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Condensed Answer: If the derailleur and the cassette are designed for the same number of speeds, then in most cases, it’s totally acceptable to combine a 105 rear derailleur with an Ultegra cassette.

Cassette and Derailleur Compatibility

Modern shifters are indexed. In simple terms, this means that the shifter’s movement is segregated into individual clicks. Each click results in a shift that occurs via a cable pull or release. The amount of cable that’s pulled or released by the shifter is known as cable pull. The purpose of indexed shifters is to make shifting fast and efficient.

A pre-determined movement of the rear derailleur is also crucial for accurate index shifting. Hence why derailleurs have rear shift ratios.

The rear shift ratio describes how much a derailleur moves per 1mm of cable pulled or released by the shifter. If the ratio is 1.7, for example, the derailleur would move 1.7mm per 1mm of cable pull.

The rear shift ratio of a derailleur depends on the number of gears on the cassette and the bike type (MTB or road).

The rear shift ratios of Shimano’s derailleurs can be found in the table below.

105 and Ultegra drivetrains come in 10/11/12 speeds. The rear shift ratios of 11 and 12-speed derailleur match but that of 10-speed models is different. This leaves us with the following possibilities:

• 10-speed 105 derailleur with 10-speed Ultegra cassette
• 10/11-speed 105 derailleur with 10/11-speed Ultegra cassette

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

Conclusion: If you plan on using a cassette with a large cog over 30 teeth, use a derailleur with a long cage.

A Note On Friction Shifting

Before indexed gears, riders relied on friction shifters. Friction shifters move freely (no clicks) and it’s up to the rider to determine the correct position of the lever for a shift to occur. This method is slower and has a steeper learning curve. Nonetheless, it has one major benefit – compatibility.

The freedom of the shifter makes the rear derailleur’s ratio irrelevant because the shifter isn’t “locked”. As a result, the rider can always find a position to trigger a shift.

If you have friction shifters, you can use a 10-speed derailleur with an 11 or 12-speed cassette provided that the derailleur has the needed capacity to operate with the cassette.