For the most part the inner workings of motorcycle suspension are not understood or confusing to many bikers. Whatever the reasons why you wish to change your motorcycle shocks, looks, lowering your bike, performance, comfort or all the above, you should understand springs, dampers, pre-load adjusters and sag. In 2 segments (other will be published next week), Progressive Suspension, since 1982 a premier manufacturer of suspension for motorcycles (Harley, Metric, Vintage, Off-road) including ATV, UTV, explains the critical shock components that you should understand before ordering. Today, Springs & Dampers.
1- 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.
2- 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. (source Progressive Suspension)