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I'm looking into replacing shifters on my MTB. Currently I have 8-speed Shimano Alivio shifers - will 9-speed Deore or SLX shifters work? I do not care if one of the shifter's position will be unreachable.

Later I may replace the cassette to 9 speed to, but I do not want to do this right now.

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I'm adding this is a comment because I'm not sure, hopefully someone else can confirm. But AFAIK 8/9 speed MTB shifters are designed for different width chains, so their indexing positions are different. In short you'll never be able to calibrate it. But 8sp derailleurs are still widely available, can't you just buy one of those? –  cmannett85 Feb 9 '12 at 7:25
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Unfortunately you cannot do that. The COGSET and the SHIFTER must have the same "distance", or more correctly, the same lateral distance (on the pulley) for each lenght of cable pulled (on the shifter).

When you go from 7 to 8 speed, the distance between cogs is the same (because the freehub shell is longer), so you could use a 7 shifter with an 8 cogset and one gear would be unreachable;

But when you go from 8 to 9 speed on the shifter, the expected distance between cogs change (because the freehub shell for 8, 9 and 10 speed is the same). Then you need a 9 speed cogset too, so that EACH shift will correspond to the CORRECT DISTANCE of lateral movement in the derailer.

If you move from 9 to 10, the same problem happens. That's a perverse kind of programmed obsolescence, in my opinion, since even cranksets end up being incompatible due to the chain being narrower and narrower... (look for the Sheldon Brown "Nanodrive" system).

So the bottomline is: if you want to replace shifters, you'll have to replace the cogset, and probably the chain and cranks. I would not advise you to do so only for the sake of shifter change. Consider doing it only as a last resouce.

(I for one never changed to the 9+ speeds framework. I have 5 bikes in current use, all of them with the "7/8 speed framework", and I can easily find spare parts like cogs. My 7 speed shifter (a very old Alivio) still works fine, and it seems to me it is very rare for the shifter to have a poor working only because it got old).

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Thanks for your answer. For completeness sake - an article here: ctc.org.uk/DesktopDefault.aspx?TabID=3946#Shiftratio - suggests that maximum error in an ideally adjusted 9/8sp combo would be about 0.6 mm which is less than the derailleur's "wobble" or margin of error, so in theory such a combination could work after all. But it definitely would require more accurate adjusting. –  Sergey Feb 9 '12 at 20:37
    
@Sergey Just some points: the article suggests you can use a 9 speed derailer with an 8 gear shifter/cogset or vice-versa, not mix shifters and cogsets of different types; The error you mentioned would exist for a correctly matched group, "within" each gear, not across gears; Since total width of 8 and 9 cogsets is the same, measured from first to last, mixing 9spd shifter and 8spd cog would be like having a shirt with 8 buttons and 9 button houses. Unfortunately, it won't work :o( –  heltonbiker Feb 9 '12 at 23:06
    
Sorry, I linked to the wrong chapter within the article. I was referring to the "Sprocket Pitch" chapter: "...some "incompatible" combinations actually work okay – at least when new. ... If, for example, the mech shifts 4.55mm instead of 4.35mm, the most it'll be out is only 0.6mm, and only in next-to-top or next-to-bottom. The rest of the reason is that the guide pulley can float about half a mm side to side, allowing at least that much misalignment between mech and sprocket without any nasty noises." - but they also say it would only work when the parts are new and ideally adjusted. –  Sergey Feb 10 '12 at 0:03
    
@Sergey again, I'm pretty sure they're talking about shifter vs. derailer vs. cogset with SLIGHTLY different pitches/ratios, but with the same number of gears. –  heltonbiker Feb 10 '12 at 1:08
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Ok, I re-read the article and now I see that the max error between Shimano 8sp and Shimano 9sp will be almost 1.5 mm ((4.80 - 4.35) * 3), so this setup is not going to work well even in theory. Thanks. –  Sergey Feb 10 '12 at 1:42
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