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new rear derailleur

Old 10-23-13, 02:20 PM
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new rear derailleur

Any one here have experience running a 9 speed rear derailleur with 8 speed Shimano STI shifters and an 8 speed cassette? I suppose I'm just delaying the inevitable conversion/upgrade but all the other components are working great.

Scott
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Old 10-23-13, 02:49 PM
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I have a solo bike with that configuration (9 speed MTB rear derailleur) for years now. Works fine ! Don't change.
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Old 10-23-13, 02:54 PM
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forgot to mention: And I use 8 speed chain ! On the 9 speed derailleur and NO problems. Also possible is using a narrower 9 speed chain (0. 6-0.9 mm narrower) which last about as long as an 8 speed chain in my experience.
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Old 10-23-13, 03:01 PM
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You didn't mention whether the derailleur is a Shimano or not, but if it is then it should work just fine.
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Old 10-24-13, 04:09 AM
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No indexing happens on the derailleur side, it doesn't care whether it is 8 or 9 speeds. Furthermore there is no load on the teeth like on the crank or cassette; you will not be wearing nor stretching your chain. Shimano uses a 2:1 pull ratio, a unit of distance on the shifter pulls twice the distance on the derailleur. If you attempted to use a SRAM derailleur that is set for a 1:1 ratio, that would be another story.
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Old 10-24-13, 08:54 AM
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The existing derailleur is a Shimano xtc, lots and lots of miles on it. Ordered a Shimano Deore XT RD-M772-SGS Shadow
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Old 10-24-13, 01:20 PM
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Almost all 7-8-9-10 speed shimano derailers use the same pull ratio, and so are interchangeable. HOWEVER -- the Shadow series is different, and works only with 10-speed Shadow shifters. Sorry! See here:

https://sheldonbrown.com/k8.shtml

Though shifters are different for 8, 9 and 10-speed cassettes, the same rear derailers index properly with 7- through 10-speed Shimano cassettes. Most Shimano-compatible freewheel bodies accept 8-speed cassettes.. Note the following exceptions:
  • Aluminum 10-speed freehub bodies with taller splines work only with 10-speed sprockets
  • The wider 8-speed cassette does not fit on the narrower 7-speed body.. You could install 8 sprockets of a 9-speed cassette, though.
  • An 11-tooth sprockets requires a "Hyperdrive C" freehub and lockring
  • Shimano "Nexave" cassettes have 8 sprockets, in the narrower 9-speed spacing. A spoke-protector disk takes the place of the innermost sprocket. Nexave cassettes are listed with the 9-speed cassettes.
  • The Capreo 9-speed body has four special sprockets, with 9, 10, 11 and 13 teeth, so there's no way to use it with 8-speed sprocket spacing.
  • Pre-1997 Dura-Ace derailers work only with Dura-Ace shifters of that era; "Shadow" series MTB derailers work only with 10-speed "Shadow" shifters.
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Old 10-24-13, 01:37 PM
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This is timely, because I'm about to do the exact same thing. Thank you.
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Old 10-25-13, 01:27 AM
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WheelsNT appears to be getting Shadow and Dyna-Sys confused. DynaSys is Shimano's 10-speed mountain bike equipment, which is not compatible with other systems. The "Deore XT RD-M772-SGS Shadow" that the OP has purchased is listed as 9-speed, and not as DynaSys, so should work fine with his 8-speed setup.

The Shadow designation refers to the fact that it is a lower-profile derailleur than other models, meaning that it doesn't hang as low down as other models with the same cage length and does not protrude as far out from the frame. These are both features that MTBers riding in technical terrain desire, but the need on a road tandem is minimal, and I've heard that the non-Shadow models actually shift a little better due to the more traditional pully placement and stiffer mounting design.

There are Shadow versions of some 9-speed rear derailleurs, and maybe all of the 10-speed DynaSys rear derailleurs are Shadow (but I'm not sure about that part). I know for sure that Shadow and DynaSys do not mean the same thing, as WheelsNT appears to think.
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Old 10-25-13, 05:50 AM
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The bullet points I posted are a quote from the Sheldon Brown site which I linked; normally Sheldon Brown is authoritative.... I stand corrected.
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Old 10-25-13, 11:06 AM
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Well I'll let you know because it's on the way. Scott
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Old 10-27-13, 10:35 AM
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Originally Posted by Chris_W
WheelsNT appears to be getting Shadow and Dyna-Sys confused. DynaSys is Shimano's 10-speed mountain bike equipment, which is not compatible with other systems. The "Deore XT RD-M772-SGS Shadow" that the OP has purchased is listed as 9-speed, and not as DynaSys, so should work fine with his 8-speed setup.

The Shadow designation refers to the fact that it is a lower-profile derailleur than other models, meaning that it doesn't hang as low down as other models with the same cage length and does not protrude as far out from the frame. These are both features that MTBers riding in technical terrain desire, but the need on a road tandem is minimal, and I've heard that the non-Shadow models actually shift a little better due to the more traditional pully placement and stiffer mounting design.

There are Shadow versions of some 9-speed rear derailleurs, and maybe all of the 10-speed DynaSys rear derailleurs are Shadow (but I'm not sure about that part). I know for sure that Shadow and DynaSys do not mean the same thing, as WheelsNT appears to think.
[edit]Never mind what I said when I first posted this. It was working fine...on the stand. Didn't work so great in the real world. The chain was skipping in the highest two gears on the outside and on the 3rd from the top. The amount of clearance is indeed the issue. The chain was hitting the derailer arm. I could see the scuff marks it made.

This would be a great derailer IF your bike has the clearance for the different way that it sets up. It's not for sure that it won't work with XYZ road bike. But it didn't work with my tandem. Maybe I will try the M591 below or another traditional non-shadow derailer.

I was going to say...I have an M592 Deore Shadow that I put on my tandem due to the rated 36T capacity and it works fine...

https://www.ebikestop.com/shimano_deo...ack-RD7591.php

It's ugly as all get-out but it seems to work well as a derailer.

The M591 looks much nicer to me, more like a normal derailer, but it didn't have a "rated" capacity of 36T that I wanted since I am running the HG61 12-36. https://www.ebikestop.com/shimano_deo...ver-RD7592.php

Last edited by brons2; 10-27-13 at 07:57 PM.
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Old 10-27-13, 10:44 AM
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Originally Posted by hortan
forgot to mention: And I use 8 speed chain ! On the 9 speed derailleur and NO problems. Also possible is using a narrower 9 speed chain (0. 6-0.9 mm narrower) which last about as long as an 8 speed chain in my experience.
I have used 10 speed chains on my 9 speed systems as well, I found they shifted better with the 10 speed chains. The tandem got a 9 speed chain though for it's 9 speed system.
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Old 10-30-13, 12:12 AM
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Just to update this, I replaced the Deore M592 Shadow type with a Deore M591 non-shadow type that I found at the local Performance Bike. Works great, no interference, even though it's not rated for the 12-36 cassette. I would definitely recommend the Deore M591 for a 9 speed drivetrain if you're looking to run a MTB cassette.
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Old 11-07-13, 05:14 PM
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Originally Posted by gsms
Any one here have experience running a 9 speed rear derailleur with 8 speed Shimano STI shifters and an 8 speed cassette? I suppose I'm just delaying the inevitable conversion/upgrade but all the other components are working great.
It's an acceptable mix according to Shimano (unless you're dealing with 8-speed Dura-Ace).
https://www.shimano.com/publish/conte...ty%20Chart.pdf

Last edited by Bezalel; 11-07-13 at 05:16 PM. Reason: Added Dura-Ace
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