Chainguard

A gear case, also known as a chain case or chain guard, is a protective enclosure for the bicycle's chain and sprocket assemblies. Its primary function is to protect the cyclist from being soiled or trapped in the chain rings while also protecting the chain from dirt and moisture. Gear cases may fully or partially enclose the drivetrain and sometimes incorporate an oil bath for continuous lubrication. Modern examples are usually moulded in plastic. Although they are also used by some touring bicycles as well as some chain-drive machinery, chain cases are most frequently found on utility bicycles.