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 level, regardless of the students' age and grade. Once the appropriate spelling instructional level is established - be it alphabetic, within-word pattern, or syllable juncture - instruction can be adapted by focusing on fewer words at a time, teaching spelling patterns in an explicit manner, and providing for copious amounts of practice and review. Word sort A word sort is a developmental word study activity espoused by the "Words Their Way" curriculum as written by Donald R. Bear, Marcia Invernizzi, Shane Templeton, and Francine Johnston. The activity focuses students' attention on critical features of words, namely sound, pattern, and meaning.