Originally Posted by
Gasman1440
Everyone keeps saying big-big + but if you dont have the chain in your hand how do you know? I’m trying to decide if I need a new chain and I dont have the cassette yet. I cant do big-big because I’m trying to decide which cassette to get (10-33 or 10-36) and then if I need a new chain. So first I have to understand the small intricacies of chain fitting. I’m new to fixing bikes and have very limited mechanical ability but like to learn. I have the tools and ability to change a cassette but dont know enough about is less then one spare link in the chain ok (e.g. the calc is 53.xxx and my chain is 54).
The big-big + 1 method: the "+1" is needed to allow the chain to climb over the teeth of the chainring or the cog when shifting to the big/big combination. Otherwise the chain will jam, and the tension will mangle the RD and the hanger. Instant disaster you do not want to happen. It is not about stressing the RD pedaling in this combination, but about it being destroyed during shifting. Check it on a stand first, gently rotating the pedals by hand.
Many years ago when I had no clue about bike mechanics I destroyed an old Sears FreeSpirit this way. I shifted to the big-big, simultaneously in the front and the rear. The chain was not properly sized.
I you do not have the cassette you can use the Park formula, or this calculator:
Javascript Bicycle Chain Length Calculator (the latter does not add +1", you need to do it yourself; they also speak about adding +2" for RDs with large wheels, this is not correct, +1" should be enough).
Exceeding the B-screw capacity/specs for the large cog is a different story.