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A couple of years ago, Shimano debuted their Shadow Plus technology for mountain bike rear derailleurs. What is it and why would you want it? If it's such a good idea, why hasn't it been copied by other manufacturers or used in other groupsets?

Related: Why doesn't Shimano use shadow technology on road derailleurs? - which deals with the "Shadow" tech, which in true Shimano-style, is different from "Shadow Plus."

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    Mountain biking is harder on a derailleur than road biking is. The chain is always taut on road bikes in normal operation; less so on mountain bikes (and this can cause problems). So, Shadow Plus adds a clutch. So, its a largely unnecessary addition of complexity and weight (which weight weenies won't like) for road bikes. – Batman Jan 10 '17 at 22:58
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TL;DNR A Shimano 'Shadow Plus' or SRAM Type 2 dérailleur (aka clutch dérailleur) means you can change the chain tension by flicking a lever.

For completeness - back in the late 2000's, Shimano introduced the 'Shadow' dérailleur (refer OP's link) which brings the deraileur in behind the cassette, providing it more protection from knocks and damage. A short time (couple of years?) later they added the 'clutch' which in reality is nothing to do with Shadow, but Shadow was still new, and they still sold non-shadow dérailleurs in the lower spec ranges, so the needed to keep Shadow in the name of this new model. Even though/because all clutch dérailleurs were also Shadow, it made a lot of sense for marketing to call them 'Shadow Plus' rather than call them 'Plus'

Unfortunately because of the close time frame between introduction, Some people mistakenly believe 'Shadow' means it has a clutch, as they never saw a 'Shadow' without the clutch.

So what is Plus - As alluded to by @Batman's comment - The Plus means a clutch. But what is a clutch

Dictionary: noun: clutch; plural noun: clutches

1.
a tight grasp.
"she made a clutch at his body"
    a person's power or control, especially when regarded as inescapable.
    plural noun: someone's clutches
    "Tom had fallen into Amanda's clutches"
2.
a mechanism for connecting and disconnecting an engine and the transmission system in a vehicle, or the working parts of any machine.
"she let the clutch in and the car surged forward"
    the pedal operating the clutch in a vehicle.
3.
a group of eggs fertilized at the same time, laid in a single session and (in birds) incubated together.
4.
a slim, flat handbag

But the Shimano marketing term is none of the above. SRAM used a more generic term "Type 2"

Going back even further than that (early/mid 1990's if memory serves me right, but might have been the 80's) Shimano came up with a 'new dérailleur' with a lighter spring and lowered chain tension. This was sold as having all sorts of advantages over the heavier springs - easy wheel changes, better drive train efficiency, probably making you more attractive to opposite sex was somewhere in there (it was the 1990's). When it became apparent that MTB's were suffering because the spring was too light, its was going to be difficult to sell a "heavy spring" without Shimano getting egg on its face (as riders would become less attractive to the opposite sex). They developed the 'clutch' - a little lever that lets you have all the advantages of a light spring (easy wheel removal is the main claim to fame) and all the advantages of a heavy spring (less chain slap, better chain retention over rough ground etc).

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    Hi Matt. I'm thinking the first definition a tight grasp is what the name is about. – andy256 Jan 11 '17 at 2:28
  • A clutch seems different from a heavier spring. Explain? – RoboKaren Jan 11 '17 at 3:37
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What is it and why would you want it?

Shadow plus refers to the adition of a one-way friction clutch to the derailer cage pivot, of which its tension can be adjusted. The purpose of this mecanism is make it harder for the cage from getting pulled by the chain and swing outward when traveling on rougth terrain or when diving from a drop. The reason why you do not want the cage to swing is that it can cause the chain to hit the chainstay and damage it and cause lots of noise, or it can even jump out of the chainring. To summarize in one sentence, it reduces chain-slap (the chain impacting the chainstay) and the likehood of the chain getting loose.

If it's such a good idea, why hasn't it been copied by other manufacturers or used in other groupsets?

It has, in fact, been implemented by other manufacturers, SRAM call their clutch-equipped derailers Type-2.

As for other group sets, I take that you mean road derailers? if that is the case, road RDs do not benefit at all from this feature, as there is not rough terrain (talking about roots/ rock gardens here) or drops on the road.

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