I am looking to design a small robot which uses three omniwheels. How does one go about connecting small omniwheels to DC motors since there do not seem to be any hubs made for omniwheels.
RobotShop offers a selection of omniwheels, but you are correct in that theredo not seem to be any hubs made for them which would allow you to easily attach a motor to any of them. Since this is the case, you need to be a bit creative. The inexpensive ones are not designed to be used with any specific motor, in which case you will need to create a custom hub or shaft collar to fit between the motor shaft and the wheel (since they will be two different diameters). You can do this by finding cylindrical parts (PVC tube, wood, metal etc.) and sanding them down to make them fit. You may need several of them to create an opening for a small motor shaft. Alternatively, you can insert a small length of a cylinder (metal, plastic or wood) into the center of the wheel and use a shaft coupler or hub (or servo hub if you can use a continuous rotation servo) between this rod and the shaft of the motor. Another, less atractive option would be to modify the wheel itself in order to mount a hub. You might use polymorph to fill in the opening and drill through the center with the appropriate sized drill (of create a custom hub).
Hope this helps,