Derailers serve the job of moving the chain from one sprocket to the other and keeping it where you desire so that you have the gear ratio that you want.  This post will be about the mechanics of derailers and their parts.  I will be composing a derailer tuning how-to after this.

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Derailers are really simple. They’re composed of what engineers call a four bar linkage.  All of them are made this way including all rear derailers and all front derailers.

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A four bar linkage is basically a parallelogram shaped system.  I’ve Highlighted the pictures to show the four sides or “linkages.”  Simple four bar linkages are used when you want one face (A) to move further or closer and in a predicted direction to fixed face (B) while remaining on a parallel plane with one another.  This is simply how chains are moved back and forth and down and up given sprockets without being twisted or slanted or the derailer hitting anything.

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Here are the rear derailer basics.  I’m missing two things which are the Adjustmen barrel (where the cable enters the derailer) and the Cable Anchor.  They’re not shown very clearly on this pic. 

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Here are the basics of a front derailer, again they are the same!

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As simple users of a bicycle, one only need to know that a derailer, acting as a parallelogram or four bar linkage, uses High and Low limit adjusters to control the range of motion of the unit.   A limit adjuster is nothing more than a set screw that acts on a limit stop (built into the body of the derailer) to stop it from moving a certain direction.  Here the high limit adjuster is shown to be in contact with the high limit stop. 

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Here as we pull the derailer all the way to the left as it would if it were on the lowest gear setting, we can see the low limit adjuster (the screw) hitting the low limit stop thus limiting how far left (towards the bigger gears) it could go. 

That is really all it is to a derailer.  Next I'll show you how to tune it to get the best shifts and to stop that damn chain from falling off the sprocket.