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
Word sort Students do not need to have strong phonics skills in order to engage in sound sorts. This can be a beginning phonemic awareness activity because students need only to identify the sound in order to complete the sort. Letter knowledge is not required, and phonemic development can mature as students do acquire more print knowledge. Picture sorts are one component of word study and are used to help beginning readers develop Concept of Word, phonological awareness, and phonics. Picture sorts most often begin with focusing on initial sound (single consonant, digraphs, or blends). By using picture sorts teachers are able