Can I use a Shimano Alivio 7 speed shifter with Shimano Deore 9 speed drivetrain? Basically the bike originally came with everything being set up for 7 speed. Somebody switched out my tire/rim and left a 9 speed tire/rim as a replacement. I'm hoping to find the cheapest way to correct this? Also my chain was originally for a 7 speed, does it matter now that there is a 9 speed drivetrain? Do I need to replace it? Thank you to anyone that can offer some info on this and point me in the right direction.
The cheapest way would probably be to remove the 9 speed cassette from the wheel and replace it with a 7 speed cassette (of the same ratios you previously had) and the appropriate spacer (4.5mm thick, the spacer goes on the freehub first, followed by the 7 speed cassette). This is likely the cheapest approach.
A Bike Exchange question from years back can offer insight:
Reason: to convert to 9 speed, you would need a different front shifter if it is index shifting. You would also need a new chain because a 9-speed chain is narrower than a 7-speed chain. You may even need to change out your rear derailleur if it does not have the range to handle the 9-speed cassette you have.
You also did not mention the state of the 9-speed cassette you have. If it is worn, then you would be replacing that as well to go 9-speed.
Note that your rear wheel can natively accommodate 8, 9, and 10 speed cassettes, and some 11 speed cassettes if they are large enough (a 34 tooth [or larger] largest cog in the Shimano realm) without the need for spacers (the exception being 10-speed road cassettes, which use a 1.0mm spacer to fit an 8/9/10 freehub but would still fit).