Step 2: The mechanical part

In the circuit, i left two 3mm holes at the back, so anyone can hook up it's own mechanical part.
I made mine from a 1mm thick brass plate, in the photos you can see how i cut and bent it. This structure can hold the mini motors and the wheels together and it makes a pretty decent rpm reducer. Then i threaded the holes so i can bolt this structure to the PCB without nuts.
The mini ball roller is used at the front of the robot as an all direction wheel, you can just solder a 5mm LED in that part, it has more friction, but it should work.

With this mechanical design, there are a few problems, the robot turns too fast the front part when there's a speed change in a motor, resulting in a instable direction, without changing the design, the only solution is making the robot slower, that can be done lowering the gain with the pot. But i realized that a better solution would be to made a different robot design, with the wheels in the same part as the  sensors, so the feedback would be much better. But for my first robot, this is OK.