Super Pet CritterTrail Snap-on Comfort Wheel Cage Accessory for Hamsters, Gerbils & Mice The CritterTrail Snap-on Comfort Wheels is extra silent and has a solid running surface to provide a safe exercise source for your little friend. The wheel features smooth threads that allow for a surefooted grip while creating a comfortable running surface. This 6.75in wheel is sized perfectly for hamsters and gerbils. The enclosed wheel attaches directly to the side of any CritterTrail Home and also attaches directly to CritterTrail Fun-nel Tubes for added exercise fun. Measures 7in L x 4.5in W x 7.75in H. For Hamsters Mice & Gerbils. Please note: Color specification is unavailable on this item