Rear mech chain wrap capacity and jockey wheel size

[Fatal Swan]

I've just swapped a long cage 10-speed SRAM X0 rear mech for a long cage SRAM XX rear mech (because I like the silver finish more) and discovered that the long cage of the new one is not as long as the long cage of the old one. About 94mm vs 100mm.

As a result, it seems that chain wrap capacity of the new mech is less. Looking at the specs of the XX rear mech, it seems to be 40 teeth which accounts for its intended use on a 24/42 chainset with 11-36t at the back (14t+25t=39t total wrap). The setup I'm using it on is with a road crankset of 34/50 with 11-36 cassette, giving 16t+25t=41t total wrap, so just too much.

Since fitting the new XX mech I haven't reduced the chain length yet to be completely sure but it definitely looks like SRAM's specs on rear mech capacity are less conservative than Shimano's notorious underestimates, and I could really use a little extra wrap capacity if I'm going to avoid having to swap for a smaller cassette.

The question is, if I add a larger lower jockey wheel, will the chain wrap capacity increase? I imagine the top one could easily impact on the shifting so I'm not desperate to mess around with that one, but am I right in thinking a larger lower wheel will add to the length of the derailleur and hence capacity? So the current 11t jockey wheel is about 44mm, and a 14t one for example is about 58mm. So if I'm right, that should add to the effective cage length by 7mm, more than giving me back what I lost in the cage length between the two mechs? The cage itself of the new mech seems to have at least 8 or 9mm of space to accommodate the extra diameter of a larger lower jockey wheel:



Can anyone tell me if my thinking is right, or anything I haven't considered? Thanks!

 

[Frankenorange]

If the cage can take up the slack in small 2 small and not be stretched out way too close to horizontal in big to big then you are good. The max capacity of both these mechs can be pushed well out of spec so just eyeball the mech old school.

 

[Fatal Swan]

If the cage can take up the slack in small 2 small and not be stretched out way too close to horizontal in big to big then you are good. The max capacity of both these mechs can be pushed well out of spec so just eyeball the mech old school.

Cheers - in big to big the mech is not maxed out quite as far as it could be, though there really isn't a lot to play with by shortening the chain for big to big, maybe I could drop two links but that's probably all without stretching it right out. On the other hand small to small is definitely too slack at present, and I'd need to lose at least 4 links there (ideally maybe 6). So it does feels like I've reached the viable wrap capacity of the mech already, which is why I want to try and steal a little more!

 

[bikeworkshop]

The chain is barely wrapping round the bottom jockey in big-big, so a larger wheel will not make any difference this end.
Small to small, you have added 3 teeth so potentially could hold another link and a half(the chain only wraps round half the jockey in this position..
So there's a slight difference in your favour.
Trouble is, the derailleur isn't designed that way, so there are probably negative effects to be discovered.

The right derailleur always works best.