- A bike's drivetrain is a pretty simple, straightforward system. Assuming all parts are in good repair, it will allow you to shift gears quickly and easily when you encounter different grades of terrain that demand different gears. Gears allow you to pedal efficiently at the same general rate, despite alterations in terrain that would otherwise impede your pedaling. There are 4 basic parts of the system: left and right shifters, front and rear derailleur, cogs (gears) and cable that connects the shifters and derailleurs. Pedaling is also a necessary part of the shifting process as the chain only moves up and down gears when it's moving forward.
- The two types of gear shifting are upshifting and downshifting. Upshifting means to shift to higher gears and downshifting means to shift to lower gears. Higher gears are more difficult to pedal, particularly on an incline but translate your energy into more forward motion and speed. Lower gears are easier to pedal up inclines and rough terrain but provide less forward motion. You upshift prior to a downhill or easier section of trail and downshift prior to an uphill or harder section of trail. What makes the concept confusing is that your higher gears in front are the larger gears. However, in the rear, higher gears are actually the smaller gears. Therefore, front and rear derailleurs work in reverse to perform the same upshifting and downshifting functions.
- The first step of shifting gears is done manually. The left shifter shifts the front derailleur and the right shifter moves the rear derailleur. Shifter design varies but each shifter allows for two separate shifts: one for downshifting and one for upshifting. Often shifters are marked so that you can see what gear each one is set to. When you click the shifter, it adjusts the tension of the cable that runs to the given derailleur.
- When you downshift using your left shifter, the cable tension is loosened and a spring moves the front derailleur over the lower (smaller) gear. Conversely, when you upshift it tightens the cable, pulling the derailleur over the higher (larger) gear. The chain is located in the middle of the front derailleur and is carried onto the appropriate gear by the derailleur.
- In the majority of rear derailleurs, downshifting increases cable tension and moves the derailleur toward the direction of the lower (larger) gears. The derailleur carries the chain onto the larger gear that you've shifted to. If you upshift, the cable is loosened and a spring in the rear derailleur moves it back down to the higher (smaller) gear that you've shifted to.
