Build a robot wheel with this part. An O-ring can be used as "tube". There is a variant for mounting on a plastic TT Motor and a second design for mounting the wheel at a 3mm axis (e.g. mini metal gear motor). For the latter a M3 suare nut and grub screw is needed. The design allows to use a wheel encoder based on the ITR8307 optocoupler for rough control of the motor movement.