"Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel (ship, boat) or vehicle (car, motorcycle,bicycle) to follow the desired course. An exception is the case of rail transport by which rail tracks combined together with railroad switches (and also known as 'points' in British English) provide the steering function."in wikipedia
G. W. Lucas has the equations for an elementary trajectory model for the differential steering system of robot wheel actuators. Check his website for equations and this one for the java applet.