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I'd like to replace a Shimano 105 5800 11-28 cassette on one of my bikes (CAADX 2016) with a CS-HG800-11 11-34 to get a wider range of gears.

My rear derailleur is Ultegra RD-6800-SS, which can't take a cassette this big. I am thinking of replacing it with RD-R8000-GS (current generation of Ultegra, medium cage, takes up to 34t).

Are there any potential issues to be aware of with this combo (6800 shifter, R8000 derailleur, CS-HG800-11 cassette)?

P.S. The bike has a 44/34T cyclocross crankset. Both the front shifter and the derailleurs are Ultegra 6800 (11-speed). I plan to keep all of that as-is.

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    Beyond needing a longer chain, I can’t think of anything. – Rider_X Jul 16 '18 at 20:04
  • If your bike has a standard 53-39 crankset, also look into replacing either the entire crankset or just the chainrings and get a 50-34 compact crankset. FWIW, I have 5700 (10-speed) shifters with a MTB rear derailleur and 11-34 casette on my commuter bike. It works but shifting is nowhere near as good as straight 105/Ultegra. (I have never noticed any bit of difference performance-wise between those two...) – Andrew Henle Jul 17 '18 at 14:19
  • @AndrewHenle: My bike has a 44/34T cyclocross crankset. Both the front shifter and the derailleurs are Ultegra 6800 (11-speed). I had no plans to change any of that but would love to hear if there are potential issues. (It seemed that changing the gearing at the back was easier than at the front, but am happy to be proved wrong on that.) – NPE Jul 17 '18 at 15:49
  • @NPE If you're already running a 44/34 crankset and still need lower gears, there's no point in changing that. I was asking because some people might not think about changing out the crankset or even just the chainrings. – Andrew Henle Jul 17 '18 at 16:50
  • @AndrewHenle: That makes sense, thanks for suggesting! – NPE Jul 17 '18 at 19:28
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Should be OK. Both shifter and derailleur are 11 speed, only 1 generation apart. AFAIK Shimano did not change the cable pull or derailleur movement ratio.

All the other derailleur specs check out:

As you say, the R8000-GS goes up to a 32 tooth max sprocket.

Your front ring tooth difference (10) is within R8000-GS max of 16

Your total capacity required (32-11 + 44-34 = 31) is well within max of 39.

  • Thanks for spelling out the specs that needed to be checked out. That's what I wasn't sure of. – NPE Jul 17 '18 at 19:29

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