Onmi wheels

Omni wheels allow free movement in the direction of the axis, as the small dark grey wheels rotate freely. I used 48mm omni wheels from robotshop.com (http://www.robotshop.com/en/48mm-omniwheel-lego-nxt-servo.html). They come with a hub for lego axles, which is useless for the 4mm fischertechnik axles, but luckily they have an 11mm hub that fits snugly over the fischertechnik hub nut (31915+35113), which can be tightened very firmly on the metal axles using two Z15 wheels (35695) to provide grip. To make a very stiff connection between the wheel and the hub nut, I forced a small copper nail in the little hub key hole.