• For front derailleurs, capacity is the maximum chainring difference it can shift, e.g. the common compact double crankset with a 50t big ring and a 34t small ring needs a capacity of 50-34 = 16. Modern Shimano and Campagnolo road front derailleurs are rated for that capacity, with SRAM using a smaller capacity deliberately.
• For rear derailleurs, capacity is the difference above, plus the difference between the largest and smallest cog. For example, with the compact crankset above and a 11-34 cassette, the required rear derailleur capacity would be 16 + (34-11) = 16 + 23 = 39. This is the exact capacity of Shimano's R7000 and R8000 long cage rear derailleurs.
• Rear derailleurs may be offered in several different cage lengths. Longer cages have higher capacity. In theory, they should shift a bit more slowly than short cage derailleurs, but the difference may be noticeable. There is often a maximum size for the largest cog, e.g. short cage Shimano R7000 and R8000 rear derailleurs are rated for cassettes with up to 30t big cogs, which means that 11-32 and 11-34 cassettes are out of manufacturer specification.
• However, you can commonly exceed the stated capacity slightly. Shifting will be worse than if you run equipment in spec, and the quality of shifting should decline as you go further from spec. However, not all bikes will be affected the same way, and not all cyclists will notice.

• For front derailleurs, capacity is the maximum chainring difference it can shift, e.g. the common compact double crankset with a 50t big ring and a 34t small ring needs a capacity of 50-34 = 16. Modern Shimano and Campagnolo road front derailleurs are rated for that capacity, with SRAM using a smaller capacity deliberately. For triple cranks, use the difference in tooth counts between the big and smallest ring.
• For rear derailleurs, capacity is the difference above, plus the difference between the largest and smallest cog. For example, with the compact crankset above and a 11-34 cassette, the required rear derailleur capacity would be 16 + (34-11) = 16 + 23 = 39. This is the exact capacity of Shimano's R7000 and R8000 long cage rear derailleurs.
• Rear derailleurs may be offered in several different cage lengths. Longer cages have higher capacity. In theory, they should shift a bit more slowly than short cage derailleurs, but the difference may be noticeable. There is often a maximum size for the largest cog, e.g. short cage Shimano R7000 and R8000 rear derailleurs are rated for cassettes with up to 30t big cogs, which means that 11-32 and 11-34 cassettes are out of manufacturer specification.
• However, you can commonly exceed the stated capacity slightly. Shifting will be worse than if you run equipment in spec, and the quality of shifting should decline as you go further from spec. However, not all bikes will be affected the same way, and not all cyclists will notice.

• For front derailleurs, capacity is the maximum chainring difference it can shift, e.g. the common compact double crankset with a 50t big ring and a 34t small ring needs a capacity of 50-34 = 16. Modern Shimano and Campagnolo road front derailleurs are rated for that capacity, with SRAM using a smaller capacity deliberately.
• For rear derailleurs, capacity is the difference above, plus the difference between the largest and smallest cog. For example, with the compact crankset above and a 11-34 cassette, the required rear derailleur capacity would be 16 + (34-11) = 16 + 23 = 39. This is the exact capacity of Shimano's R7000 and R8000 long cage rear derailleurs.
• You can commonly exceed the stated capacity slightly. In theory, shifting should be worse the more you exceed the capacity. Not all bikes will be affected the same way, and not all cyclists will notice.

Rear derailleurs also have a screw (the B-tension screw). These set how close the top pulley is to the cogs, and they are usually adjusted when in the largest cog. If the top pulley is too far away, this will compromise shifting. If it is too close, you nay be unable to shift to the largest cog, as the chain will rub. Manufacturers typically specify an acceptable distance. Park Tools also has written guides (for the rear and front derailleurs respectively), and their YouTube channel also has videos with explanations.

• For front derailleurs, capacity is the maximum chainring difference it can shift, e.g. the common compact double crankset with a 50t big ring and a 34t small ring needs a capacity of 50-34 = 16. Modern Shimano and Campagnolo road front derailleurs are rated for that capacity, with SRAM using a smaller capacity deliberately.
• For rear derailleurs, capacity is the difference above, plus the difference between the largest and smallest cog. For example, with the compact crankset above and a 11-34 cassette, the required rear derailleur capacity would be 16 + (34-11) = 16 + 23 = 39. This is the exact capacity of Shimano's R7000 and R8000 long cage rear derailleurs.
• Rear derailleurs may be offered in several different cage lengths. Longer cages have higher capacity. In theory, they should shift a bit more slowly than short cage derailleurs, but the difference may be noticeable. There is often a maximum size for the largest cog, e.g. short cage Shimano R7000 and R8000 rear derailleurs are rated for cassettes with up to 30t big cogs, which means that 11-32 and 11-34 cassettes are out of manufacturer specification.
• However, you can commonly exceed the stated capacity slightly. Shifting will be worse than if you run equipment in spec, and the quality of shifting should decline as you go further from spec. However, not all bikes will be affected the same way, and not all cyclists will notice.

Last, rear derailleurs also have a screw to control how close the top pulley is to the cogs (the B-tension screw). If the top pulley is too far away from the cogs, this should reduce shift quality. If it is too close, you may be unable to shift to the largest cog, as the chain will rub. Manufacturers typically specify an acceptable distance. Park Tools also has written guides (for the rear and front derailleurs respectively), and their YouTube channel also has videos with explanations.

Derailleur or derailer

Almost all derailleurs are actuated by shift cables. Because cables stretch a bit over time, new bikes require a minor adjustment after a few weeks' riding. While there is a bit of a learning curve, it is possible to adjust your derailers yourself, a process outlined later. The shift cables and housing are the main wear parts in derailer drivetrains, as they will accumulate contamination over time. Very active riders may replace these as often as annually. In many cases, poor shifting can be rectified by changing the cables.

The rear derailer is mounted to the frame with a derailer hangar, usually replaceable and made of aluminum. Even a minor drop can bend the hangar, which will affect shifting. However, the hangar is designed to break before anything more valuable, i.e. the derailer itself or the frame, does.

Derailleur or derailer

Maintenance

Almost all derailleurs are actuated by shift cables. Because cables stretch a bit over time, new bikes require a minor adjustment after a few weeks' riding. While there is a bit of a learning curve, it is possible to adjust your derailers yourself, a process outlined later. The shift cables and housing are the main wear parts in derailer drivetrains, as they will accumulate contamination over time. Very active riders may replace these as often as annually. In many cases, poor shifting can be rectified by changing the cables.

The rear derailer is mounted to the frame with a derailer hangar, usually replaceable and made of aluminum. Even a minor drop can bend the hangar, which will affect shifting. However, the hangar is designed to break before anything more valuable, i.e. the derailer itself or the frame, does.

Key concepts and terms

• For front derailleurs, capacity is the maximum chainring difference it can shift, e.g. the common compact double crankset with a 50t big ring and a 34t small ring needs a capacity of 50-34 = 16. Modern Shimano and Campagnolo road front derailleurs are rated for that capacity, with SRAM using a smaller capacity deliberately.
• For rear derailleurs, capacity is the difference above, plus the difference between the largest and smallest cog. For example, with the compact crankset above and a 11-34 cassette, the required rear derailleur capacity would be 16 + (34-11) = 16 + 23 = 39. This is the exact capacity of Shimano's R7000 and R8000 long cage rear derailleurs.
• You can commonly exceed the stated capacity slightly. In theory, shifting should be worse the more you exceed the capacity. Not all bikes will be affected the same way, and not all cyclists will notice.