A little late responding but if you're still around...
Originally Posted by
Calle
I tried swapping in an 11-40 in the rear but the 105 RD did not like the 40T sprocket even though its a long cage. Im trying to figure out how others have done it with the 105.
I haven't checked to see if someone got the 5800RD to work on such a big cog but I highly doubt it. The long cage distinction does not directly mean it can handle larger cogs, what it can handle is more chain wrap. It's confusing because they are both rated in teeth.
Specs
RD-5800-GS-L
It appears it's designed for around a 32 tooth maximum sprocket (that's what you're dealing with), and 37 teeth total capacity (that is chainwrap, and completely overkill since you're eliminating front shifting). You can often squeeze a tooth or two larger cog on shimano derailleurs I've heard, but I highly doubt you'd be able to get even a 36 in with a stock 5800 (but someone correct me if I am wrong). The problem is the top pulley will crash into the gear since it doesn't have enough clearance to go below it. The length of the cage is the distance between the other pulley that sticks out further - we don't care about that one. The only way I see you can use your road derailleur and get a 40 tooth cog to work is go the route I did with an extended hanger that physically drops the entire derailleur down lower. I'm not sure if there are other companies that do this, but I used a Wolf Tooth Roadlink adapter.
And just to add in a quick update on mine... I've got several shorter rides on it now <30 miles, a 50 miler and 64 mile group ride. I love it! For the small hills around here the 36t is just fine, and similarly the 11t with 44 chainring is plenty except for if I wanted to push really hard going downhill, but I am fine coasting in that scenario, it's not a race bike. It's been extremely rare I even shift into the 11t, I don't go much faster than low to mid 20's (mph) that often.