no such thing...
The only thng that matters is the total wrap capacity. You have to add the front difference to the rear difference to get the total. If the total does not exceed the stated wrap capacity, then you'll have no problem. I run a setup that exceeds the 36T wrap capacity of a campy medium cage RD (36T), with my 53/39/28 and 12-25 (a 38T wrap). This never causes a problem, because no one would ever use the 28/13 or 28/12, where the chain will hang loose.
I'll have to diagree with orange_ julius. Shifting from a large cog in the 50T down to the 34T on a compact should never be a problem, because there is plenty of chain tension at the start of this shift. The rear deraileur tension does not "yank the chain clean off the chainrings". That's pure conjecture. It may indeed be wise to shift 2-3 cogs smaller before making the chainring shift, but what this does is improve the chainline, preventing the dreaded chain drop. It has nothing to do with excessive chain tension. The same thing can be seen on a triple, shifting from the 39T to the 30T. It's not a large jump and the tension is not high in the 39T and even the largest cog, but if the chain tends to drop off to the left, despite proper adjustment of the FD, performing the cog shift first can cure the problem. The real culprit in this problem is probably the shifting ramps being a bit too agressive. Short chainstsy also seem to make this problem more pronounced. I never has this problem on a triple until I installed one ona Cervelo R3 with ultra short chainstays. I could not get the chain to quit dropping unless I was in the third from largest cog (or smaller) when the shift to the little ring was made.
