Road Shifters with MTB rear derailleurs?
 

Would like to put a 11-34t cassette on a 105 triple equipped road bike (for touring). I believe I would need a 10 speed rear mountain derailleur such as the SLX long cage to accommodate the extra gearing. My question is would the 105 road shifters correctly actuate the SLX rear Derailleur?

Yup. Road shifters with a mountain bike rear derailleur is pretty much a standard factory spec for tandems.

Rear works fine as the spacing and actuation ratios are the same between road and mountain... the front shifting is problematic and mtb and road components do not play well together.

You need a Shimano 9 speed mountain RD. The new 10 speed RD's use different cable pulls.

It's not clear to me that the OP doesn't already have 10 spd...shifter specifically. Changing to a lower geared 10 spd cassette to use with a 10 spd shifter would thus require a 10 RD, no? Is there something I'm missing? Are these parts not available in 10 spd? Is 105 level not available in 10 spd?

No, 10sp mountain derailleurs are not compatible with 10sp road shifters (for shimano... the SRAM 10sp road/mtn groups are compatible). Shimano changed the cable pull ratio on their 10sp mountain derailleurs, so they will not work. A 9sp rear derailleur will work, because it uses the same pull ratio as the road groups (the 9sp really just refers to the width of the chain it can handle, not anything to do with the number of gears it can reach). I guess in theory you should change the jockey wheels to a 10sp one, but I doubt it really matters that much, as I have seen others do it with no issue.

Pull ratio is the amount the derailleur moves for each mm of cable pulled... if the derailleur wants to move too much or too little for each shift (the shifter will pull a specific amount of cable each shift), the gears won't line up and it won't shift properly.

EDIT: Make sure you get a long cage model. Assuming a 50/39/30 triple, you JUST meet the maximum chain wrap of that derailleur by 2 teeth (and are WAY over the medium cage capacity). If you have a wider range (53/xx/28, for example), there could be issues.

10 speed road shifters are compatible with 9 speed MTB RDs, for 10 speed shifting. The cable pull in 10 speed MTB RDs is different from everything else, and will only work with 10 speed MTB shifters.

igknighted and slim, thanks for the explanation. i take it this means that 9 spd mtn and 10 spd road cassette bodies are the same width.

Yes. 8, 9, and 10-speed cassette bodies are all-ee same-ee.

Nope, it's all marketing.