Harold25 said:
My shifters are Shimano Deore RapidFire. So I just need a Shimano deore cassette. Any thoughts on using a 12/25 9 speed cassette? there is one at the local bike shop. They don't have an 12/36 cassettes. Would it be easier to just swap out the smallest 2 cogs?
Any Shimano cassette will work, as long as the sprocket count matches the shifter. Also any reasonably priced SRAM cassette.
It will cost you about the same to get a 7-speed cassette and 7-speed right side shifter as it will to get just a 9-speed cassette, and 7-speed equipment will last longer and shift better. The sprockets are thicker, the chain bushings are wider, and wider sprocket spacing does not demand as much precision in its adjustment. 6-speed would be even better, but 6-speed trigger shifters are not available as far as I know (though 6-speed twist grip shifters are).
At my shop, 7-speed cassettes start at $15, 7-speed chains at $12, and 7-speed trigger shifters at $20. 9-speed cassettes start at $36 and 9-speed chains at $23. When you factor in the faster wear of 9-speed parts, you could easily be spending 3 to 4 times more in parts to keep a 9-speed drivetrain in business. And you will require more frequent shifting adjustment, whether you do it yourself or pay a professional for the service.
Either Shimano's CS738 cassette (13-34t) or CS739 (14-32t) would be a fine choice for the job. They have proportional gear intervals and high quality materials.
If you want to use a 7-speed cassette on an 8/9-speed wheel, you have to put a 4mm wide spacer behind the cassette to take up the extra space. Bike shops have them.