Derailers intended for MTB have longer cages to accommodate wider gearing 
and are designed to track the angle of a cogset with at least a 30 tooth 
big cog.  Road derailers are usually rated to 28 or fewer teeth but this 
can often be cheated by a few teeth.  The cage will likely be shorter since 
overall gearing is not as wide.  

The reality is there is enough variability in stuff that surprising 
combinations of components may work just fine.  Don't worry about locating 
an 8 speed specific derailer.  One marked "9 speed" will also work, and 
probably some 7 speeds as well.  

Your selection should take into account your maximum cog size.  If you're 
max cog is 30+, life will be easier with MTB stuff.  Certainly as you get 
up to 34 and 36 tooth cogs, a road RD could be problematic (although I've 
seen a short cage Ultegra shift an 11-34 9 speed).  If I were buying stuff, 
I'd go with the MTB parts for versatility.  If I had something in the parts 
bin, I'd try it first.  

dougP

On Tuesday, June 17, 2014 7:56:49 PM UTC-7, Michael wrote:
>
> MTB derailers or Road derailers.
>
> What's the diff, if anything?
>
> I know RBW seems to like MTB derailers for all their current build 
> kits (Deore).
>
> Any benefits to Road 8-speed derailers over MTB?
>
> I am under the assumption that any 8-speed Road parts are going to have to 
> be NOS or used, because looks like 8-speed derailers don't exist anymore in 
> current production offerings as of 2014?
>

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