Solid modelers, as the name implies, are good at "solid" modeling. Components such as timing belts and cables that have to be "flexible" during a motion actually change their shape during the motion. They do not maintain their initial shape, and this makes it difficult to study the way a timing belt interacts with two or more pulleys, or a cable system moves a lever or bell crank with a conventional solid assembly model.
The following concepts and techniques describe a method for creating a sketch that can be used to simulate the motion of these flexible components by just dragging different points on the sketch with the mouse (see example at http://rbi.ims.ca/4913-534). The ability to dynamically drag the sketch separates this technique from other tools such as Link Values or Equations.
This is really just a starting point. From this type of dynamic layout sketches entire systems of belts and pulleys can be studied. These concepts can be combined with the SolidWorks 2006 Sketch Blocks to really make it interesting.
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 Adding a geometric relationship Equal between two lines in a sketch makes the length of both lines the same. Adding the same geometric relationship Equal between two arcs makes the radii of both arcs the same. The length of these arcs (along the circumference) will still be underdefined. For two arcs of the same radii, add the line between the endpoints of the arc, and then add an Equal relationship between them to get the arc length the same (see figure center). Notice the included angle of the two arcs is also the same.
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The included angle of any two arcs can also be made the same by using the previous concept as construction geometry for an arc of a different radius (see figure bottom).
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Two separate line segments with an overall length controlled by the 15 inch dimension. If the 6 inch dimension (controlling Line 1) is changed to 10 inches, the overall length will stay 15 inches, and Line 2 changes to 5 inches.
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You need this trick if: You are designing timing belts or chains, pulleys or cables and need to study the motion dynamically.
Online resources: One good source for more information is the SolidWorks Discussion Forum at
http://forum.solidworks.com/swforum/logon.asp
(you will need to log in). Search for tips and information on related topics such as layout sketches, skeleton parts, assembly sketches and Sketch Blocks.
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