I have a 105 compact crank double with Ultegra deraileurs and Ultegra flightdeck brifters. I want to switch to a triple crank for loaded touring. What do I have to do and replace. I am thinking of the 6603 Ultegra cranks. I currently have a SRAM OG-1070 11-28t cassette. Do I need a long cage rear deraileur? I saw that the Ultegra recommends not to use more than 27t cassette. Any help would be great. I put this in Bike mechs too, but I thought I may get a lot of help here too.

Well, you could just swap out your chainrings on the double to get you into those lower gears. You might not need the tall gears for loading touring. GEt two smaller chainrings, you would be surprised at the spread you can have on them in a compact double set up. Make sure you have your lowest needed gear, I bet you will not feel cheated with your highest gear when touring loaded. Touring is, after all, about enjoying the trip? This would be less expensive and more easily reversed. One ring to climb, one to cruise, what more would you need?

You didn't say which series components you are using so I can't say for sure. You may need a new crank, front shifter, a new long cage derailleur, and a new chain, as well as a new bottom bracket plus cables, housing and tape. Some shifters are double/triple compatible, some are not. Generally your bottom bracket if internal will not have sufficient spindle length for the triple. You said you were thinking of using a 6603 crank (external brackets) but you didn't say what you presently have. 6600 series front derailleurs are for doubles, 6603 for triples, but it has been my experience that the front derailleur will work both ways (usually enough pivot distance that it will work, not as well as a triple designed front derailleur but it works). As for rear derailleur, the long cage will probably be needed to take up the slack in the chain when on the front small chain ring. You want the GS series rear (if sticking with ultegra) not the SS. Chain will definitely need to be longer.

Having said all that if it were me I would go with a compact crank and leave it at that. What in the world do you need a 28 rear and a small chain ring for********** With that gearing you could climb a wall.

You'll probably get a lot more authoritative answers in mech.

If you have a compact crank already, you may not have to change much. My guess is that your current rear derailleur is already a medium cage and not a short.

Check Shimano's site on your current shifters to see if they're triple compatible. Many are.

You will need a new bottom bracket.

People who go on long loaded tours in hilly areas still prefer triples and mountain gearing. You might be able to climb a wall with a 28 in the back but if it's your 3rd wall of a long day on the 6th day of the tour and you're carrying 50 lbs of gear, lower gearing can mean the difference between riding and pushing.

I have 105 5600 series compact crank 50-39T. The deraileurs are Ultegra 6600 front and rear. I have 6600 Ultegra flightdeck brifters. What does Maximum sprocket 27t mean when refering to rear deraileurs?

Largest cog count. There's also wrap capacity to deal with. Triple with big cogs in the rear is going to work best with a long cage RD. As a commuter that also loaded tours, I say pooh on the idea of the double compact. You want gear choices, and you may want to look at MTB options. Standard option on a Trek 520 for instance is to swap the front crank to MTB.

If I use the XT rear deraileur, will it work with my 10 speed set up?

Shimano rear derailers are all the same pull ratio from road to MTB. The front derailers differ.

I think the BCd is 135mm on your crank so 39 tooth is the smallest you can go on it. You can put a Sram MTB cassette and a long cage derailer so you can go up to 39 in or so in the back but if you have a 10 speed now, the MTB cassettes only go up to 9 speed. That will make you have to chainge the chain. To do the tripple you need ALL new stuf!

I have the complete XT series hydraulic disc and all on my Kona Blast Ballon tire grocery hauler and child puller utility bike. I just don't want to tear it all apart. Besides I like zooming down hills in the big mean gears. I think I am going to have to get a total new drive train.

The only cranks that are 135 BCD are Campy and track Miche.

Compact always almost means 110 BCD (Shimano, FSA, Campy, etc)

My ultegra, dura-ace and 105 cranks are all 130 BCD and not 135 but you still cant go smaller than a 39 tooth chain ring. One tooth smaller in the back is like 3 and 1/2 larger in the front so you can get the most bang for your buck in the back cassette. So you can use a 9 speed cassette that is 11 to 34 tooth to help zip yourself up the hills.