Motor vehicle suspension is comprised of two systems. One deals with absorbing bumps in a controlled manner and the other deals with returning the wheel to its correct location relative to the vehicle. It is the job of the spring to perform this latter task.
A spring has a state at which it is described as being “at rest”. In the case of springs used in motorcycle suspension, the spring can be compressed or expanded from this position. This only occurs through the application of force. For the most part, simple metal springs conform to Hooke’s law. The more force (which can be measured in Newtons) applied to the spring, the more it compresses. Hooke’s law is a linear equation. So, if you apply twice as much force to a spring, you will compress it twice as far.
When a spring is released, it will return to its state of rest. But, because the end of the spring is moving and has a mass, it has inertia. This inertia is a force, which causes the spring to extend beyond its length at rest. The spring continues to extend until the inertia can no longer overcome the force required to continue extending the spring. At that point, the spring then starts to compress again to reach its state of rest. If the spring were not losing energy in other ways, this expansion and compression would continue forever. Here is a neat-o demo of springs doing what springs do.
Because the gradual dissipation of energy through air resistance and heat loss takes too long to keep motorcyclists happy, the shock absorber is used to overcome the spring’s inertia. It dampens the rate at which a spring is compressed and expanded.
The more mass you attach to the end of a spring, the more momentum / inertia the spring end gains as it moves. Because of this, the pogo effect of the spring is increased and stopping it requires more damping. This in turn, reduces the efficiency of the shock absorber to do other things, such as absorb shocks from bumps! On any vehicle, the mass attached to the end of the spring (such as the wheel, brake components, etcetera) is referred to as “un-sprung mass”. As already indicated, unsprung mass compromises the effectiveness of shock absorbers and as a result, motorcycle designers and after-market specialists place an emphasis on reducing this mass as much as possible.
When replacing springs for a motorcycle, there is a measurement referred to as the “spring weight”. The larger this number, the more force is required to compress the spring, meaning the spring feels “stiffer”.
The other factor in motorcycle springs is the preload adjuster. Preload, is as it sounds. It is the amount of extra load placed on the spring before the motorcycle hits a bump. When you increase the preload, you are compressing the spring and taking it away from its state of rest. Because of this, increasing the preload, increases the upward force the spring provides when the motorcycle suspension is at rest. As you may have guessed, the mass of the motorcycle and rider (excluding the unsprung mass) also loads the motorcycle suspension, compressing the springs. If the springs are compressed too much when the bike is at rest, it will not be able to adequately deal with any bumps you come across when riding. Motorcycle designers can pick a spring weight suitable for the mass of the bike, but as they do not know the mass of the rider, they cannot be 100% accurate in their choice of springs. Therefore, preload adjustment is used to fine-tune the suspension.
As per usual, this article is based upon my observations and my understanding of the laws of physics. If I have made an error, please feel free to leave a comment.