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 be introduced at a very early stage and develop with a child’s individual ability. Sorting pictures or oral vocabulary is a manipulation of sounds, and this manipulation increases awareness. A simple introductory sort is by initial sound and this can develop to ending sound, vowel sounds, and word families sorts. The root of importance is student motivation and involvement in the sort. By “setting the scene with sounds," sorts may include concrete materials and pictures linked to learner interest. Phonemic awareness, not phonics, is the understanding that our spoken sounds work together to make words. This is a very early