When you add a gear, you should be able to choose all the activities type you have, not only 5 type of generic activities and "other". Now, you only can "Automatically Add to...": Running, Cycling, Hiking, Swimming, Walking and Other. At least, I think that Cycling should be split in two activities like road cycling and mountain biking.
Now, you can only add a bike as a default to cycling, although you can add several "other" gear type as a default for cycling.
Then you should be able to link a road bike to new "road cycling activities" and a MTB bike to new "Mountain biking activities", and you should be able to automatically add other gear types to the correct "new activities" and automatically know exactly when it’s time to change out your mtb or road bike chain and other gear.