A gear case, also known as a chain case, is an enclosure for the bicycle chain

Bicycle chain

A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are nickel-plated to prevent rust, or simply for aesthetics. Nickel also confers a measure of...

 and sprocket

Sprocket

A sprocket or sprocket-wheel is a profiled wheel with teeth, cogs, or even sprockets that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it...

 assemblages commonly employed by utility bicycles. It serves to protect the chain from mud and dirt and tends to fully enclose the drive train. It may also contain an oil bath to keep the chain lubricated

Lubrication

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

. Modern examples are usually moulded in plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

. Similar devices may be found in connection with chains used on larger vehicles and machinery.

There are also a variety of smaller structures called chainguards which, rather than protecting the chain, serve only to protect the rider's clothes from being soiled or trapped in the chain rings.

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