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9 gear derailleur on 8 speed bike

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9 gear derailleur on 8 speed bike

Old 07-02-20, 11:07 AM
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Chefs20
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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!
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Old 07-02-20, 11:42 AM
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"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.
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Old 07-02-20, 11:57 AM
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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.
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Old 07-02-20, 12:23 PM
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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
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Old 07-02-20, 02:16 PM
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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.
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Old 07-02-20, 02:48 PM
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I have an XTR derailleur of unknown origin hooked to an 8 speed rapid rise trigger shifter.

No issues
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Old 07-02-20, 03:38 PM
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Originally Posted by Pop N Wood View Post
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.

Last edited by Doug64; 07-02-20 at 05:51 PM.
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Old 07-03-20, 06:19 AM
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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?
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Old 07-03-20, 07:57 AM
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Originally Posted by Chefs20 View Post
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 Your existing 8 speed chain should work fine unless you felt it needed replacement anyway. IF I keep the 3 gear crankset, will that work out? If your new 9s RD has the correct cage length to handle the chain wrap needed for a triple crank.What shifters should I buy? Unless there is something wrong with your current shifters there is no need to replace them. Just use the existing 8 speed shifter so it never wants to go to a ninth gear? There is no 9th gear if you're using a 8 speed cog and shifters. The RD doesn't know what speeds it's being used on or care. The shifter controls # of speeds. I run a 9 s RD on my daughters 6 speed bike.

See above
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Old 07-03-20, 08:02 AM
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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?

Last edited by Chefs20; 07-03-20 at 08:12 AM.
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Old 07-03-20, 08:21 AM
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Originally Posted by Chefs20 View Post
What chain length would be appropriate if I go single crank?
Unknown, you'll need to measure and cut.
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Old 07-04-20, 10:01 AM
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Originally Posted by Pop N Wood View Post
I have an XTR derailleur of unknown origin hooked to an 8 speed rapid rise trigger shifter.

No issues
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.

Originally Posted by fietsbob View Post
"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.
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.

Originally Posted by Chefs20 View Post
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?
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.
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