Road Cycling - Derailleurs for Compact Drive

Want to build a bike with 34/50 compact drive crank and 12-27 cassette. You guys told me that a front derailleur for a double should work fine with this. Now I'm wondering about the rear derailleur. I would like to use a Dura Ace 7700 for a double if I can.
The specs are as follows;

Largest cog 27t
Total capacity 29
Maximum front difference 14

I need a total capacity of 31, but I'm not too concerned with that as I plan to shift it as a crossover so will never be in the large-large, or small-small combinations.

What I'm wondering about is the Front difference spec(which I am exceeding). Why does the rear derailleur care what the difference is in front as long as total capacity is not exceeded?

Anyone else using this gearing combo? What do you use for a rear der.?

Thanks.

Want to build a bike with 34/50 compact drive crank and 12-27 cassette. You guys told me that a front derailleur for a double should work fine with this. Now I'm wondering about the rear derailleur. I would like to use a Dura Ace 7700 for a double if I can.
The specs are as follows;

Largest cog 27t
Total capacity 29
Maximum front difference 14

I need a total capacity of 31, but I'm not too concerned with that as I plan to shift it as a crossover so will never be in the large-large, or small-small combinations.

What I'm wondering about is the Front difference spec(which I am exceeding). Why does the rear derailleur care what the difference is in front as long as total capacity is not exceeded?

Anyone else using this gearing combo? What do you use for a rear der.?

Thanks.

I'm using an FSA Energy compact 50/34. I started with 105 double front and 105 rear der and I had problems not dropping the chain going big to small on the front, I had to be very deliberate making that change, I also had chain rub using the 2 smallest rear cogs when on the small ring but that's not a good combo anyway so I didn't worry about it. My normal cassette is an 11/23 DA but I do use an SRAM 12/26 if there are a lot of hills.

I upgraded to DA 7700 series FD and 7800 series RD using the same cassettes I only get a touch of rub when on the small ring on the smallest cog and the front shifting problem is a thing of the past.

I'm using an SRAM PC69 chain.

I couldn't find a 9spd rear der any cheaper than the 10spd one so I snagged it and haven't looked back. It's a 9/10 speed component.

Want to build a bike with 34/50 compact drive crank and 12-27 cassette. You guys told me that a front derailleur for a double should work fine with this. Now I'm wondering about the rear derailleur. I would like to use a Dura Ace 7700 for a double if I can.
The specs are as follows;

Largest cog 27t
Total capacity 29
Maximum front difference 14

I need a total capacity of 31, but I'm not too concerned with that as I plan to shift it as a crossover so will never be in the large-large, or small-small combinations.

What I'm wondering about is the Front difference spec(which I am exceeding). Why does the rear derailleur care what the difference is in front as long as total capacity is not exceeded?

Anyone else using this gearing combo? What do you use for a rear der.?

Thanks.
A short cage will work as the wrap spec is conservative, and you may not even be using the next to smallest cog when in the small ring due to possible chain rub on the big ring. The standard double front also works. The rear derailer doesn't care what rhe fronti s doing. That 14t spec just relative to the tooth difference in front.

I just put an FSA carbon compact crankset (34/50) on my DA equipped Merlin. With either a 12/25 or 12/27 in the back I have no problems at all, save for a bit of chain rub when in the 34 on the front and the 12 or 13 in the back, as Shadco said also.

The front der needed to be lowered a bit, and the limit screws needed to be adjusted, but it was a very easy install and setup.