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Old 04-29-21, 05:04 AM
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Badger6
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There are a bunch of ways to do this...I'm only going to offer the simplest (meaning fewest new required parts) to make this happen.

1. Crankset. You'll need a different crank and a 1x chainring. The chainring MUST be of a type that has a "narrow wide" or some other type of tooth profile that promotes chain retention. Shimano GRX crank with GRX chain ring is probably the easiest route, because it will work with your existing BB, so you won't need to change that also. There are other options, but all require changing the BB also. Number of teeth is up tot you, more below.

2. RD, you will need a new RD, period, with a clutch. Your current RD has plenty of capacity to "wrap" the chain (39t total) and could easily handle an 11-42 cassette (31t btw low/high) with an extender link, but, the RD you have is unclutched and you would likely experience a lot of chain drop on rougher surfaces. (a) Easiest (best?) option is the GRX RD-RX-812, it'll bolt right up, and is designed to work with 11-42 cassettes in 1x configuration. (b) Using an extender link, the RD-RX800-GS (essentially same RD you have now, with a clutch) should also work, but this is a two part solution where you are using the derailleur outside it's spec'd range, though there is a lot of experience doing it and it is known to work.

3. Cassette. Of course, easiest option for immediate out of the box functionality with little fuss using the GRX RD I mentioned above is an 11-42 cassette, CS-M8000 being the more economical choice, unless you want the trick and super light XTR. I am assuming you have a 11-speed Shimano Hyper Glide road hub, so you'll need a 1.85mm spacer if you don't have it, but it is cheap. It going on first, then the cassette.

This is the most straightforward way to do it, there are some other ways to get even lower gearing, but it'll require a bit more in depth study to get the right parts combos and get it tuned. Not impossible, just requires more effort initially. See this for a discussion on some options.

Lastly, gearing. I don't know how low you want to go, but if you choose to use an 11-42 cassette and pair it with a 40t chain ring, you'll gain a bailout gear below your current lowest ratio, but you'll lose your top end, topping out just past 50/14 in your current configuration. You could go a bit lower with a 38t ring (even lower than current), or slightly higher (though still lower than current) with a 42t. Try out this Gear Calculator to get a sense for how a 1x configuration would compare with your current setup.
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