Bicycle Mechanics - 105 STI with Shimano SLX rear Derailleur

I am building a Soma Double Cross with 105 STI and a compact double up front. I was going with a 12-27 10 speed cassette to begin with, since it will be a commuter. If I ever want to tour I thought being able to put on a 12-34 might be nice. So before I buy the RD is there any down side to putting on a Shimano SLX instead of a 105 rear derailleur? This would allow me to have a commuter wheelset and a touring wheelset with different ratios.

You will need a 9-speed rear mountain derailleur to work with your STI shifters. Shimano 10-speed mountain type rear derailleurs have a new actuation ratio to work with their new 10-speed mountain shifter cable pull. Be sure to get a "high normal" type RD so that the shifters work the same way as with the road derailleur. A "low normal" RD would make the shifter work backwards which is OK but most road riders don't like the change.

Why not just use a GS model of the 5700 RD?

The Maximum Sprocket of the GS is only 28 like the SS. I need more Max Sprocket (34 or more)not more Total Capacity. The TC of a 12-34 with a compact crank (50-34)is only 28.

Thought you would say that, I am using a 5700 GS with a 12-32, which is more than enough gears, not sure I would need the 34 unless I was going vertical.

So your 5700 works on a 12-32? Shimano says Maximum is 28. I am building this for someone else. He had mention he might want to do some light touring. So I wanted to gear it for some vertical and added weight.

re-checked, am using a 5700 compact crank, and 5700 GS RD with a SRAM 11-32 10 speed cassette, all works fine together.

This setup needs a 39t capacity, with is the max the 5700 GS can take (the 5700 SS is 33t)

Have been using this for working out the max t capacity needed http://home.earthlink.net/~mike.sherman/shift.html

Sorry bad math.

There's no set-in-stone maximum cog compatibility with any rear derailleur. The exact position of the derailleur hanger relative to the rear axle will vary some, and that will affect the maximum cog size you can use without the upper derailleur pulley hitting the largest cog. Shimano's specs on this are known to be conservative, and again, are really only an educated guess.

I recently decided I wanted to use a SRAM Apex-like drivetrain on my "day touring" bike, but had mostly Shimano parts on hand and for the most part prefer Shimano over SRAM anyway. So I came up with what I call a "Shimapex" :D group. I used Shimano 105 shifters, 105 compact double crankset (50/34), 105 front derailleur, Shimano "trekking" LX rear derailleur ("9 speed" with an official max. cog compatibility of 32t), ten speed 12 x 32 SRAM cassette, and 10 speed SRAM chain. I'm loving this drivetrain, it gives me the kind of low gear I need with the simplicity and smoothness of double shifting up front. We've sold quite a few bikes equipped with SRAM Apex, and have retrofitted quite a few bikes with it as well, and for some applications it is just awesome IMO. I'm certainly enjoying my "Shimapex" setup.

http://i414.photobucket.com/albums/pp226/wellbiked/88002.jpg

So you use a "9 speed" rear derailleur with a ten speed cassette??

So you use a "9 speed" rear derailleur with a ten speed cassette??

Yes, as Al1943 mentioned earlier in the thread, if you use a Shimano mountain bike derailleur with your 10 speed drivetrain (including 10 speed Shimano road shifters), you can't use the new "10 speed" Shimano mountain bike derailleurs. Shimano uses a different cable pull ratio on the 10 speed mountain bike derailleurs/shifters, the new system is called "Dyna Sys." The cable pull ratio of all other index-compatible Shimano shifters/rear derailleurs is the same, road or mountain, with the one exception being 8 speed and earlier Dura Ace. And now Dyna Sys is different, too. So yeah, just use any Shimano mountain bike derailleur you like with your road shifters except a 10 speed Dyna Sys model.

Super, thanks for the help.