I'd like to try an eight speed system on my 9 speed hard-tail mountain bike (Raleigh M80). Can I just replace the 9 speed cassette and chain (SRAM 950) with 8 speed (e.g., SRAM 850 cassette and 68 chain) and leave the 9 speed shift levers? (I've heard the 8 speed system was better in some ways.)
2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
What ways have you heard the 8 speed system was better? I've only heard that the 8 speed chain is beefier and thus less likely to break and the shifters are less complex and thus more robust. If you aren't breaking 9 speed chains, I wouldn't worry about it. You could always go test ride an 8 speed bike at the LBS.
If you really want to try 8 speed on your exact bike, you'd need the chain, cassette, and an 8 speed rear shift lever. You could probably get a 9 speed rear shift lever to shift an 8 speed cassette but any benefit of the 8 speed shifter is lost then.
Not to mention that chain rub can become an issue with 9-speed front derailleurs and the wider 8-speed chains. I find 9-speed to be superior in most respects anyway, except that the parts are more expensive.