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Read derailleur compatability

Old 05-31-21, 08:19 AM
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_ForceD_
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Read derailleur compatability

I know there are (limited) lists available out there that give some of this info. But...is there a way to visually compare, or a measurement of the derailleur itself that can tell you if a rear derailleur is compatible, or interchangeable with an indexing shifting system?

Dan
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Old 05-31-21, 08:28 AM
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Mount it on the bike and see if it works. That is one visual method. The other visual method is to read the manufacturer specs.
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Old 05-31-21, 09:31 AM
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Id say - under realistic conditions - no.
Unless you recognize the derailer by design, and by experience or previous knowledge can say what its compatible with.
Since the error is usually cumulative, adds up from gear to gear, it really has to be pretty darn close for a random derailer to line up with each sprocket across the width of the cassette. Similarly, this is why measuring to determine travel, pull ratio etc is somewhat difficult. Merely a ruler is unlikely to be accurate enough.
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Old 05-31-21, 09:54 AM
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It sounds like you have the derailleur 'in hand' and want to test it?
Originally Posted by Iride01 View Post
Mount it on the bike and see if it works.
Though it may sound like a flippant answer, I agree with this.
You'd have to make a pretty accurate testing jig/fixture to measure the lateral movement of a derailleur jockey wheel for a given amount of cable pull.

Measuring the cable pull of an indexing shifter is easier - there's info on line showing how to make a test jig with a cheap digital caliper to do that. Even with that, the shifter has to be mounted on a spare bar or piece of pipe.

Not your question, but :For 'non-standard' combinations of shifter and derailleur, one solution is to move to old-style friction shifters, I guess. I don't think I have the skill to shift a 9+ speed cassette without indexing.
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Old 05-31-21, 10:09 AM
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Originally Posted by _ForceD_ View Post
I know there are (limited) lists available out there that give some of this info. But...is there a way to visually compare, or a measurement of the derailleur itself that can tell you if a rear derailleur is compatible, or interchangeable with an indexing shifting system?

Dan
For Shimano, anything that isn’t marked “Dynasys” is cross compatible from 5 speed to 9 speed (10 speed for road bikes). All Dynasys components are clearly marked but, if all else fails, look at the model number and check with Shimano.

Shimano and SRAM aren’t cross compatible at all.
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Old 05-31-21, 04:13 PM
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This has the specs for most derailleurs to compare with. You might want to ask here as well just to confirm if you find something on the list that will work as another poster corrected an error he found on one of the derailleur specs. Bicycle Cassettes & Drivetrains Chart - Google Sheets
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Old 05-31-21, 06:37 PM
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That is a pretty good list, Cranky. IRT the suggestions above to just put it on the bike and see if it works. Yeah...but if Im buying the derailleur at a swap meet, for example, I can really just put it on to see if it works.

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Old 06-01-21, 09:16 AM
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If you are buying at a swap meet you better know that it will do what you want. Why don't you just learn how to understand the specs and where to find them?

For Shimano stuff I go here: https://si.shimano.com/#/

There are all sorts of useful stuff. Many times just putting the part model number in the search box gets you an SI that will have the pertinent specs. And if you need to know compatibilities there are docs for that too, but sometimes a little confusing. And they understandably don't tell about mix and match frankenstein stuff.
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Old 06-01-21, 11:40 AM
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The specs of all the indexing shifters are available online. To test derailleurs I have lightly clamped a pair of vice grips on the shifter cable, measured to a stationary point with digital calipers, shifted across a few gears and measure again and then calculate the cable pull needed. This isn't going to catch the older derailleurs that have variable movement or the old Accushift Suntour that I understand didn't have consistent pull. And you are not going to be able to do it at a bike show.
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