>>1920346>What makes a mountain bike derailleur different from a road bike derailleur?Originally deore was a 'touring ensemble' meaning parts meant for touring bikes. Touring bikes were road-esque and had classically been equipped with road parts. So mtb derailers really are a subset of road derailers. They are practically the same thing. And mtb derailers still are widely specced on touring, hybrid, and other road going bikes. And road derailers now have morphed into shadow profiles, clutches, and long cages.
If you compare a couple of earlyish classics, pic related, both relatively ubiquitous and basically peak derailer, there are some differences.
The main one is the XT has a longer cage, so will shift wider gears, although XT actually did come in a short cage version as well, that struggles with even a 28t cog (kinda neat, i have one). And many road derailers have long cages. But that's generally the main difference.
The XT has more complicated rubber seals on the jockey wheels meaning it has more drag and should spin smoother for longer.
The tricolour mech has more more aero details particuraly on the cage plate, which would not be great on a mtb as they inhibit mud shedding.
The XT mech is about 50 grams heavier, or 1/5 heavier, so it's stronger.
For more modern examples, the shadow profile (which puts the derailer body more out of the way so it's less prone to impact damage), and clutches (which eliminate chain slap and chain drop over rough ground) come in far earlier on mtb derailers than road, although this technology does exist (somewhat gratuitously) later in road derailers.
Mtb shifters have barrell adjusters so high spec mtb derailers deleted their barrell adjuster. This is quite irritating for custom nonsense.
After the 9 speed era the pull ratios change between them too. Before then it's all the same. So compatibility is different. Although this now is looking to return to a (different) (stupid) standard shared between them.
