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The number of gears supported changes
The higher the grade of Shimano rear derailleur, the more gears it can handle. For both road bikes and MTBs, 12-speed is the maximum number of gears supported. Of course, upgrading between 11-speed derailleurs will not change the number of gears supported.
It is important to note that Shimano officially specifies compatible rear derailleurs for each rear speed, so it does not mean that “an 11-speed 105 RD-5800 can be used in an 8-speed Claris component, which is less than 11 speeds.
Shimano components have different chain thicknesses for each number of gears (the more gears, the thinner the chain), so the front and rear derailleurs are designed to be sized accordingly.
Can I use a deprecated combination?
The derailleur is a simple mechanism that moves only the distance that the STI lever pulls the inner wire, so it can move even if the number of gears is different.
If you are using a rear derailleur with a smaller number of gears than the original one (such as an 8-speed derailleur in an 11-speed compo), you may be able to use it, although it is not recommended, because the chain thickness is larger than expected.
On the other hand, on the other hand, using a derailleur that is larger than the original number of gears (such as an 11-speed derailleur inside an 8-speed component) may cause a failure and should be avoided.
Either way, it is best to use an officially compatible combination, as it will definitely not provide the shifting performance envisioned by Shimano.
Compatible sprockets change
Shimano sprockets are available in various combinations for each number of steps, such as wide ratio and cross ratio.
Each rear derailleur product has its own “minimum number of teeth” and “maximum number of teeth” for the corresponding sprocket, so upgrading the rear derailleur may change the numbers here.
For example, if you upgrade from the 105 RD-5800-SS to the RD-R7000-SS, the minimum number of teeth will remain the same, but the maximum number of teeth will increase by 2T, from 28T to 30T.
The RD-5800-SS and RD-R7000-SS have the same 105 11-speed and short cage, but if you upgrade the rear derailleur to the RD-R7000-SS, you will be able to choose a wider ratio sprocket.
Smoother chain movement and sound
Shimano’s rear derailleur, ULTEGRA and above for road bikes, has ball bearing pulleys. The bearing pulleys make the chain move smoother and make less noise.
Of course, the resistance of the pulley rotation is negligible when you consider the resistance (power loss) generated by the entire bike, but it is one of the advantages of upgrading.
It is important to keep in mind that upgrading to ULTEGRA or higher rear derailleur has these advantages. It will help you to make a decision when you are wondering “105 or ULTEGRA?
Lever operation becomes slightly lighter
When using a higher grade rear derailleur, the lever operation when shifting gears may feel a little lighter.
If you don’t shift much and ride with torque, you may not feel the need to shift much, but if you ride with a constant cadence, you will shift frequently, and the lighter the shifting, the easier it will be to ride.
If you try using a new higher grade rear derailleur with the same STI lever as before the upgrade, you will feel a little lighter winding up (of course, using it as is is is not officially recommended by Shimano).
If you upgrade from 105 to ULTEGRA or any other grade, the difference may be marginal, but if you jump up a grade or generation, such as from SORA to 105, or from the 5700 series to the R7000 series even with the same 105, you will notice that the shifting is a bit lighter.
It is hard to tell whether the lighter shifting is due to the STI lever or the rear derailleur, since the STI lever needs to be replaced when the number of shifts changes with the higher grade, but at least the spring action of the rear derailleur is lighter and smoother with the higher grade.