But they don't even have one. Probably because nothing smaller than a 32 will fit on a 104 BCD. Just my guess. They make 20 T for the smal, which only fit XT with some filing, so they would probably make a 30T middle if it was possible.
Homebrewed Components makes 31t rings for 104BCD cranks, but that is the smallest you can go before the crank spider can start interfering with the chain. Extralite makes a 30t ring but you might have to do some filing or grinding to your crank spider to make it work
I have a HBC 31t ring on my HSCC 104bcd 4-bolt crank. Had to do a small bit of filing to keep the chain from riding up on the bolt hole shoulders. Some cranks need this, some don't, but if they require removal of material (most do), it doesn't take much for the chain to clear.
What is the purpose of a smaller than 32T middle anyway? It seems like the ratios overlap with the small ring ratios anyway,a nd you wouldn't be getting anything new over having a 32.
There are also granny rings smaller than 22t 20/30 or 20/30/40 is really well liked by some people as a way to make 29er gearing feel similar to 26er gearing. Even if you don't go smaller on the granny, the smaller middle could be enough of change to prevent having to shift to the granny ring in many situations. On that note, they're pretty popular for single ring setups too.
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