Unsprung mass is the mass of the portions of bicycles that is not supported by the suspension systems. At one extreme are road bicycles with no suspension in the frames, very little in the tires, and none in the saddles. By raising themselves off their saddles, riders may provide suspension with their knees, making their mass be sprung mass, but all of the mass of the bicycles remains unsprung mass. At the other extreme are full suspension mountain bikes. With front and rear suspensions the only parts unsuspended are the wheels and small parts of the front forks and rear chain-stays. Even then, as mountain bikes have large low-pressure tires which allow much more travel than small high-pressure road tires, the wheels are sprung to some extent as well.
In general, bikes are so light compared to their riders that travel is a much bigger motivator than unsprung mass in determining where to put the suspension and how much to use. The exception to this is that on recumbent and tandem bicycles where the riders are either unable to lift themself out of their seat or unable to see in advance when that will be needed, the riders' mass can no longer be expected to be supported by their knees over road irregularities. These bicycles generally have some sort of suspension system to reduce unsprung mass.
Read more about this topic: Bicycle Suspension, Terminology
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