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
Interrogative word Interrogative word An interrogative word or question word is a function word used to ask a question, such as "what", "when", "where", "who", "whom", "why", and "how". They are sometimes called wh-words, because in English most of them start with "wh-" (compare Five Ws). They may be used in both direct questions ("Where is he going?") and in indirect questions ("I wonder where he is going"). In English and various other languages the same forms are also used as relative pronouns in certain relative clauses ("The country where he was born") and certain adverb clauses ("I go where he goes").