Condensed Answer: Dura-Ace 9000 and 9100 are 11-speed road group sets by Shimano. As a result, the shifters have matching cable pulls, and the derailleurs have identical rear shift ratios.
Consequently, it’s possible to combine most 9100 parts with 9000 and vice versa. The only exception would be the 9000 front derailleur – it isn’t officially compatible with 9100 cranks.
Cable Pull & Rear Shift Ratio
Two factors have a major impact on the compatibility between different group sets – the cable pull and rear shift ratio.
The movement of modern index shifters is segregated into clicks depending on the number of speeds that the bike has. Each click equals a shift achieved by pulling or releasing the gear cable. The amount of pulled or released cable is known as cable pull and depends on the number of gears, the bike type (MTB or road), and the manufacturer of the component.
The rear shift ratio is a property of the rear derailleur and indicates how much the rear derailleur moves per 1mm of pulled or released cable. If the rear shift ratio is 1.4, the rear derailleur would move 1.4mm for every millimeter of cable pull.
The pre-determined values of the cable pull and the rear shift ratios allow engineers to position the rear derailleur at a precise location for an accurate and fast shift to occur.
If the designated derailleur is replaced with one that has a different rear shift ratio, the shifting will be poor or even fully non-functional.
How Compatible Are Dura-Ace 9100 and 9000
Dura-Ace 9100 and 9000 are both 11-speed group sets in Shimano’s road segment. Consequently, their shifters have a matching cable pull of 2.7mm. The rear shift ratio of the derailleurs is about 1.4.
The matching values indicate that 9100 and 9000 shifters and derailleurs are compatible.
Shimano’s compatibility chart confirms that.
The images above are taken from Shimano’s compatibility chart.
The first one shows that 9100 shifters (ST-R9100) are compatible with the 9000 rear derailleur.
The second one shows that the 9100 shifters are compatible with the 9000 front derailleur too.
One can also see the compatibility between the front derailleurs and the cranks. (The cranks are in the third column from left to right).
The 9100 front derailleur is compatible with both 9100 and 9000 cranks. But the 9000 front derailleur is only compatible with 9000 cranks.
Why? The small chainring of 9100 cranks is 0.4mm closer to the frame so that the bike has a straighter chain line. Meanwhile, the larger chainring of 9100 matches the location of 9000’s large chainring. Thus, there’s a 0.4mm extra distance between the 9100 small and big ring in comparison to the 9000 units.
The front derailleur of the 9100 group set reflects the new spacing of the cranks. Thus, it’s not recommended to combine a 9000 derailleur with 9100 cranks.
Having said that, the 0.4mm difference is not enough for a massive incompatibility. Some people have combined 9100 cranks with a 9000 derailleur with acceptable results. The combination could be made to work to a fairly acceptable level by adjusting the in-board limiting screw of the derailleur.