9 gear derailleur on 8 speed bike
 

I have an 8 speed cassette in the back, bought a 9 speed rear derailleur - I plan on using a single crank with an 8 speed shifter. By doing this will the 9 spd derailleur work ok?

Thank you!

"Speeds" are in the shift lever clicks , not the rear derailleur
9 speed cassettes are about as wide as 8 speed , Just the spaces between them are thinner to add that 1 cog.

Shimano? No issue. "May" have an possible issue with the 8 speed chain and cage width but running a narrower 9 speed chain would solve that.

No problem with chain width on 9sp RD with 8 speed chain. I run 9 speed RD’s with 7 and 8 speeds with 8 speed chain.

I always hear how Shimano narrowed cages and haven’t found anything to support it. This thread got me to measure the cages of 8 and 9 speed RD’s. Both are 9mm +/-. Measured a 9 speed XT that was slightly wider than an 8 speed XTR. Now there maybe some that are narrower.

That said, an older 6/7 speed Deore measured 10.5mm +/-.

Maybe 10/11 speed, but not 8 to 9.

John

Many years ago, I upgraded one of my bikes from 7 speed to 9-speed. The derailleur was the only component that I didn't change.

I have an XTR derailleur of unknown origin hooked to an 8 speed rapid rise trigger shifter.

No issues

I used a 9 spd. rear derailleur on my 8 spd. commuter bike for 8 years. I ran a 11/34 8 spd. cassette (lots of hills), with a Shimano Deore rear derailleur. Never had any problems with it. I also has a 46/36/26 crankset.

Thanks guys for your quick replies. So, I will keep the 8 speed cog and put the 9 speed DR on with a 9 speed chain. IF I keep the 3 gear crankset, will that work out? What shifters should I buy? Just use the existing 8 speed shifter so it never wants to go to a ninth gear?

See above

Thank you! The DR is a Shimano Deore RD-M592-SGS. I imagine the cage length will be adequate? What chain length would be appropriate if I go single crank?

Unknown, you'll need to measure and cut.

The shifter isn’t Rapid Rise. The derailer is...or, more technically, the derailer is a low normal rather than a high normal. The “high normal” is the “normal” derailer.

True up to 10 speed mountain with both Shimano and SRAM. 10 speed road can still be used with 9 speed mountain in Shimano. I’ve no experience with SRAM road so I can’t say.

Just use the 9 speed derailer with your existing 8 speed. You won’t even have to change the chain. If you want to change gearing, you’ll need to match the number of cogs with the shifter. That’s a much more expensive proposition, however.