Making a kilometer blurry
Join Date: May 2006
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Well, the frame flex between the der. hanger and the wheel's axle should be pretty minimal. There is some give in the rear triangle, but not much.
I think it's more likely to be lateral wheel flex. It could also be drive-side trailing spokes pushing the leading spokes outward under torque. Check to see if the last spoke crossing has the trailing spoke under the leading spoke. When you pedal, tension rises slightly in the trailing spokes, and lowers slightly in the leading spokes. This effect will push the spoke cross out to the right slightly.
Make sure your wheel is dished properly. In my experience, it's unlikely to be dished too much to the drive side, but it's worth checking anyway.
Try to figure out what part of the der. is hitting the spokes. You can mess with your der. B-screw adjustment (that adjusts the body angle to keep the upper pulley off your biggest cog). The B-screw is supposed to just prevent the upper pulley from contacting the big cog. You might be able to fix your problem by going further from or closer to the cog.
You might also try shortening or lengthening your chain to swing the der cage backwards or forwards.
Lastly, you can tray cold-forming the cage a mm or two outward, which will likely ben the der's pivot. Do this at your own risk, but It worked for me one time on the trail when an MTB crash bumped it the other way.