According to Wikipedia, Chinese mechanical engineer Ma Jun (c. 200–265 CE) is credited with inventing a chariot with a direction pointer, though some references indicate that it was invented even earlier in China. The pointer at the top of the chariot always indicates a set direction, regardless of what twists and turns the chariot takes. This device was one of the first mechanical navigational devices, used before the introduction of the magnetic compass (which the Chinese employed in the 11th century).
I remember seeing a small toy South Pointing Chariot but could not figure out how it worked. I understood it worked by means of a differential gear set, but even so, its working was a mystery. I had the idea to create one, rather than to just buy one to better understand how it works, because the available toys use gears that are too small to see. I tried to find hobby gears that would work, but found the same problem- all the gears were too small, and crown gears were a rarity. I purchased a 3D printer to build the gears large enough to see. Gear design cad files are available for free at McMaster-Carr, and can be imported into the 3D cad design program, Fusion 360. The gears had to be modified to work in the Chariot, but having the basic gear teeth shape saved lots of time.
In designing the Chariot, the differential was built first. As soon as the differential was built, it was easy to see how the pointer maintained a fixed position if the top and bottom crown gears moved the same amount but in opposite directions. The rest of the build was dedicated to insuring that one turn of the left wheel turned the top crown gear one turn, and one turn of the right wheel turned the bottom crown gear one turn in the opposite direction. Finally, the wheel and tire diameter must equal the Chariot wheelbase.
Step 1: Parts
A 3D printer, I used a Anycubic i3 Mega with 1.75 mm PLA and stock 0.4 mm nozzle
2 ea. 5 mm x 100 mm stainless steel shafts.
1/4" brass stock for bearing spacers.
14 ea. sealed ball bearings, 625zz.
6 ea. 5 mm collars with set screws.
2 ea. M5 x .8, 40 mm long bolts for axles.
1-1/2" wide x 1/4" thick bar stock for counterweights (optional).
2 ea. rubber bands 1/4" wide for tires (optional).
Step 2: Construction
I printed the parts using PLA filament, the straight cut gears required a bit of work to clean up because the hot PLA spread out at the bottom of the gear where it touches the printer bed. The bevel cut gears came out fine with no extra clean up. Maybe another version of this could be built where the straight cut gears have a tapered side that touches the printer bed before the gear section begins. The chassis required around 6 hours to print, and the total time to print all the gears was also around 6 hours.
Two bearings were installed in each wheel flange to support the wheels. A 5 mm x 40 mm bolt is the axle. I added washers to adjust the bevel gear clearance so they mesh but don't bind.
The spider shaft of the differential had to be trimmed down shorter than I wanted because the top bearing holder wasn't removable, thus I had trouble fitting the differential into the chassis without shortening the spider shaft. In the future, I might make the top bearing holder removable, with a design similar to the wheel flange bearing support.
I added weights (just cut sections of 1-1/2" x 1/4" bar stock) under the chariot chassis to counterbalance the gears such that the chariot doesn't need a support to remain upright on 2 wheels. That also helps keep the wheels in contact with the ground.
Step 3: Conclusion
I am amazed that the chariot actually points a constant direction as well as it does. It will drift a bit after 4 or 5 complete turns unless you get the distance between the wheels to exactly match the wheel diameter. I made the wheel width narrow so that there would be less chance the wheel could ride up on one side or the other. If the number of turns in on direction or the other is reasonably the same over a long distance, the pointer will point in a consistent direction.