Shimano Understanding SHIMANO RD Spec's
How to make sense of Shimano's Rear Derailleur specifications
How to make sense of Shimano's Rear Derailleur specifications
When selecting a rear derailleur, there are several compatibility specifications that must be maintained with the rest of the shifting
system and the drivetrain. If any of these do not match, performance will be compromised or the system may not function at all.
This sheet should will help with understanding the different specifications associated with choosing the right derailleur for the job.
Low Sprocket Max (sometimes referred to as Max Cog) and Minimum:
The maximum and minimum sizes of the first cassette sprocket (the easiest to pedal and the highest tooth count) that the derailleur
can accommodate. If you attempt to use a derailleur with a cassette with a higher tooth count than it is designed for, you will
experience severe difficulty in shifting to the largest sprockets. You may also damage the derailleur, wheel, or frame. Attempting to
use a derailleur with a cassette that has too small of a large sprocket can result in decreased shifting performance. This may also
affect the warranty coverage.
Top Sprocket Max and Minimum:
The maximum and minimum sizes of the last cassette sprocket (hardest to pedal and the lowest tooth count) that the derailleur can
accommodate. Using a cassette outside of these ranges will result in poor shifting and may result in increased likelihood of chain
skip.
Total Capacity, Max Capacity, Chain Capacity, or Chain Wrap:
The maximum amount of chain slack that the derailleur can take up (or let out) in order to accommodate the range of sprocket sizes
on the bicycle. This value is expressed as a number of sprocket teeth. To find the minimum total capacity needed for a combination
of cassette/freewheel and front chainrings, you must add the size difference between the largest and smallest sprockets on the
cassette and the smallest and largest chainrings. The rear derailleur’s total capacity must be greater than or equal to this number
for the system to function properly. As a math problem:
Total Capacity ≥ (Largest Rear Sprocket-Smallest Rear Sprocket) + (Largest Chainring-Smallest Chainring)
For a system with an 11-32 cassette and a 50-34 crankset:
≥ (32-11) + (50-34)
≥ 21+16
≥ 37
In this situation, a RD-R8000-SS, with a 35T total capacity, would not work. A RD-R8000-GS, with a total capacity of 39T, would be
the correct choice. Using a derailleur with insufficient total capacity will either require a chain that is either too long or too short for
the system. A chain that is too long works well on the largest cassette sprocket and chainring, but will be excessively loose when
on smaller combinations. This will lead to a decrease in shift performance along with increased risk of skipping chain and chain
suck. A chain run too small will shift into the big-big sprocket combination with difficulty or not at all, it may become stuck there,
and it may damage the derailleur, wheel, or frame.
Rear Speeds:
This refers to the number of sprockets on the cassette. The number of sprockets on the cassette must match the number of
indexes in the shifter, and the number of sprockets for which the rear derailleur is designed. Apart from the obvious need for
each component to have the same number of positions, different numbers of sprockets require different chains, and this must be
accounted for with derailleur cage and pulley dimensions.
Cable Pull/Actuation Ratio/Shift Ratio/Family
All of these terms are ways that technicians talk about the relationship between shifter, derailleur, and cassette. The amount of
cable pulled by the shifter, the way the derailleur moves when the cable is pulled, and the distance from one cassette sprocket to the
next all interact with one another. All three of these components must work together to move the derailleur just the right amount
to make a perfect shift for each “click” of the shifter. When components are all of one “family” – 11-speed MTB, for instance – the
parts are designed to match up. The compatibility charts at si.shimano.com show the SHIMANO-tested combinations and are the
definitive guide as to which works with which. Other combinations may be functional, but the performance of these combinations
cannot be guaranteed.
SHIMANO Rear Derailleur / Shift Families:
` • 12-Speed MTB
• 11-Speed MTB
• 11-Speed Road
• 10-Speed Road (New)
• 10-Speed Road (Legacy)
• 10-Speed MTB
• 9-Speed MTB
• 9-Speed Road
• 8-Speed MTB
• 8-Speed Road
• 8-Speed DURA-ACE
• 7-Speed MTB
• 7-Speed Road
• 7-Speed DURA-ACE
• 6-Speed MTB
• 6-Speed Road
• 6-Speed DURA-ACE
system and the drivetrain. If any of these do not match, performance will be compromised or the system may not function at all.
This sheet should will help with understanding the different specifications associated with choosing the right derailleur for the job.
Low Sprocket Max (sometimes referred to as Max Cog) and Minimum:
The maximum and minimum sizes of the first cassette sprocket (the easiest to pedal and the highest tooth count) that the derailleur
can accommodate. If you attempt to use a derailleur with a cassette with a higher tooth count than it is designed for, you will
experience severe difficulty in shifting to the largest sprockets. You may also damage the derailleur, wheel, or frame. Attempting to
use a derailleur with a cassette that has too small of a large sprocket can result in decreased shifting performance. This may also
affect the warranty coverage.
Top Sprocket Max and Minimum:
The maximum and minimum sizes of the last cassette sprocket (hardest to pedal and the lowest tooth count) that the derailleur can
accommodate. Using a cassette outside of these ranges will result in poor shifting and may result in increased likelihood of chain
skip.
Total Capacity, Max Capacity, Chain Capacity, or Chain Wrap:
The maximum amount of chain slack that the derailleur can take up (or let out) in order to accommodate the range of sprocket sizes
on the bicycle. This value is expressed as a number of sprocket teeth. To find the minimum total capacity needed for a combination
of cassette/freewheel and front chainrings, you must add the size difference between the largest and smallest sprockets on the
cassette and the smallest and largest chainrings. The rear derailleur’s total capacity must be greater than or equal to this number
for the system to function properly. As a math problem:
Total Capacity ≥ (Largest Rear Sprocket-Smallest Rear Sprocket) + (Largest Chainring-Smallest Chainring)
For a system with an 11-32 cassette and a 50-34 crankset:
≥ (32-11) + (50-34)
≥ 21+16
≥ 37
In this situation, a RD-R8000-SS, with a 35T total capacity, would not work. A RD-R8000-GS, with a total capacity of 39T, would be
the correct choice. Using a derailleur with insufficient total capacity will either require a chain that is either too long or too short for
the system. A chain that is too long works well on the largest cassette sprocket and chainring, but will be excessively loose when
on smaller combinations. This will lead to a decrease in shift performance along with increased risk of skipping chain and chain
suck. A chain run too small will shift into the big-big sprocket combination with difficulty or not at all, it may become stuck there,
and it may damage the derailleur, wheel, or frame.
Rear Speeds:
This refers to the number of sprockets on the cassette. The number of sprockets on the cassette must match the number of
indexes in the shifter, and the number of sprockets for which the rear derailleur is designed. Apart from the obvious need for
each component to have the same number of positions, different numbers of sprockets require different chains, and this must be
accounted for with derailleur cage and pulley dimensions.
Cable Pull/Actuation Ratio/Shift Ratio/Family
All of these terms are ways that technicians talk about the relationship between shifter, derailleur, and cassette. The amount of
cable pulled by the shifter, the way the derailleur moves when the cable is pulled, and the distance from one cassette sprocket to the
next all interact with one another. All three of these components must work together to move the derailleur just the right amount
to make a perfect shift for each “click” of the shifter. When components are all of one “family” – 11-speed MTB, for instance – the
parts are designed to match up. The compatibility charts at si.shimano.com show the SHIMANO-tested combinations and are the
definitive guide as to which works with which. Other combinations may be functional, but the performance of these combinations
cannot be guaranteed.
SHIMANO Rear Derailleur / Shift Families:
` • 12-Speed MTB
• 11-Speed MTB
• 11-Speed Road
• 10-Speed Road (New)
• 10-Speed Road (Legacy)
• 10-Speed MTB
• 9-Speed MTB
• 9-Speed Road
• 8-Speed MTB
• 8-Speed Road
• 8-Speed DURA-ACE
• 7-Speed MTB
• 7-Speed Road
• 7-Speed DURA-ACE
• 6-Speed MTB
• 6-Speed Road
• 6-Speed DURA-ACE
