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I'm putting a 46x30 chainring with a pretty narrow tread on a frame. On the advice of my LBS I got a Sun XCD double FD, but I'm starting to think it's not right for this application. When I have the derailleur on the big ring and far enough out to avoid rubbing, there's razor-thin clearance between the outer plate and the crank arm. And that's just on the stand -- I'm worried that once I'm on it and things are flexing, the crank is going to hit the cage.

I think a narrow-cage derailleur, and maybe one that is designed for smaller chainrings (the Sun is listed at max 56T) will be a better fit. Does anyone have any recommendations?

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what kind of shifters are you using? –  Booker Apr 1 at 16:41
    
Dura Ace bar-ends. I have a 9-speed 11/34 on the back, if it changes the equation any. –  Josh French Apr 1 at 16:54

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There will be only very small differences in the widths different FDs (assuming you are talking about the inside to outside dimension). One that is designed to be minimum sized so that it provides a best fit for 46t large chainrings is Shimano's CX-70, intended for cyclo-cross: http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/cyclocross/cx70/product.-code-FD-CX70-B.-type-.fd_road.html It does have a 16t capacity. It is also compatible with the bar ends (which don't index the front anyway). I have purchased one of these to use with a similar chainring setup and 105 dual-STI brifters, but I have not yet installed it.

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FWIW I moved the mech up 5-6mm and the clearance is a little better. If it's not enough I'll see if I can find the Shimano for cheap somewhere. Thanks for the tip! –  Josh French Apr 2 at 4:51

If you compare new and old derailleurs, you'll see that the new ones have cages made from thinner metal, but stamped to make them torsionally rigid. Older ones like this Campagnolo FD

Vintage Campy FD

have a nice flat low profile cage that works well with cranks that resemble the older Campy with low Q-factor and relatively straight crank arms. Of course vintage Campagnolo is insanely priced, but you may be able to find similar Dura Ace or Sun Tour FD's that would work just as well.

Also, another trick that can help is grinding the cage slightly to better match the curvature of the chainring.

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The grinding suggestion is a good one. You may also be able to shim it in some way to make it fit better. –  Batman Apr 2 at 22:12

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