How does the Rear Derailleur coordinate the chain with the front derailleur system to achieve power transmission and speed change operation?

The guide wheel system of the Rear Derailleur consists of two upper and lower guide wheels, which are designed to dynamically adjust the chain path. The upper guide wheel is located on the side of the larger sprocket of the flywheel. When the rider needs to downshift, the guide wheel moves outward and actively pushes the chain to the larger sprocket to ensure sufficient pedaling power. The lower guide wheel is close to the smaller sprocket of the flywheel. When upshifting (high-speed cruising), the Rear Derailleur releases the chain, and the spring tension causes the guide wheel to return inward. The chain naturally transitions to the smaller sprocket to reduce energy loss. The guide wheel surface is usually designed with tooth-shaped grooves to increase the contact area with the chain, prevent the chain from falling off under bumpy road conditions, and reduce friction noise during speed change.
The speed change process is triggered by the rider operating the speed change handle, and the Rear Derailleur adjusts the guide wheel position according to the command. The rider presses the speed change handle to release the speed change cable or send an electronic signal. The spring tension inside the Rear Derailleur dominates, the guide wheel moves inwards towards the cassette, and the chain jumps to the smaller cog under the action of tension.
The shift cable is pulled (or the electronic motor is activated), overcoming the spring tension, and the guide wheel moves outwards, actively pushing the chain to the larger cog. Electronic shifting systems use sensors to detect the chain position in real time, compensate for wear or terrain changes, and ensure accurate shifting. Some Rear Derailleur designs are equipped with a tension wheel, which uses adjustable spring pressure to keep the chain moderately tight to prevent loosening or skipping teeth. The tension of the front and Rear Derailleurs needs to be adjusted in coordination to prevent excessive tilting and friction of the chain during cross-shifting. The cassette range of the Rear Derailleur needs to match the number of teeth on the front derailleur to ensure the appropriate chain length. Extreme gear ratios may result in insufficient chain tension, which needs to be adjusted by B screws or chain cutting.
Modern Rear Derailleurs optimize shifting efficiency and reliability through technological innovations. The inward-moving guide wheel design shortens the chain path, reduces the risk of impact in the event of a fall, and improves the speed of shifting response. For example, in mountain bike off-road, the Shadow Rear Derailleur can avoid deformation of the guide wheel caused by hitting rocks. The guide wheel position is driven by a motor, combined with sensor feedback, to achieve millisecond-level speed change response. The Rear Derailleur uses a carbon fiber guide arm or an aluminum alloy body to reduce weight, while enhancing bending strength to meet high-intensity off-road needs.
In different riding scenarios, the coordination between the Rear Derailleur and the front derailleur needs to be adjusted specifically. The front derailleur is used less frequently, and the Rear Derailleur needs to respond quickly to high-frequency upshifts, and the tension wheel pressure needs to be lowered to reduce friction. The Shadow Rear Derailleur reduces the risk of impact, and the guide wheel needs to be cleaned of mud and sand regularly to prevent speed jams. The front derailleur needs to be adjusted in conjunction with the Rear Derailleur to prevent the chain from falling off during bumps. The guide wheel movement range is set by the limit screw to ensure that the chain does not rub the maximum/minimum sprocket of the flywheel. Lubricate the guide wheel bearing regularly and check whether the speed change cable is worn.