The Wheels

This is an important choice. There are plenty of omni-wheels available on the internet, but I recommended those ones.

The contact (yellow) material is rubber, not plastic. Therefore, these wheels don't slip on nearly any surface, even over glass or similar. The elements conform a perfect circle. This allows a nice turning. There is no "bumpy" movement due to uneven sections on the wheels.

The central axis is designed to fit LEGO elements, but there is a cylindrical cavity (9 mm diameter / 8 mm deep) that allows you an easy adaption to any possible motor axis you may need to connect.

You can purchase them here: http://www.holonomicwheel.com/index.jsp