Condensed answer: Officially, the maximum large cog capacity of the Dura-Ace 9000 rear derailleur is 28 teeth. This means, that the derailleur isn’t compatible with an 11-32 cassette and will have a hard time reaching the largest cog.
Maximum Low Sprocket Capacity
Each derailleur has a maximum low sprocket capacity. If the cassette exceeds it, the derailleur will not be able to reach the largest cog. The longer the cage (body) of the derailleur, the higher the maximum low sprocket capacity.
Derailleurs are divided into three segments based on cage length – short (s), medium (m), and long (l). Dura-Ace 9000 has a short cage length that comes with a fairly limited low sprocket capacity (28 teeth). The advantages of short cage derailleurs are lightweight and snappy shifting.
Road bikes often come with derailleurs that have a short or medium cage. The main reason for this engineering is that road bikes have a lower rolling resistance and are used on paved roads. Thus, they operate with higher gearing and come with cassettes that often have under 30T large cogs.
The advantage of those cassettes is that the gear transitions are smaller. For example, an 11-speed 11-28 cassette would have much smaller jumps between each gear than an 11-34 cassette.
The smaller transitions allow the rider to maintain a smooth cadence. The term cadence indicates the number of crank rotations per 1 minute. Consistently high cadence such as 90RPM is associated with efficiency and higher average speeds.
Even though the Dura-Ace 9000 derailleur doesn’t support an 11-32 cassette, there are methods that can increase the derailleur’s maximum cog capacity.
The most common solution is to replace the derailleur cage with an aftermarket solution that’s longer.
The longer cage will automatically increase the max. cog capacity of the derailleur, but will void its warranty.
If you want to increase the capacity of the rear derailleur even further, you can get a derailleur hanger extender.
The derailleur hanger extender is a middle-man that attaches to the original derailleur hanger and lowers the derailleur. The result is an even higher low sprocket capacity.
The Advantages Of Combining Dura-Ace 9000 with an 11-32 Cassette
The main incentive to combine Dura-Ace 9000 RD with an 11-32 cassette is to get an even lower gear (32T) that would make climbing easier.
The so-called gear ratio determines the pedaling difficulty of a gear combination.
The gear ratio indicates the number of rotations that the rear cog and wheel make per 1 full revolution of the cranks.
To determine the gear ratio, one needs to know the number of teeth on the chainring and the cog in use.
For example, if the chainring has 36 teeth and the cog has 32, the gear ratio is 36:32 = 1.12. This means that the rear wheel will rotate 1.12 times for every spin of the cranks. The lower the gear ratio, the easier it becomes to pedal and climb.
That said, getting a cassette with a larger rear cog is not the only way to lower the gear ratio and improve the climbing capabilities of a bicycle. You can also lower the gear ratio by using smaller chainrings.
For instance, the lowest gear ratio of 50/34+11-32 is 34:32 = 1.06.
But if you switch to a 48/32 crankset and keep the original 11-28, cassette, you will have a 32:28 = 1.14 low gear ratio which is pretty close to 1.06.
That way you’re eliminating the need to install an aftermarket cage to your derailleur and thus preserving the original looks and warranty.