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Old 03-12-18, 09:11 PM
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Originally Posted by Waltsmith View Post
The (front derailleur) shifting cable comes from under the bottom bracket, but the wraps around part of the derailer to fasten in front. If you follow, the cable pulls down, but because it wraps around it is actually pulling up on the pivot. So I donít know what kind of derailer that makes it.
Sounds like you're describing a "dual pull" derailleur. Those are designed to work in bottom-pull setups like yours, where the cable comes up from the bottom bracket, or in top-pull setups where the front derailleur cable is routed along the top tube and down the seat tube to pull the derailleur linkage from above. The cable is routed/attached differently depending on the pull direction.

Originally Posted by Waltsmith View Post
Thinking deore or 105 but Iím really stuck on what ďtypeĒ of derailer I need.
We've got some variables to narrow down...

What type of shifters are you going to use? Shimano road and mountain bike front derailleurs have traditionally used different pull ratios, so you'll generally want to match your shifter and derailleur. Use a road derailleur with a road shifter (e.g. Ultegra, 105, Tiagra, Sora) or a mountain derailleur with a mountain shifter (e.g. Deore, Altus, Acera, Alivio). Since doubles have seldom if ever been fitted to mountain bikes, you'll notice a dearth of double mountain bike shifters.

Clamp-on or braze-on? If you use a clamp-on derailleur, you may need to know the diameter of your seat tube so you can select a derailleur with the right clamp size (or shims).

What are your chainring sizes? The curvature of a derailleur's cage will ideally match the curvature of the outer chainring, so you'll notice that derailleur specifications note chainring sizes. Derailleurs are also rated for the maximum difference between chainring sizes. And triple-specific chainrings have sculpted cages to fit prescribed inner/middle/outer ring size combinations.

Or, putting all the minutiae aside... If you're not going to use your inner chainring but you're otherwise happy, you can use the front derailleur's low limit screw to prevent shifting into the small ring, effectively giving you a double.
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