Gebaut mit Hugo
Somebody who had built a (non ft) ball balancing robot gave me a recepe for making a smooth, light stiff ball, based on a 25cm styropor one (http://www.amazon.de/gp/product/B00ARFRO8E/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A30YUXLJNYIIR6).