Derailleur gears several standards in use, and in each the product of the derailleur's actuation ratio and the length of cable pulled must equal the pitch of the rear sprockets. The following standards exist. Some rear derailleurs, especially for mountain bikes, incorporate a clutch to keep the lower length of chain in sufficient tension to prevent the chain from striking the bottom of the chain stay, called "chain slap", and which can damage the chain stay. Clutches are also helpful in preventing the chain from derailing from the chain ring on systems without a front derailleur. The front derailleur only has to
English translations of Asterix "Why is that man wearing a melon?" This relies on the fact that the French word for melon is "also" the name for the iconic British bowler hat; with no way to convey this in the English translation, in the British edition Obelix says, "I say, Asterix, I think this bridge is falling down" referring to the children's rhyme "London bridge is falling down", leaving the original joke incomplete. In the panel shown, the reply of the British man on the right was "Rather, old fruit!", in some publications of the book; a good pun and typical of the way