So I recently put a bash guard on my bike. Now I am thinking to change my rear derailleur from a long to a med or short. Just for clearance and shortening my chain a little more. Any tips or advise on this subject??
Thanks
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First determine the minimum derailer capacity that you'll need given your current setup. (biggest chainring tooth count - smallest chainring) + (biggest cog - smallest cog)=minimum derailer capacity (this is specified by the manufacturer). Then make sure that the derailer is compatible with your largest cog (i.e. just because a derailer has the capacity to take up slack doesn't mean that its physical range of movement will 'clear' the largest cog on one's bike).
For chain length: wrap your chain around the biggest chain ring and largest cog. Don't thread it through the rear derailer; shift the FD to the largest chain ring and run the chain through the guide to be sure the guide doesn't interfere with the chain line. Pull the chain tight and note the closest position that the chain could be connected, add one inner and one outer link and cut the chain. Install everything and go ride!
A personal note is that shimano is conservative with their capacity specifications-at least with their road derailers. I've read other manufacturers are as well-YMMV. Also if you're careful not to cross chain you can go with an even lower capacity RD. I've pushed my shimano ultegra short cage derailer to its limits riding in the Colorado rockies!

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It is not enough to fight for the West; it is even more important to enjoy it. While you can. While it's still here. -Edward Abbey

Sweet thanks for the info!!