Mountain bike hubs have the same diameter and width as that of the thickness and inner diameter of 11-speed cassettes, which means this setup is possible and is compatible with MTB, but for the road bike category, this setup is non-compatible due to different dropout spacing and the thickness of 9-speed hub. the main reason for the non-compatibility of the road bike category is the value of different cable pull and cable pull ratios, these two values would play a major role in fitting and putting different speed cassettes, hubs, and derailleurs, and the compatibility of these bike components are totally dependent on these values.
Cable pull values are much more important and necessary in shifting gears from lower to higher and higher to the lower sprocket of the 11-speed cassette, these values are specific for specific bikes, and also there is a large difference between the distance of these sprockets, and when you shift gears, chain of your bike would work abruptly according to the values of the cable pull and cable pull ratios and when you have installed different speed cassette, then you will feel much trouble and frustration in shifting of gears, this bad shifting of chain within the cogs and teeth of your bike cassette would also depend on the type and brand of gear shifters, this setup I mean 9-speed hub and 11-speed cassette would work fine and accurately without any issue if your bike has friction shifters, but if you have installed indexed or other types of shifters, you will face a lot of difficulty and problems with them.
Necessary Components Which Are Needed For 11 And 9-Speed Compatibility:
You have to spend many bucks on buying and purchasing many components of bikes based on this setup, first of all as you are installing an 11-speed cassette, so you have to buy a new cassette online or at any of your bike mechanic stores, before buying a check the teeth and cogs, although it is necessary there were many complaints and issues in the past, even a single broken, cracked or bent teeth of 11-speed cassette would run your bike chain.
You also have to replace or interchange your previous 9-speed derailleur with a new 11-speed derailleur, because there is a positive response and a much better relationship between an 11-speed derailleur and an 11-speed cassette for smooth running of the bike chain. non-compatible derailleurs would constantly move right and left side and also shows the movement up and down, to avoid or maintain these movements of derailleurs you have to buy a new compatible 11-speed derailleur, as I have described above in my post derailleur is also depend on the values of cable pull and cable pull ratios, you have to make all of these things compatible with each other.
The old and previous 9-speed derailleur is not capable to work with the greater number of cogs and cassettes.9-speed derailleurs have normal limits and acceptable reach of the maximum number of 34 cogs cassettes, which is the 9-speed cassettes, but larger 11-speed cassettes have a greater number of 46 cogs of the cassette, so it is not possible for the derailleur to work accurately with the greater number of cogs cassette.
You also need new shifters, the reason is the that old 9-speed cassette contains 9-speed shifters, but now you have installed an 11-speed cassette, so it is not possible to shift gears and bring the chain in 10 and 11 cogs of the cassette with the same 9-speed shifter, you have o buy 11-speed shifters for this setup to make the shifters compatible with the 11-speed derailleur and 11-speed cassette.
New 11-Speed Chain:
It is a must and necessary to buy a new 11-speed chain because the old or previous 9-speed chain is not compatible with the 11-speed cassette and 11-speed derailleur. this difference is present within the manufacturing and engineering of 11-speed and 9-speed chains. width and diameter of 9-speed chains are thicker and wider as compared to 11-speed chains, which means the cogs and teeth of 9-speed cassettes are also wider and thicker as compared to 11-speed cassettes, so 9-speed chains are not installed on 11-speed cassette bikes, and also 11-speed chains are not installed and are not compatible for 9-sped cassettes.
Difference In Width And Diameter Of 11-speed And 9-Speed Hubs:
All the modern, expensive, and a greater number of gear hubs are wider and thicker in diameter as compared to smaller gear hubs, due to which 11-speed hubs have a width of 1.85mm and are wider and thicker than 9-speed hubs.
There are a few types and brands of 11-speed cassettes which are compatible with the 9-speed hubs as described below:
Shimano XT CS-M8000:
It is an 11-speed Shimano company brand. it can easily be installed and is compatible with the 9-speed hubs. but if your bike has 11-speed hubs then you need compatible-sized small washers or spacers so that these spacers would make this Shimano brand 11-speed cassette compatible with the 11-speed hubs, but if your bike has 8 or 10-speed hubs then these Shimano 11-speed cassettes are also compatible with these hubs, and you don’t need to buy spacers or washers for 8 and 10-speed hubs.
SUNRACE CXMSX8 11-Speed Cassettes:
It is SUNRACE brand, this company is also manufacturing and making 11-speed cassettes, it is the better and broad company all over the world, it has a big advantage, these 11-speed cassettes are also compatible with the two wide and largest cassettes company brands that are SRAM and Shimano companies. if you have a 9-speed hub body of these brands then you have easy access and a bonus if installing these SUNRACE 11-speed cassettes.