South Pointing Chariot.
A Chinese chariot with a figure mounted on top, forever pointing towards the south, irrespective of which way the chariot is heading. A series of gears driving a differential driven by the 2 wheels creates the movement of the figure relative to the chariot.
Actually not too accurate because of the need for the extreme dimensional accuracy of the gears and wheels but good for impressing the people on grand public occasions.
It was thought to have been developed first in 2600 BC the first historically confirmed version was created by Ma Jun around 200. Its secret was lost and reinvented many time through history but in 1947 George Lanchester designed a model that used a differential to drive the pointing figure. It is that design upon which this model is based.
The design uses a 1:1 ratio between the wheels and the differential and a wheelbase equal to the wheel diameter. All the calculations for the design can be found here.
For more details go to http://www.3dprinterclocks.com and view the video at https://www.youtube.com/watch?v=YzUKr621gr4&feature=youtu.be
All the gears need to be free running on the shaft so drill out to Ø4.1mm.
The shafts need to be a tight fit in the frames