bumperm

My Trek Madone 6.9 has DA 7800 and compact crank 50-34, SRAM 11-26. I live next to the eastern flank of the Sierra, lots of mountainous terrain. At 64 and less than a year riding, I need a reasonably low gear for the steeper hills. Shimano says the the rear derailleur will handle 27 cog max. I changed to SRAM 11-28 that other posts on BF said would work. However, even with the B screw adjusted for max tension, the upper pulley would "bounce" on the larger cog, resulting in noise and probably wear - long term.

The fix was relatively simple. I disassembled the main derailleur pivot per Park Tool's website:

https://www.parktool.com/repair/readhowto.asp?id=76

and found that there were two holes for the tension spring in the upper pivot body. Looking inside the barrel body with an imaginary center point between the two holes at 6 O'clock, the hole used for the spring anchor would be at 4 and the unused hole at 8 O'clock. The 8 O'clock hole provided way too much tension. Drilling a new hole for the spring anchor point at about 5 O'clock solved the problem.


New hole was drilled with #45 drill (.082") from inside the barrel using a drill press. If doing this remove pivot bolt and O-ring seal first, use caution not to allow drill tip to damage O-ring seal area as it exits.