment on the shock that allows the rider to quickly set up his shocks to match his bike/rider weight combination. This is where Preload Adjustment comes into play.

By compressing or uncompressing the spring a small amount, a shock can be perfectly tuned to suit the conditions for which it will be used. In addition to this level of tuning, most PSI shocks are also offered in Heavy Duty applications, recommended for bikes that are operated at or near the manufacturer’s maximum load rating over 50% of the time.

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