Rat bikes are motorcycles that have fallen apart over time but been kept on the road and maintained for little or no cost by employing kludge fixes.Survival bikes look similar to but are different in purpose from rat bikes — they are modified for stylistic reasons.

The concept of keeping a motorcycle in at least minimally operational condition without consideration for appearance has probably characterized motorcycle ownership since its earliest days. The essence of a rat bike is keeping a motorbike on the road for the maximum amount of time while spending as little as possible on it. This calls for adaptation of parts that were not designed to fit the model of bike in question. While the origin on the term ratbike is unclear, it may attributed to custom motorbike magazines, and retrospectively applied. Most Rat bikes are painted matte black but this is not a requirement.

Survival bikes are bikes that may appear to be rat bikes, but are not. They are influenced by the Mad Max films. The term survival bike itself originated in the British motorcycle press particularly Back Street Heroes, and the now-defunct AWoL in the late 1980s and early 1990s.

Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus. Many members of other rodent genera and families are also referred to as rats, and share many characteristics with true rats.

Rats are typically distinguished from mice by their size; rats are generally large muroid rodents, while mice are generally small muroid rodents. The muroid family is very large and complex, and the common terms rat and mouse are not taxonomically specific. Generally, when someone discovers a large muroid, its common name includes the term rat, while if it is small, the name includes the term mouse. Scientifically, the terms are not confined to members of the Rattus and Mus genera, for example, the pack rat and cotton mouse.

The best-known rat species are the black rat (Rattus rattus) and the brown rat (Rattus norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams (1.1 lb) in the wild.

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