Condensed Answer:
Shimano 105’s 11-speed road cassettes are compatible with the 11-speed Ultegra series (R8000 & 6800).
Shimano 105’s 10-speed road cassettes are compatible with the 10-speed Ultegra series (6700).
Indexed Drivetrains
Understanding the following principles will help you determine when a cassette can be integrated into a drivetrain with indexed gears.
- Indexed shifters
Modern shifters are indexed. This means that their movement is split into a number of equally spaced clicks. Each click indicates a shift and pulls or releases a pre-determined amount of gear cable.
That amount is known as cable pull and depends on the number of speeds, the bike type (MTB or road), and the component manufacturer.
- Rear shift ratio
Derailleurs have a rear shift ratio that indicates how much the unit moves per 1mm of cable pulled or released by the shifter. For instance, if the rear shift ratio is 1:1.7, the derailleur moves 1.7mm for every millimeter pulled or released by the shifter.
Just like the cable pull, the rear shift ratio depends on the gearing, the bike type, and the company making the component.
When combined the values of the cable pull and the rear shift ratio move the derailleur to the precise location needed for a shift to occur.
Since the cable pull and the rear shift ratio are speed-specific, it’s not always possible to mix shifters and derailleurs.
Cassette & Shifter Compatibility
If you’re going to use index shifters, the number of speeds has to match the cogs on the cassette. (e.g., 10-speed shifter for 10-speed cassettes).
If the cassette gears and the shifter’s speed do not match, the shifter will have extra or an insufficient number of clicks and won’t control the derailleur accurately.
The spacing between the cogs on a cassette decreases as the number of speeds increases. In other words, the cogs on an 11-speed cassette are closer together than those on a 10-speed cassette.
Hence 10 and 11-speed shifters have different cable pulls. The rear shift ratio of 10 and 11-speed derailleurs is dissimilar too.
The cable pull of 10-speed Shimano road shifters is 2.3mm.
The cable pull of 11-speed Shimano road shifters is 2.7mm.
The rear shift ratio of 10-speed Shimano rear road derailleurs is 1.7.
The rear shift ratio of 11-speed Shimano rear road derailleurs is 1.4.
All 10-speed road cassettes of Shimano are spaced equally. Hence they can be easily integrated into a road drivetrain with an equal number of speeds. The same applies to the 11-speed cassettes, although they have a different spacing as already mentioned.
Rear Derailleurs and Maximum Cog Capacity
An incompatibility occurs when trying to mix short cage derailleurs with a cassette that has a large cog with 34 teeth.
Short cage derailleurs usually go up to 30 teeth. After that, the shifting degrades as it’s harder for the derailleur to reach the largest cog.
If your derailleur has a short cage, it’s recommended to avoid using a cassette with more than 30 teeth on the largest cog.
Shimano’s Chart Confirms The Compatability
Shimano’s compatibility chart confirms that 105 cassettes can be integrated into an Ultegra drivetrain.
The first image shows that a 10-speed 105 cassette (CS-5700) can be used in an Ultegra 6700 drivetrain.
The second image shows that an 11-speed 105 cassette (CS-R7000) is compatible with 11-speed Ultegra (6800 & R8000) drivetrains.
