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Shimano has been using their 'Shadow' technology on their high-end mountain bike rear derailleurs (> Deore) for many years. In addition to the benefit described in the images below (from here), Shadow derailleurs have a more direct housing routing near the derailleur, which results in a lot less cable friction, and therefore, better and smoother shifting.

lowgear highgear

Why doesn't Shimano use this technology on their road rear derailleurs? Interestingly enough, SRAM also went out of their way to avoid direct housing routing near the rear derailleur when they came out with their road derailleur, even though they'd been using direct routing for many years on their mountain bike derailleurs.

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3 Answers 3

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Simple answer would be that it's heavier, which is very important in road-bikes.

Regarding the cable friction and smoother shifting, road shifters are going towards electronics

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Electronic shifters are a whole other level, and cost over $2000 from what I've seen. There's nothing inherently expensive about electronic shifting, so maybe it will come down to a reasonable price at some point, but for now it's out of the price range of all but the most dedicated cyclists. –  Kibbee Apr 8 at 16:38

a) profile reduction is more or less irrelevant to road bikes.

b) the cost of retooling all their road bike dérailleur manufacturing to get a very marginal gain in reduced cable friction probably makes it not worth it. It might be more expensive to produce dérailleurs in that configuration too.

c) (tinfoil hat mode) they deliberately want to keep shifting on their cabled road dérailleurs poor so as to boost sales of di2

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d) Reducing compatibility means you have to buy more parts rather than reusing compatible old parts. –  Emyr Apr 8 at 11:52
    
@Emyr, Isn't changing things around (e.g. tire sizes) a leading strategy to stimulate purchases? –  Vorac Apr 8 at 12:17
    
Possibly, the 29er trend meant you could use the same rims for MTB, cyclocross, heavy touring, but couldn't use your old 26" forks on your new 29er. Manufacturers need durability to sell their products, but if the parts are too durable they need other ways to trigger subsequent purchases. –  Emyr Apr 8 at 12:34

The benefit of the Shadow Plus friction clutch is that rough terrain doesn't allow the chain to pull against the rear derailleur cage spring, so the chain stays taught and in control.

Key words rough terrain here. The whole purpose of the shadow tech is to reduce chain slap, which isnt a problem for pavement road racing.

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The question was not about the Shadow plus plus technology with the friction clutch but about the Shadow technology, i.e. the design where the derailleur is designed to be positioned more below the cassette than beside. –  Benedikt Bauer Apr 8 at 13:09

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