I have a robot that it's down part is something like this photo: http://www.sanatbazar.com/components/com_jshopping/files/img_products/full_Capture3.JPG . This chassis has 4motors and 4wheels are connected directly to motors, but my robot only has 2motors and 2 rear wheels are connected to motors directly. And the problem is about forward wheels! I am looking for a simple and cheap(but good) way to connecting them.
As I saw in some toy cars, they used a shaft through 2wheels and connecting them to shaft, but as I have electrical parts inside the body and need smoother movements, I prefer to use another way. I searched about bearings but don't know how to attach them to my chassis?
At the end I should mention my robot has made by 3D printed parts and I can change the down part's design to matching something like bearings.Also the chassis dimension is about 16*16*6 CM^3.
EDIT: These photos are my chassis, wheel and two 3D printed white parts that I wanted to use as connection, but I saw this mechanism is very unstable and wheels slipping:
I can change my chassis design and 3D print it again, but I am looking for a proper design for my purpose. I don't like to pass a shaft trough side holes, because I want to put my electronic board there!