Bicycle Mechanics - Frame flex? Or something else..?
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I have a frame made of columbus ultrafoco steel tubing, with a columbus carve carbon monostay in the rear. When climbing out of the saddle in my 26 tooth in the rear, once every two pedal strokes, it sounds like the rear derailer is just barely making contact with a spoke... If I do it seated...nothing. I tried trimming the rear der out a little, but I don't want to mess up the shifting at all... I suppose its just something I need to live with...or shift to a higher gear when standing....but I thought I would throw it out there.
06-14-06, 06:32 AM
Frame/bottom bracket flex can cause a bike to "ghost-shift". So can a dirty bottom bracket cable guide. If the cable hangs up on it, it can release when the frame flexes under a standing load, and voila!, your bike has an "automatic transmission".
Other than making sure that the derailleur cable moves freely under your bottom bracket, there is no fix for frame flex (except maybe to weigh 135 lbs.)
06-14-06, 07:13 AM
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.
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