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I want to buy a new frame that supports Direct Mount. Now I read that Bottom Swing is preferable for hardtails. At the same time, it was also mentioned that Bottom Swing is mounted higher onto the saddle tube than Top Swing. Are there two versions of Direct Mount to accomodate these few centimeters height difference, or do the top and bottom swing "Direct Mount" front derailleurs position their "Direct Mount" screws at different heights internally, so that I don't have to worry about compatibility?

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"Top swing" versus "bottom swing" is defined by the position of the derailleur cage in relation to the clamp. There are indeed two completely different versions of direct mounts: high direct mount is for bottom swing derailleurs (since the derailleur is below the clamp), and low direct mount is for top swing. The two versions are not compatible.

Bottom swing is in general preferred unless there is a compelling reason to need the more compact size of a top swing; typically this is only the case on full-suspension bikes.

This is not to be confused with "top pull" and "bottom pull", which only reflects the direction of the cable entering the derailleur - from above or below. Both top pull and bottom pull (as well as dual pull which supports either) mechanisms can be found in both types of derailleur mounts.

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There are most certainly front derailleurs that are convertible between top and bottom pull (both Shimano and Sram make many different models in both direct mount and clamp mount.) –  WTHarper Aug 19 '13 at 12:43
    
Top pull and bottom pull are not the same as top swing and bottom swing - but that's a good point. I've updated the answer to reflect this. –  lantius Aug 19 '13 at 22:13

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