Algodoo

All of the tracers make a perfect circle. And yes, I used the grid...
How to make:
1. Make sure your rope (or whatever you used) size is 0.200.
2. Make a rope that is 2.00 meters long (should say beside your mouse how many meters).
3. Make all of the axles a motor and don't mess with the speed, if it doesn't say 10 axles at the top, do number 2 again.
4. After you make the rope, make the 2, 4, 6, and the 8th axle reversed.
5. Add a center tracer to all of the ropes, I suggest you make the fade time of the tracer 60 seconds.
6. Press play and if the last rope doesn't change its angle then enjoy your new circle maker!