Mistakenly ordered medium cage 6800 rear der.should i exchange or put on bike

[dddd]

Well, considering that the 9100 RD is now only available in one longer cage length that goes to 30t, the mid-cage 6800 mech shouldn't look too "fred" on most bikes.
But Shimano is conservative with their recommendations and 6800 is a bit different in ways than 9100. It may come down to the tiny details that I mentioned, and even the shifting speed (toward smaller cogs, presumably) has been mentioned in company literature to be a function of the weight of the cage together with the length of chain that it carries.
Just like similar claims that newer, lighter shift levers improve a bike's steering, really!

[hobbanero]

my only gripe with my medium cage setup is that it does not seem to hold the same chain tension as my short cage. On bumpy roads, I lose the chain off the front big chainring with the medium cage but not the short cage (same road, same bike, same FD setup). Could be just a weak spring in my particular derailleur, because I don't know why they would design it that way.

[Mark McM]

Originally Posted by hobbanero

my only gripe with my medium cage setup is that it does not seem to hold the same chain tension as my short cage. On bumpy roads, I lose the chain off the front big chainring with the medium cage but not the short cage (same road, same bike, same FD setup). Could be just a weak spring in my particular derailleur, because I don't know why they would design it that way.

The derailleur cage spring tension may be adjustable. From the Rear Derailleur Overhaul instructions on the Park Tool web site:

7. As cage is removed from body, make note of the spring hole in the derailleur cage and body. There are commonly two or more hole choices. New derailleurs are typically assembled with the spring in the hole that allows the most relaxed position of the spring. If more cage tension is desired, the spring end can be installed in the other hole.

[dddd]

It's been my experience that a shorter-cage derailer, as with a "GS" vs. "SGS" cage, does a bit better with controlling chain slap noise on an off-road bike.
But as far as having the chain come off (not while shifting) on a road bike, it sounds like something else must be wrong, such as a bent chainring/tooth, bent chain or poorly setup front derailer.

[DrSpoke]

This has happened to me a lot on my gravel bike. So much so that I try to remember to shift to the big ring when descending. I coast along and then when I try to pedal the chain is off. I switched to Di2 and the problem seemed to go away. But has come back again on the last couple of rides. I'm hoping Shimano makes a gravel RD - basically an Ultegra w/a clutch. But am not holding my breath.

Originally Posted by hobbanero

my only gripe with my medium cage setup is that it does not seem to hold the same chain tension as my short cage. On bumpy roads, I lose the chain off the front big chainring with the medium cage but not the short cage (same road, same bike, same FD setup). Could be just a weak spring in my particular derailleur, because I don't know why they would design it that way.