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I upgraded following parts to Shimano 5800 (front and rear derailleurs, crankset, chain, STI shifters). The last thing left to replace is 8 speed Shimano CS-HG51 cassette. Unfortunately I cannot index the new 105 5800 shifter with this cassette. I guess the space between the rear cogs are not the same as in 10/11 cassettes. Could you please confirm that? Also do I need specific 11 cassette for my Shimano 105 5800 group or maybe 10 would be enough? Changing to 11 would force me to change the hub as well, as it is only compatible for 10 cassettes.

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    Going from 8 to 11 is going to be different spacing. – paparazzo Feb 8 '15 at 14:08
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    You'll need an 11-speed cassette. The cogs are thinner and the spacing between two is narrower. But putting an 11-speed on the rotor might not work if your hubs are older than 2-3 years. The specific 8-speed rotors don't take 10 or 11-speed cassettes. You'll have to change the rear hub or the rear wheel. – Carel Feb 8 '15 at 14:11
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    If you run n speed index shifters, you need the n speed cassette in the back. Unfortunately, if your wheel cant take a 11 speed cassette, you're going to need to rebuild the wheel with a new hub or more easily/economically, buy a new wheel. – Batman Feb 8 '15 at 14:39
  • Thanks @Carel for confirming that. One thing to clarify if my 5800 shifter will fit (index) with 10 speed 5700 cassette (this is the top cassette compatible with the hub I have) – Milosz Frejnik Feb 8 '15 at 14:41
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Yes, 8/9/10/11 cassettes have different cog spacing.

You have several options:

  1. Replace freehub body to Shimano/SRAM 11 speed compatible, if hub manufacturer provides such an option. It's 1.85mm longer than 8/9/10.
  2. Replace freehub body with Campagnolo one. The trick is that all 11 speed cassettes are cross compatible, but Campy freehub body has same width for 10/11 speeds.
  3. Use 10 speed cassette. This blog entry states that works good enough after some derailleur and cable tension adjusment.
  4. Use 11 speed cassette without one cog and using additional spacer.
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Okay, super old thread here, but the cable pull of 11-speed shifters (2.7 mm) is relatively close the the pull of 8-speed shifters (2.8 mm), so you might get away with combining 11-speed shifters with a 7-8-9-10-speed (1.7 ratio) derailleur and an 8-speed cassette. The cable pull of the ST-4700 shifters (which are sort of a hybrid designed for a combination of 11-speed derailleurs and 10-speed cassettes), is even closer to 2.8 mm by my calculations so should work flawlessly with a combination of 7-8-9-10-speed derailleurs and 8-speed cassettes.

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