Wolftooth makes this little nugget of aluminum. In a nutshell, it allows you to increase how big of a cog your rear derailleur can shift to. It only works on certain derailleurs, but if you have an old school, sorta normal one, like a Shimano 105 from 2016, or a 9 speed Shimano Deore, or a 10 speed SRAM GX unit, this thing will work. The main stuff it does not like are the newer shadow derailleurs from Shimano, and 11 speed SRAM stuff. We only use it on two derailleurs, the 10 speed GX for touring bikes that are gunna be ridden off road, and the 9 speed Deore, for touring bikes that are not gunna be ridden off road.
With the Roadlink and a long cage GX or Deore rear mech, you can shift across an 11-40 rear cassette. Usually, without a Roadlink, this would top out at 36T. If you limit the chainrings up front to a tooth count different of 20 teeth or less, this roadlink will allow you to have both decently high enuff gears and super low gears, which is great for mixed condition touring. There’s some wiggle room when you install it. Don’t by this if you are not a fiddler by nature. It’s not plug and play. You’re gunna have to monkey around with it a bit. Sometimes we rotate them back a bit on the derailleur hanger, to wrap up more chain. If you do that, make sure you snug down the mounting bolt so it doesn’t wiggle around and leave you with a wizard sleeve chain situation.
Read more about ultra wide range gearing with our Turkey Vulture Supreme write up.
Wolftooth has written a bunch of technical info about this doodad, and if you want to see what else it can be used for, go there. We don’t care about the other stuff, which mainly relates to 1x systems (there are better ways to go) and gravel bikes (we put touring gearing on e’rything).