Spoke

A spoke is one of several rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. 

The term originally meant the portions of a log which one had split lengthwise into four or six sections.  The radial members of a wagon wheel were made by carving a spoke (from a log) into their finished shape.  Eventually, the term spoke was applied to the finished product of the wheelwright's work.

 Other websites 
These pages are in German:
Hub dynamo assembly instructions with spoke length formula and a nice diagram 
Online spoke length calculator, uses slightly different measures than the above formula 

Category:Engineering