Historically, rear derailleurs merely shoved the chain across the rear cogs laterally, without the pulley cage adjusting for the graduating size of the cogs as it shifted over. The Japanese company Suntour, devised something known as a Slant Pantagraph or Double Servo-Panta mechanism. These derailleurs move the pulley cage up and down as well as over. In this way it keeps the guide pulley just a couple of millimeters away from the cogs as it shifts through them. This style of derailleurs provides a much faster shift, with an exceptional sure-footedness as it moves. There are four major parts to a rear derailleur. Naming them from the bike rear wheel drop-out they are; the left link (including the mounting bracket), the body (the piece that facilitates the double pivot), the right link, and the pulley cage (which holds both pulleys). The two pulleys have names. The upper pulley is known as the guide or jockey pulley (it guides the chain in through the cage), the lower one is the tension (or idler) pulley (it takes up chain slack). The Suntour rear derailleurs we sell actually have precision sealed cartridge bearings, none of the Shimano models do. Derailleur pulley cages often come in short or long cage models. Each has their benefits and drawbacks. The long cage model will shift to, and permit the use of larger lower geared cogs.
The drawback is that the long cage moves sluggishly compared to short cage models, because it has more chain slack to compensate for. The short cage model moves very swiftly, but will generally not accommodate low geared cogs beyond the 26 tooth size on road racing rear changers and 28 tooth on mountain. All rear derailleurs have a maximum tooth capacity or Max Capacity. This is the number of teeth that appear on the largest (lowest gear) cog. Larger than this size, the derailleur has trouble lifting it onto the cog. However most derailleurs are under rated by two teeth from the real maximum of what they can truly accept. It is common for a 28 tooth max rear capacity to be pushed easily to use a 30 tooth cog. Keep it in mind. There is second capacity in rear changers. It is known as Total Capacity. The total capacity is calculated by subtracting the size of the smallest chainring number from the largest chainring and adding it to the difference between the largest and smallest cog in the rear. For example, if the front chainrings were 24-36-46 and the rear cluster were 13-28t then, the front difference of 22 (46 minus 24 = 22) would be added to the rear difference of 15 (28 minus 13 = 15) to achieve a Total Capacity of 37 teeth. Total Capacity is a rating to express the most extreme comfortable working conditions for a given rear derailleur. Again manufacturers are forgiving on this one. Keep it in mind. Overshift is when the derailleur pulley cage advances farther than the operator wanted. It could be either a mis-shift or the derailleur being out of adjustment and permitting the cage to move to far. B-Tension spring or body tension spring adjustment refers to the tension of the hardened spring enclosed in the mounting bracket. Some rear derailleurs have a screw that can adjust and move the entire derailleur, as it's mounted, to provide more or less resistance in taking up chain slack. We've included a column in the Rear Derailleurs table that has the measurement center to center of the pulley bolts of each of the derailleurs so you can evaluate the length for yourself.