One Chain w/Two Sets of Chainrings?
 

I have two sets of chainrings that I would like to use on the same cassette utilizing the same chain. One most of the time, the other on occasion---provided it will work.

The main set is 52/38 with a total capacity of 32, the secondary set is a 56/44 that ends up needing 30. (Cassette is 12-30)

The RD is a 105 short cage (RD-5700) that Shimano shows "33 or less" for a total capacity. Will it handle both gearsets with one chain? I usually use big chainring to big sprocket + 2 links to fit a chain (per Shimano tech docs), should I fit the chain with the 56t or 52t ring? Or, measure for each and split difference?

Or, should I use one chain for each?

Thanks, Greg.

Here's the problem: Your main chain is almost surely going to be too short to cover the big/big combination. If you ever accidentally shift into that combination (or even the next couple biggest rear cogs), it can cause a TON of damage to your bike in less time than it took me to type this sentence.

If you size the chain to fit the 56 chainring it will probably be too long when using the 52 chainring. If it was my bike I'd switch chains when I switched chainrings.

If the chain is measured for the 56 it will probably be OK for the smaller crankset. The issue may be chain take up riding small/small with the 38 but that's easy enough to find out.

Keeping chains matched to the cassette is more important than to the rings,so from a wear standpoint staying with the same chain makes sense. OTOH if you ride both combinations roughly equally, having two chains that you rotate will keep them both matched for wear. Rotating chains is SOP for many high mileage riders, so you'd be in good company doing so.

sounds like you have all the parts so it wouldn't cost anything to just try it out and see.

That's it. I'll just fit it to the large ring then try the smaller set. I was just wondering if it was even worth the time to try.

Thanks.

I see the biggest potential issue is the ability of the short cage RD to handle the chain wrap - what you want to do is trivial for a long cage RD.