32 REAR DERAILLEURS ABOUT THIS CHAPTER Outermost gear: The cog on the rear wheel that has the fewest teeth and is closest to the dropout. This This chapter is about installing, adjusting, and ser- term will be used instead of high gear, top gear, or first vicing rear derailleurs. The procedures for installation gear, or a letter code (described in the following sec- and adjustment make references to installing the wheel, tion NAMING COGS AND GEAR COMBINATIONS, page 32- chain, shift-control mechanism, and cable. These items 3) will be used. are fully covered separately in preceding chapters. This Low gear: With regard to rear derailleurs, low gear chapter also covers repair of derailleur-hanger threads. typically means the rear cog with the greatest number The procedure assumes that the front derailleur is of teeth. It is called low gear because it results in the installed. The front derailleur need not be precisely lowest number when calculating gear ratios. It is con- adjusted, but must be capable of moving the chain to fusing because the cog with the fewest number of teeth the innermost and outermost chainrings. It may seem sticks up the least, and fewer teeth may seem to some like a good idea to install and adjust the front derailleur to be lower. For this reason, this book will always first, because of this. However, the front-derailleur pro- use the more wordy alternative, innermost gear, or a cedure requires that the rear derailleur be able to shift letter code (described in the following section NAMING the chain to the innermost and outermost positions, COGS AND GEAR COMBINATIONS, page 32-3) will be used. as well. Whichever is done first, to complete one de- Bottom gear: Same as low gear. railleur adjustment it may be necessary to do some Last gear (or last position): A gear is called last preliminary work on the other derailleur, as well. gear because it is the last one counted when counting There is some confusing and contradictory termi- cogs on the freewheel/freehub, this term is avoided nology used in regard to derailleurs, so be sure to be- because the outermost cog provides the highest gear come acquainted with the following terminology sec- ratio, which might also be called last gear. tion to become clear on the terms used in this book. Innermost gear: The cog on the rear wheel that has the most teeth and is closest to the spokes. This term will be used instead of low gear, bottom gear, or last GENERAL INFORMATION gear, or a letter code (described in the section NAMING COGS AND GEAR COMBINATIONS, page 32-3) will be used. TERMINOLOGY Mounting bolt High gear: With regard to rear derailleurs, high Guide pulley B-screw gear typically means the rear cog with the fewest num- Parallelogram ber of teeth. It is called high gear because it results in H-screw H the highest number when calculating gear ratios. It is L-screw L confusing because the cog with the greatest number of teeth sticks up higher, and more teeth may seem to some to be higher. For this reason, this book will Pinch always use the more wordy alternative, outermost gear, mechanism or a letter code that is described in the following sec- Cage Cage tion NAMING COGS AND GEAR COMBINATIONS (page 32- (inner plate) (outer plate) 3). Tension pulley Top gear: Same as high gear. First gear (or first position): Called first gear be- 32.1 This side view and back view show the major parts of the cause it is the first one counted when counting cogs rear derailleur. on the freewheel/freehub, this term is avoided because the innermost cog provides the lowest gear ratio, which might also be called first gear. 32 1 32 REAR DERAILLEURS Limit screw: Adjustable stops that are used to stop smoothly over its full range of motion without any the inward and outward motion of the derailleur at incremented stops. It is up to the operator to decide points that enable the chain the shift to the innermost what the correct amount of lever motion is to get from and outermost cogs without going too far. one gear to the next. H-screw: A limit screw for stopping the derailleur Derailleur hanger: The plate that the derailleur from shifting the chain out past the outermost cog. attaches to. Sometimes it is integral to the right rear L-Screw: A limit screw for stopping the derailleur dropout. Sometimes the right rear dropout is a two- from shifting the chain in past the innermost cog. piece constructions so that the hanger can be replaced. B-Screw: A screw used to adjust the spring ten- On the most inexpensive bikes, the derailleur hanger sion on the mounting pivot, which affects the distance is the plate that mounts between the dropout and the between the cogs and the guide pulley. wheel-retention mechanism. Pulley wheel: A toothed wheel in the derailleur Stop tab: The tab near the bottom of the derailleur cage that the chain runs on. hanger that stops the forward rotation of the derailleur. Jockey wheel: An alternate name for a pulley Mounting bolt: The bolt through the topmost wheel, generally the upper one. Guide pulley will be part of the derailleur that attaches the derailleur to the used instead. derailleur hanger. Guide pulley: The upper pulley wheel in the Mounting pivot: The derailleur pivots around the derailleur cage that guides the chain from one cog mounting bolt at the mounting pivot. This pivoting to the next. allows the derailleur to change position to accommo- Tension pulley: The lower pulley wheel in the date changes in gear size as the derailleur moves in and derailleur cage that pulls back on the lower section of out. This pivot also allows the derailleur to be rotated chain to keep it under tension. back to allow rear-wheel removal. Derailleur cage: The assembly at the bottom of Return spring: When the tension on the inner the derailleur that encloses the chain, consisting of two wire is released, this spring inside the parallelogram plates and two toothed wheels called pulley wheels. causes the derailleur to move out as far as the outer- Outer plate: The plate in the derailleur cage that limit screw will allow. is outward of the pulley wheels. Cage pivot: The pivot that the derailleur cage ro- Inner plate: The plate in the derailleur cage that tates about. The cage rotates so that the tension pulley is inward of the pulley wheels. can move forward or backward. This keeps the chain Parallelogram: With regard to the rear derailleur, taught when its effective length changes as it is moved this is the part of the body between the mounting pivot to gears of different sizes. and the cage pivot (consisting of two arms on four Cage stop screw (or pin): A screw (or pin) in the pivots) that actuates to move the derailleur cage in- outer cage plate that bumps into the cage pivot hous- ward and outward. ing to keep the cage pivot spring from completely un- Adjusting barrel: A hollow screw in the de- winding when the chain is not in the derailleur. railleur that the inner wire passes through and the Over-shift: When the chain moves too far, and housing stops against. As the adjusting barrel is does not align with the intended cog. screwed in and out, the relative length or tension of Under-shift: When the chain does not move far the cable system is changed. enough, and does not align with the intended cog. Pinch mechanism: The mechanism that attaches the In-shift: Any shift to a cog that is more inward inner wire to the derailleur. The inner wire is usually than the one that the chain is currently on. routed through a groove in a plate on the derailleur, and a Out-shift: Any shift to a cog that is more out- bolt or nut presses a washer or plate on top of the inner ward than the one that the chain is currently on. wire to trap and compress it in the groove. The groove in Up-shift: This term will not be used because there the plate is often hidden by the pressure washer/plate. are two opposite ways that it could be understood. On Indexing: This describes a type of shifting in a rear cogset, an up-shift could be an in-shift because which the shift mechanism moves in distinct incre- the chain is moving up onto a cog of larger diameter. ments. These increments are designed to be just the An out-shift could also be called an up-shift because right amount to get the chain to move precisely from the chain is being moved to a cog that will create a one gear to the next. Indexing has virtually replaced higher gear ratio. The terms in-shift and out-shift will friction shifting. In friction shifting, the lever moves be used to avoid this confusion. 32 2 32 REAR DERAILLEURS Down-shift: This term will not be used because Shifter and cable installation there are two opposite ways that it could be under- To adjust the rear derailleur, the shift-control stood. On a rear cogset, a down-shift could be an out- mechanism and the cable system must be installed. shift because the chain is moving down onto a cog of smaller diameter. An in-shift could also be called an Chain sizing down-shift because the chain is being moved to a cog Rear-derailleur performance is influenced by chain that will create a lower gear ratio.