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Old 10-18-07, 09:23 PM   #1
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What's the real Rear Derailleur deal?

I know I read somewhere that the rear derailleur doesn't really care how many speeds you have...8-9-or 10..Is this true? I've searched on Sheldon Brown's site and others but am still somewhat puzzled.
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Old 10-18-07, 09:26 PM   #2
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Yes, it's true. The number of "speeds" are in the shifters, if they're indexed, and the number of cogs. The derailleur just moves the chain and keeps it tensioned.
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Old 10-18-07, 10:17 PM   #3
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so I guess when I put the new 10 speed cassette, chain, and brifters on (the 105 crank is already on) then the Deore r.derailleur should work just fine..
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Old 10-18-07, 10:44 PM   #4
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so I guess when I put the new 10 speed cassette, chain, and brifters on (the 105 crank is already on) then the Deore r.derailleur should work just fine..
If that's what you want to do, then yes.
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Old 10-18-07, 10:46 PM   #5
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Well, I would prefer to have a 105 long cage rear der. to go with my 105 triple front der. and my 105 cassette and my 105 crankset. The main thing is to get riding first and if I can score a rear der. at a good price I will.
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Old 10-19-07, 06:41 AM   #6
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Well, I would prefer to have a 105 long cage rear der. to go with my 105 triple front der. and my 105 cassette and my 105 crankset. The main thing is to get riding first and if I can score a rear der. at a good price I will.
i dont think a rear derailleur would cost you much at all on ebay... 10-30$ tops? for shimano 105
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Old 10-19-07, 06:54 AM   #7
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i dont think a rear derailleur would cost you much at all on ebay... 10-30$ tops? for shimano 105
Not if the OP wants it new. They're going for about $50-$60 shipped NOS, for the previous generation. Some of the used previous generation are going for about $40 shipped.

What is largest cog on the rear cassette? I've heard some people going to 32T with a long cage 105, but others have had problems. I'd say 30T is the conservative max on the 105.

EDIT: Hmm, very strange. The newest generation of 105 GS can be had for $35 shipped on ebay, at least in this BIN auction. Wonder why the previous generation is going for more? Personally, I do like the finish better on the previous...
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Old 10-19-07, 06:55 AM   #8
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so I guess when I put the new 10 speed cassette, chain, and brifters on (the 105 crank is already on) then the Deore r.derailleur should work just fine..
The derailleur still has to match the shifters. That is, the shifter pulls the cable a precise amount for each click, and the derailleur has to move exactly the right distance to go from one cog to another. So there is a lever-arm ratio on the derailleur that has to match what the shifter expects.

I think that in general you can't mix road and MTB components, because they're built to different specs, the same as for different brands. It's also the reason why even putting the cable under the pinch-bolt in the wrong way can affect shifting accuracy.
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Old 10-19-07, 07:18 AM   #9
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The derailleur still has to match the shifters. That is, the shifter pulls the cable a precise amount for each click, and the derailleur has to move exactly the right distance to go from one cog to another. So there is a lever-arm ratio on the derailleur that has to match what the shifter expects.

I think that in general you can't mix road and MTB components, because they're built to different specs, the same as for different brands. It's also the reason why even putting the cable under the pinch-bolt in the wrong way can affect shifting accuracy.
I wouldn't think that should matter. The RD will move what ever distance the shifter tells it to by how much cable is moved. Derailleur's are not indexed themselves, it is the shifter. What you need to make sure is that the cassette and the shifter match speeds. This is my take on it, maybe I'm wrong.
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Old 10-19-07, 07:40 AM   #10
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As long as you're not dealing with pre-nine speed Dura Ace, and as long as you're dealing with index-compatible Shimano or Shimano-compatible rear derailleurs , Shimano or Shimano-compatible shifters, and Shimano-compatible cassettes/freewheels,then all is good in regard to cable pull ratio. But yes, you do need to match the number of "speeds" between the shifters and the number of cogs. Other than the separate issues of largest cog compatibility and the chain wrap capacity of the derailleur, that's it. For front derailleurs/shifters, the cable pull ratios are different between "road" and "mountain."

Last edited by well biked; 10-19-07 at 07:46 AM.
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Old 10-19-07, 08:03 AM   #11
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I wouldn't think that should matter. The RD will move what ever distance the shifter tells it to by how much cable is moved. Derailleur's are not indexed themselves, it is the shifter. What you need to make sure is that the cassette and the shifter match speeds. This is my take on it, maybe I'm wrong.
The rear derailleur's geometry has to match the shifters and cassette. If it does, the number of "speeds" is strictly determined by the shifters. If the geometry differs, there will be incompatibility.

For example, Campy rear derailleurs won't work properly with Shimano shifters as the geometry differs. Some SRAM rear derailleurs are Shimano compatible and some are not. Jtek has made a nice living making these otherwise incompatible components work together.
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Old 10-19-07, 08:16 AM   #12
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right now I have an 8 speed cassette and 8 speed shifters with the deore rear derailleur. My new cassette is a ten speed 12-25 105 and it will be set up with 105 triple crankset, 105 ten speed chain, 105 triple front derailleur, and 105 ten speed shifters. With all that said, I'm hearing everyone say that the current rear derailleur can handle this job at least until I get a decent buy on a 105 rear derailleur.
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Old 10-19-07, 08:27 AM   #13
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That is correct. You can use your Deore RD without issue.
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Old 10-19-07, 08:28 AM   #14
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right now I have an 8 speed cassette and 8 speed shifters with the deore rear derailleur. My new cassette is a ten speed 12-25 105 and it will be set up with 105 triple crankset, 105 ten speed chain, 105 triple front derailleur, and 105 ten speed shifters. With all that said, I'm hearing everyone say that the current rear derailleur can handle this job at least until I get a decent buy on a 105 rear derailleur.
Yes the current rear derailer will handle the job. There's really no need to change it except for cosmetic reasons. It's as good as the 105.
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Old 10-19-07, 09:15 AM   #15
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hey thanks, that makes it almost ready for the road once I get the travel agents in.
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Old 10-19-07, 09:27 AM   #16
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The rear derailleur's geometry has to match the shifters and cassette....
What do you mean by geometry? I have never seen a campy derailleur up close, so I don't know how they differ from shimano. I always thought the compatibility was a cassette issue, like spacing, and not a geometry one. I'm not doubting you, just trying to understand. thanks.
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Old 10-19-07, 10:09 AM   #17
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I wouldn't think that should matter. The RD will move what ever distance the shifter tells it to by how much cable is moved. Derailleur's are not indexed themselves, it is the shifter. What you need to make sure is that the cassette and the shifter match speeds. This is my take on it, maybe I'm wrong.
You're assuming that 1mm of cable pull will move any derailleur an equal amount. That's not how it is. Sram rear derailleurs, for example are designed so that 1mm of cable pull will move the derailleur 1mm. Shimano rear derailleurs are designed so that 1mm of cable pull will move the rear derailleur 2mm. That's why the derailleur and the shifter have to match.
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Old 10-19-07, 10:41 AM   #18
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What do you mean by geometry? I have never seen a campy derailleur up close, so I don't know how they differ from shimano. I always thought the compatibility was a cassette issue, like spacing, and not a geometry one. I'm not doubting you, just trying to understand. thanks.
See Retro Grouch's explanation, it's a very good description. "Geometry" means the distance to the cable anchor bolt, the parallelogram's dimensions, etc. Derailleurs can look very similar but detail design differences have a big effect.

When posters write "speeds are in the shifters, not in the derailleur", they mean if the shifters/derailleurs are from the same manufacturer and/or said to be compatible with other maker's components. Not that everything from anybody works together.
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Old 10-19-07, 10:56 AM   #19
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With regard to rear derailleurs: Shimano rear derailleurs are interchangeable across the board except for one generation of Dura-Ace. They will operate with any Shimano shifter and with all SRAM 1:2 shifters.

Shimano front derailleurs are separated into two groups, the road group and the MTB group. They have different cable pull ratio's and are not interchangable without a matching shifter change. The Shimano flat bar road group used the MTB ratio but has a special derailleur designed to fit a road bike but work with an MTB shifter.

SRAM 1:1 shifters and rear derailleurs most operate with each other. (these groups are MTB and are characterized by the X.0, X.9, X.7 etc name)

SRAM road groups RED, Force and Rival have matching shifters and rear derailleurs. They must be used together because of a special cable pull ratio even though the end result is to move the same distance as Shimano road groups. (The cassettes are interchangeable between SRAM and Shimano)

Campy equipment has a different spacing between both cassette cogs and chainwheels and is brand specific. (There is a company that makes an inline adaptor to modify the cable pull)

Anyone with more info than this feel free to add on as getting this right is one of the big keys to mix and match when converting bikes from one to another.
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Old 10-19-07, 12:02 PM   #20
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ok, this is all becoming clearer...so I assume the long cage rear derailleur is designed for use on a triple and the short on a double?
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Old 10-19-07, 12:11 PM   #21
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ok, this is all becoming clearer...so I assume the long cage rear derailleur is designed for use on a triple and the short on a double?
Yes.
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Old 10-19-07, 12:46 PM   #22
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Wonder why the previous generation is going for more?
Because Shimano has starved the channel of 9-speed parts. Scarcity drives up price.

The channel is flooded with 10-speed parts. Bounty drives down price.
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Old 10-19-07, 02:55 PM   #23
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in my case, my first road bike was purchased when everything was becoming 10 speed. I never even rode a 9 speed road bike until the other day when I took a spin on someone's Campy equipped Allez...
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Old 10-19-07, 03:33 PM   #24
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Because Shimano has starved the channel of 9-speed parts. Scarcity drives up price.
That's really not fair as it makes it seem like Shimano intentionally withheld product to drive up prices.

In the last year or two of 9-speed, before Ultegra and 105 went to 10-speed, Shimano experienced an unexpected great increase in demand for 9-speed components, mostly in Europe. They got caught in a stock-out situation everywhere. You couldn't get 9-speed cassettes for a while and the popular 12x27 cassette was backordered for months. They would rather have had the product available but got caught by demand that wasn't anticipated.
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Old 10-20-07, 08:59 PM   #25
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So what's the problem now?
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