Spokes
The spokes of a conventional wheel are metal rods of about 2mm diameter which connect the wheel hub and the rim, normally with tangential lacing. A typical spoke length is 300 mm including the threaded section that mates with the rim nipple. At the hub end, the spoke turns through about 90 degrees and has a raised flange at its end. The diagram shows a typical layout. The nipple is not fixed to the rim, and is pushed upwards if the spoke attempts to go into compression. The mechanical behavior of spokes is, therefore, concerned to their response to tensile forces. Periodic loading occurs as the wheels rotate, even on a smooth surface, and fatigue behavior must be taken into account in spoke design.

From: G. Apps, "The Bicycle Book," Smithmark (1996)