Motorcycle suspension uses springs to hold the bike and rider at the proper height and position.
The spring rate, or "stiffness" of these springs needs to be correct for the bike,
rider weight, rider height, ability, and application.
These springs are designed to have a certain amount of sag, with the rider on the bike. This is absolutely crucial!
Most dirtbikes role off the showroom floor with an un-ballanced combination of suspension springs. Some brand new dirtbikes come equipped with very poor performing springs as well.
Example: If you are a 200 pound vet rider on a brand new RM125, you will absolutely need to install stiffer springs, front and rear.
This bike was designed to be riden by a 150 pound rider.
Having too soft of a spring rate makes a bike sag more than it is designed to. If this happens, the suspension will feel stiff, and soft, AT THE SAME TIME. The small bumps won't be absorbed properly, due to the increased spring preload. The large bumps will cause the bike to bottom easily, due to the lack of overall bottoming resistance and available travel.
Having the incorrect spring rates will always make the bike handle poorly,
You could have the best motorcycle on earth, but if it has the wrong springs installed,
it will never handle the way it was originally designed to.