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Offset seat tube and direct mount derailleur X dimension.

Is it possible to turn on the X dimension, available in the Mount type | Braze-on, for the Mount type | Direct mount?

I have a frame with an offset seat tube and woud like to move the derailleur to the rear. In it's current location the rear of the derailleur cage touches the chainring teeth and the chain rubs on the bottom of the derailleur cage in the low gear ratios.

With thanks


Direct mount X dimension

In lieu of an X offset dimension, the direct mount front derailleur option includes an angle theta that allows you to rotate the derailleur forward or backward as shown below. Allowing the derailleur to rotate as it is shifted forward and backward should enable the contours of the derailleur cage to conform to the chainring profile better than if the derailleur were simply shifted horizontally. Note that even if the seat tube is offset in its attachement to the BB, direct mount derailleurs are still located with respect to the BB.

Hi Brent

Hi Brent

Thanks for your reply. I have used the theta setting but in the case of the frame above, only moving the derailleur 10mm to the rear has solved the chain and chainring clearance issues. Rotating the derailleur a few degrees clockwise solved the chainring / cage clearance problem but introduced a chain rubbing on the bottom of the cage problem in low ratios.

It’s likely the frame builder was aware of this as the brazed on steel DM was aligned as far to the rear of the seat tube as the mitred connecting tube would allow. I have replaced this with a clamp on aluminium DM with a new M6 bolt hole in the grove, just by luck the derailleur ridge fits against the rear face of the mount.

I’m assuming DM is not such a good choice for offset seat tubes and wondering if an E2 mount on the BB might be a better idea. Would be keen to hear of any similar experiences. 

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