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Sprung and Unsprung Weight
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The sprung/unsprung weight ratio is a critical factor in determining how well any vehicle will hold the road on rough surfaces.
The "sprung" weight is the weight of the part of the vehicle and its payload which is held up by the springs. In the case of a suspension-equipped bicycle, the sprung weight includes most of the frame, and the rider. In the case of a rigid-frame bicycle, the "sprung" weight consists of the rider's trunk and head while the rider is off the saddle, carrying his or her weight on bent knees/elbows.
The "unsprung" weight is the weight of the wheels and the parts of the suspension that move up and down with the wheels as they track over bumps.
When a bump lifts a wheel up, the spring compresses. After the wheel passes over the bump, the spring pushes the wheel back down into contact with the ground. In the case of a vehicle with a low sprung/unsprung weight ratio, such as one with heavy wheels and a light rider, the spring will not be able to push it back into contact with the ground as fast as the same spring could push a lighter wheel.
If the rider is light, the spring force will also be light, so it will not be able to push the wheel back down as fast as the stronger spring that would be used with a heavier rider could push the same wheel.
If the rider is light, but the spring is strong, the rider will not get enough benefit from the suspension, and the strong spring will lift the rider as well as the wheel.
