Shimano 7 speed shifter with 9 speed drivetrain

Question

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.

Answer 1

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:

7-speed MTB cassette on 8/9-speed wheel hub, where does spacer go?

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).