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Condensed Answer: If the 105 derailleur is designed for the same number of speeds as the Ultegra shifters, they can be combined. If the gears do not match, however, the combination will not work and will result in subpar shifting.

Understanding Index Shifting

Modern drivetrains have indexed gears. In simple terms, this means that the movement of the shifter is segregated into clicks depending on the number of gears that the bike has. Each click equals a shift.

The amount of cable that’s pulled or released by the shifter during a shift is known as cable pull and depends on the shifter type (MTB or road) and the number of gears.

In order for the derailleur to move to a pre-determined location after a click of the shifter, derailleurs have a specific shift ratio.

The shift ratio indicates how much the derailleur moves per 1mm of cable pulled or released by the shifter. For example, if the rear shift ratio is 1.7, then the derailleur will move 1.7mm per 1mm of cable pulled or released by the shifter.

The fixed cable pull per 1 click and the rear shift ratio allow precise movement of the derailleur across the cassette. The result is quick and easy shifting.

The downside, however, is that the system fails when using shifters or derailleurs with the wrong settings.

The table below shows the rear shift ratio of Shimano derailleurs:

105 and Ultegra drivetrains come with 10/11/12-speeds. The rear shift ratio of 10 and 11/12-speed derailleurs is different.

Consequently, the following options are possible:

• Ultegra 11/12 shifters with 11/12-speed 105 derailleurs.
• Ultegra 10-speed shifters with 10-speed 105 derailleurs.

Derailleur Max Cog Capacity

Another property that one needs to take into consideration is the size of the cassette and the derailleur’s max cog capacity. Derailleurs with shorter cages are light and offer snappy shifting, but their maximum cog capacity is lower, and they cannot be combined with big cassettes.

The tables below contain the max cog capacity of 105 and Ultegra derailleurs:

105

Ultegra

Make sure that the derailleur you choose has the capacity to cover your cassette.

FAQ: What about combing an electronic 105 derailleur with Ultegra shifters?

105 electronic derailleurs are 12-speed and can also be integrated into an Ultegra drivetrain. For example, the R7150 Di2 derailleur is known to work with Ultegra R8150 Shifters.

FAQ: What are the advantages of using Ultegra shifters in this case?

Ultegra is only one step above 105 in Shimano’s hierarchy. Thus, the quality and the performance of both group sets are very closе. Тhe average rider wouldn’t be able to tell the difference. That said, people report that Ultegra shifters feel more precise and ergonomic.

Another benefit of mixing parts from different systems is simply availability. If you need to replace your derailleur, and you can find a 105 model that fits the criteria, you get one more option to choose from.

It’s also possible to be in a situation where you can’t find an Ultegra derailleur with a cage that’s long enough to cover your entire cassette whereas a compatible 105 model is available.

Summary: What You Need To Know

• If the number of speeds that the Ultegra shifters are designed for matches the speeds of the 105 derailleur, the combination can work. Of course, the derailleur should have the capacity to cover the cassette.

• It’s possible to use an 11-speed derailleur in a 12-speed system or a 12-speed derailleur in an 11-speed drivetrain. However, it’s not possible to integrate a 10-speed derailleur in an 11/12-speed drivetrain due to the dissimilar rear shift ratios.