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I would like to retrofit an 11-gear cassette onto my bike which currently has a 9-gear cassette. Both cassettes are Shimano, and both are relatively modern. I know I will need a new rear shifter, and possibly a new rear derailleur. Will the 11-gear cassette be wider than my current 9-gear cassette? Will this cause any issues? Do I need to buy a new wheel?

Apologies if this is a duplicate! I saw a similar question asking about the derailleur (Can I just fit a cassette with more gears with the same derailleur?) but couldn’t find anything about the wheel…

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Since current >7 speed cassettes have so fine spacing between the gears, about the only reason why you'd want to install a cassette with more gears is wanting to get more range.

A 9-speed freehub will take 11-speed "MTB" cassettes (that have a large big cog) but not 11-speed "road" cassettes (that have a small big cog). So 11-42 is a go, but 11-28 is probably a no-go. The reason is that 11-speed cassettes are wider. "MTB" cassettes with a large big cog have the big cog shaped to overhang the hub flange, but "road" cassettes have a small big cog that can't be shaped in such a manner, there's simply not enough space for that.

The main difficulty is replacing the shifters and rear derailleur. You also need to replace the chain, which might necessitate replacing the chainrings as 11-speed chain on 9-speed rings may be asking just a little too much.

If you have a drop bar bike, and have to use 11-speed "road" shifters, they use a different cable pull ratio incompatible with "MTB" equipment. So you may not find a compatible "MTB" rear derailleur, and "road" rear derailleurs don't support a 42-tooth big cog.

If you have a flat bar bike, things are easy. Replace the shifters, rear derailleur, cassette, chain and chainrings.

Another possibility is replacing the rear hub with a "road" 11-speed hub if you use drop bars. In such a case, you need to replace shifters, rear derailleur, cassette, chain and chainrings and additionally the rear hub which requires rebuilding the wheel.

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    Minor exception, the Hg700 and 800 11-34 ‘road’ cassettes will fit. Their last cog is cantilevered over the freehub body like the MTB cassettes.
    – Weiwen Ng
    Jun 18 at 21:43
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Adding to juhist’s fine answer, there is one exception to this and it depends on the wheel you are using. If your wheel is a Mavic, it can be possible to put a road 11-speed cassette on it. Mavic wheels from the early 2000’s had a wider freehub and to run a 9-speed cassette you would put the Mavic 1.75mm spacer on first to get it positioned correctly. One can remove the 1.75mm spacer and the Mavic freehub will take an 11-speed road cassette just fine (0.1mm out of spec, which is trivial). It was rather prophetic of Mavic to have done this, well before the wider 11-speed spec was announced.

It only applies to Mavic, but if that is the case, you are in luck.

And all the other derailleur and shifter details of juhist’s answer still applies.

(This actually was my experience: I had a 2004 Mavic Ksyrium wheelset that I successfully ran with 11-speed road cassettes for a couple years until I changed wheelsets)

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