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I have an 8 speed set up currently but it's time to replace the crankset. I really like the weight , price, and availability of the fsa carbon crankset ISIS but don't see many 8 speed versions. Can I use a 9speed crankset with my current set up? Could I just use a 9speed chain? Or, do I really have to find an 8 speed crankset to go with my 8 speed shifters, cassette and chain. BTW, converting to 9speed is not an option.
 

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Yes, you can use a 9 speed crank with an 8 speed drivetrain. A lot of people do it and it works fine. Some people report that if they are in the middle chainring and try to use the smaller cogs on the cassette, the wider 8 speed chain will rub against the large chainring. This will depend on the chainline of the crank, it would happen more with non-external BB cranks with chainline of 47.5 mm compared to new external BB cranks with 49-50 mm chainlines. If rubbing does occur, it is easily resolved by using a 9 speed chain. 9 speed chains will shift fine on 8 speed cassettes so no worries there. If you do use a 9 speed chain, your front derailleur may shift better if it was replaced with a 9 speed FD too as it will now be shifting a 9 speed chain on a 9 speed crank. I am sure you would be able to get the 8 speed FD to work good enough though.
 

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yes you can do it, but to do it RIGHT you need to space the chainrings to mimicate the distances of the 8 speed shifter..

if i remenber correctlly 8 speed sistem is space 3,2MM apart (on center) and 9 speeds are space 2.83mm, i will recomend you to get some small shims to compensate for that diference and then space the big chainring outwards and also the granny ring, leaving the middle ring in the original possition..

i hope this makes sense, i warranty you perfect shifting and aligment.
 

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I have a Catamount with a 9 speed crank and 8 speed "everything else" and it shifts fine and this set up has been together through 3 dif type frames.
 
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