Piff 

It seems that this should be simple, but I wanted to make sure before I break my chain to the proper length.

I have a rear derailleur that can wrap 36t (suntour Vx-GT). I am planning to change my gearing that would require a derailleur that can wrap 38t.

In order to make sure my chain isn't too long, instead of using the technique of subtracting two links from the length of a chain wrapped around the big ring and big cog, I should instead wrap the chain around the big ring and second largest cog and subtract two. Right?

If it helps, my gearing is: 52/42/28, 14-28 (14/16/18/21/24/28).

Thanks! And it goes without saying that I know not to shift into the big/big or small/small.

HillRider

No, do NOT make the chain shorter than required to wrap big-big. If it's too long for the rd to wrap up the total teeth then avoid small-small. BTW, you do not subtract two links from the length needed to wrap big-big, you add two links (1 inch).

It is essential the chain allow you to shift into big-big. Yes, you know you shouldn't but some time somewhere you are going to forget and the resulting damage can be extensive and expensive.

Piff 

Ah, whoops, been a while since I broke a chain. Thank you for the correction.

Okay, that sounds good. I'll set it up as normal, but just avoid small small. Thanks!

Bill Kapaun

Make sure you have SOME "slack" when on BIG:BIG.
This is how I check in situ.

I wouldn't settle for < 3/4" slack. You need a bit for the chain to come OFF the cog when shifting to the smaller.
You may not intend to shift BIG:BIG, but as Forest Gump says......

squirtdad

I find small/small gives the max amount of chain

but you still need to make sure big/big works..... dangerous other wise

79pmooney

I'm scratching my head. You want to change the derailleur to one with I presume a longer cage, requiring more chain but are going to start with a chain length too short already for big-big. It may not even be long enough for big-next to big after the derailleur change. (I presume you have the derailleur in hand and I am wrong here.)

As for not using the big-big - I guess you do not share my love of storming up hills and rolling over the top racer-style and shifting up to get downhill speed quickly. I've pulled back the left lever many times in that frazzled state of mind. Two extra chain links seems an awfully cheap insurance against hundreds of $$s bike damage and the call of shame. Small-small rarely causes issues outside of unwanted shifts from chainring pins or teeth engaging and chain sag, marring chainstay finish. I do it regularly on short flats or downgrades on long climbs to save two double shifts. When setting up a new derailleur, I just make sure the small-small combo works. (On some of my pinned bikes, it simply doesn't unless I set the FD cage to drag on the outside plate while I ride.)

esasjl

I use big-big plus 1 or 2 " (minimum 1 link pair). If the RD can cope with the wrap then small-small will be fine. RD wrap can be increased with larger jockey wheels, e.g., replacing 11t with an 12/14t combination (e.g., BBB SRAM) if the cage has space (RD-M95x series will wrap 53t and I think the spec is 43t). If using a double it is good to have small-small in the wrap range so all ratios can be used properly. With a triple middle-small should be in range, but in the small chainring no harm will occur if one or two (or three) of the smallest sprockets are beyond wrap range. I have found that with long cage RDs in big-big (used with a wide-range double) the tension jockey wheel can get noisy on the chain as the chain length to the chainwheel gets quite short and the angle quite large on the bottom chain run. This does improve as the jockey wheel teeth erode to fit the chain (avoid metal wheels).

HTupolev

The small-small method gives the max amount of chain if the chain wrap is adequate. If not, and you size with the big-big method, the drivetrain will have enough chain that it's overly slack in small-small. But most people prefer this latter compromise... I remember one gravel ride where the rider said "I just won't use big-big" at the start, and jammed his drivetrain badly on the second steep ascent.

cxwrench

If you're going to exceed capacity ALWAYS make sure you have enough chain to go big/big otherwise you'll have big problems as HTupolev posted.