The rear derailleur.
In addition, the rear derailleur tensions the chain. Due to the different sprocket combinations the chain always has a different length between sprocket and chain ring. The built-in chain tensioner keeps the chain in any combination taut (Fig. 2).
- The derailleur cage with chain guide pulley and chain tensioning wheel (Fig. 3)
- The parallelogram to move the derailleur cage axially (Fig. 4). The chain runs between the rollers through.
The derailleur cage with wheels is right below a sprocket.
When you move the cage in the axial direction under a different sprocket, the chain also moves to that sprocket.
By turning the derailleur cage forward or backward in the guide wheel the distance can change between the chain and chain tensioning wheel.
Thus, the chain can be tensioned.
For a good switching action it is important that the top roller is as close as possible to the sprocket wheel. The derailleur pulleys must be directly under the sprocket.
The derailleur cage is hinged by a torsion spring or tension spring suspended from the parallelogram.
This allows the chain to be tensioned.
The parallelogram is designed so that the distance of the guide wheel to the chain wheel in all positions are the same, see Fig. 4 .
The parallelogram is hinged by a torsion spring suspended from the derailleureye in the frame.
Between the pivot points of the parallelogram a spring is placed hat pulls the derailleur cage outwards and a tensioned cable which can move the parallelogram inwards.
By the combination of tensile force of the spring and the cable, the derailleur cage can be placed just below the sprocket wheels.
The cable is operated by a lever on the steering wheel. Image 5 is a rear derailleur with adjustable spring force.
The derailleur is equipped with four jackscrews in order to adjust him (Fig. 6):
- Two adjusting screws are arranged on the housing of the parallelogram which is secured to the frame.
These adjusting screws ensure that the parallelogram do not put the derailleur cage pass the outermost sprockets. If this does happen, the derailleur can put the chain between the wheel and the largest sprocket or between the dropout and the smallest sprocket. The set screw to ensure that the derailleur can not over turn the largest sprocket is provided with an L (for light). The set screw to ensure that the derailleur can not over turn the smallest sprocket is provided with a (heavy). By turning the adjusting screws the derailleur cage can be put right by the smallest and largest sprocket.
- In the house at the derailleureye a screw is inserted that can be turned against the ridge of the derailleureye. This allows the derailleur can be turned in the frame plane, and can guide the chain wheel as close as possible to the chain wheels.
- The outer cable rests against a cam on the housing of the parallelogram. With the adjusting screw between the cam and the outer casing, the cable length can be adjusted. By changing the length of cable, also changes the position of the derailleur cage. With the adjusting screw the derailleur cage is just right to put under the sprockets. This screw is essential for the adjustment of the indexed shifting. See fig.7.
The guide roller should be provided with deep teeth, because pushing in the lateral direction against the chain. The chain tensioning wheel attracts only the chain and therefore does not need deep teeth are provided. Sometimes the upper guide wheel has axial play. This allows the wheel can still find a bit and so occupy the ideal position between the sprocket and chain tensioning wheel. The adjustment of the derailleur thus is less critical. If several rollers are mounted, the upper wheel may be provided with a G (of guide lead) and the lower wheel of a T (for tension force), or the upper wheel of the indication upper and the bottom of the lower. Please note that during the assembly of the rollers they are mounted in the correct position.
The rollers can be mounted in three different ways (Fig. 8) . The most common is bearing with a sliding bus that is sealed with two dust caps. The disadvantage of this construction is that it gets fairly quickly some backlash, and that the wheel is susceptible to dirt. The wheels with steel bushings should be lubricated with thin oil. Wheels with ceramic bearings do not require lubrication. A chain tensioning pulley wheel can be used with a solid industrial groove ball bearing. Derailleur wheels with a fixed bearing are running clearance-free, highly flexible and lubricated forever.