Using a mountain bike rear derailleur with road bike shifters

Question

I am in the market for a new rear derailleur and was interested to find out if it is possible to use a mtb derailleur with road bike shifters and cassette and chain? Would it work without fault?
Does it depend on what mtb rear derailleur I have a Shimano 105 10 speed rear derailleur and Microshift front derailleur and a 10 speed Tiagra chain.
And if so I wanted to know the advantages of using a mtb rear derailleur, are they stronger, biggest cassette range capability?

Answer 1

All Shimano rear derailleurs of the 6, 7, 8, 9 speed classes, plus road 10 speed (except Tiagra 4700) use the same "actuation ratio." That's a colorful phrase to say that rear derailleurs from these speed classes respond identically to shifter input. For example, many touring bikes with drop bars and, say, 10 speed STI's (road shifters) will use a 9 speed mountain bike rear derailleur, like the Deore RD-M591 and enjoy perfect shifting. What is important is that the shifter speed matches the cassette's speed. Not only to have the correct number of detents (clicks) to match the number of cogs, but also to allow for correct amount of movement of the derailleur between cassette cogs. Different speeds of cassette have differing spacing between the cogs and the shifter's cable pull has to pull the correct amount of cable to make the derailleur move the intercog space.

Advantages of using a MTB derailleur include their ability to handle wide range gearing cassettes, up to 11-36 tooth range for your current situation (9 speed MTB rear derailleurs typically have a 34 tooth max low cog spec, however the specs are typically very conservative and exceeding them a little bit is common and function is normal). Mountain derailleurs started the "Shadow" design trend which tucks the derailleur more inside and behind the rear dropouts which affords more protection from potential damage. Road and gravel rear derailleur's have taken up this trend, but not until 11 speed, I believe. Thus I list the MTB 9 speed Shadow design as an advantage. Shimano RD-M592 is a 9 speed Shadow rear derailleur. A third advantage of MTB derailleurs, related to the first, extended range advantage, is they have options for longer cages (Shimano labels them SGS). The longer cages are able to take up more chain slack created from the larger tooth differences in a triple front and/or wide range cassette use.

I don't believe durability differences exist between MTB and road derailleurs when similar classes of these are compared head to head. Neither will there be a significant weight advantage between equivalent class road and mountain derailleur's that are compared having the same cage length (short & medium, aka GS) are the 2 lengths they would have in common. One area where a component marketed as MTB may have an advantage is the wheels' hubs, where mountain hubs may be better sealed against water and dirt ingress compared to road hubs. This was certainly true early on and would include the 9 speed era. Overall a quality comparison between like road and MTB components of the same hierarchy level would find insignificant variability in the quality of materials and performance of them.

Answer 2

I use a SLX M662 Shadow 9 speed rear derailleur with my Shimano 105 5700 10 speed shifters. The cable pull ratio is similar enough for it to work pretty well. 10 speed MTB derailleurs have a different pull ratio.

The advantage is that it allows me to use a 11–34 tooth cassette. The 105 rear derailleur is limited to 11–30 11–28.

If you don’t need the capability to use bigger cassettes I see no reason to get an MTB rear derailleur.

Answer 3

What you're describing is called a "mullet drivetrain." There are some specific combinations of parts that do work, but in general, you can't mix-and-match parts and get a workable result. The reason is that different shifters pull a different amount of cable per shift, and different derailleurs expect a different amount of cable pull per shift. And different cassettes have different spacing between gears (More on this).

Here's an article on some specific setups that do work.. There is a product called a Shiftmate that can facilitate this kind of mixing and matching by "translating" the shifter's cable pull.

You don't mention what kind of shifters you have, which is important. It sounds like probably 10-speed 105.

Mountain bike derailleurs generally cover a greater range, but they're not necessarily tougher.