Exercise Ball Chair Upgrade Wheels - Set 4 - Larger, 60mm/2.5" Office Size Wheels - Adds .5" (1/2") to Plastic Ball Chair Height (Versus 50mm/2" Wheels) NOTE: For brands other than Isokinetics Inc. Current Isokinetics Inc. brand plastic ball chairs already come with these 60mm/2.5" wheels, but similar plastic ball chairs from other brands normally come with 50mm/2" wheels. These wheels are offered as an upgrade. Please make sure you are replacing 50mm/2" wheels before you order Office size 60mm/2.5" wheels provide a smoother rolling experience, are able to better negotiate bumpy or softer surfaces, are quiter and "feel" higher quality. The also add 1/2" (.5") of height compared to 50mm/2" wheels 4 wheels per pack, 2 of which are lockable (we recommend placing them in the rear in order to avoid accidentally locking them) PLEASE NOTE: These wheels DO NOT WORK ON *metal frame* ball chairs, which have 5 wheels and require a special stem that these wheels do not have