Old 09-30-18, 09:24 AM
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daoswald
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Join Date: Apr 2008
Location: Salt Lake City, UT (Formerly Los Angeles, CA)
Posts: 1,028

Bikes: 2008 Cannondale Synapse -- 2014 Cannondale Quick CX

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Update on poor rear derailleur shifting (fixed)

I have the following Cannondale Synapse with 6500 miles on it:
  • Aluminum frame Synapse
  • Shimano 105 10-speed rear brake lever/shifter.
  • Shimano 105 3-speed front brake lever/shifter.
  • Shimano 105 front derailleur (3sp).
  • Shimano 105 long cage rear derailleur (10sp).
  • Shimano Ultegra 10sp cassette (11-28).
  • SRAM TruVativ crankset (30/39/50).
  • Fulcrum Racing Quattro LG wheel set.
  • Shimano Ultegra chain.
The rest of the hardware is probably totally irrelevant so I won't bother enumerating. The wear-and-tear parts have this mileage on them:
  • Derailleurs: Original equipment.
  • Crankset: Original.
  • Shifters: Original
  • Wheels: 250 miles
  • Shift cables: 500 miles.
  • Rear cassette: 1600 miles
  • Chain: New.
The problem I was having: Downshifting between 7 and 6, between 6 and 5, and sometimes between 5 and 4 had frequent mis-shifts. I was constantly adding tension to the indexing. Upshifting between 6 and 7, 7 and 8, 8 and 9 was also unpredictable, often missing the shift. There seemed to be no indexing setting that would get me clean shifts in both directions.

I took the bike to a mechanic who replaced shift cables, verified low friction in the shift cables, verified hanger alignment, and adjusted the indexing. It was no help. If anything, he made things worse on the road, though it looked fine on the stand.

Other observations:
  • I noticed the B-screw was all the way in to prevent the RD's upper jockey pulley from contacting the cassette when I was shifted into the 30/28 combination.
  • I noticed that in 30/11 combination the chain was ALMOST riding against the idler pulley. Not quite, but almost.
  • In the 50/28 combination the RD had room to spare.
I suspected that chain length was part of the problem. Since my chain had 1700 miles on it, I ordered a new one and then used the old one for testing. First I added one link and observed that in 30/11 the chain did ride up against the idler pulley. Then I removed two links from original length and verified that I could reliably get into 50/28. I could probably have removed one more link, but decided to stop at two. Then I installed the new chain at this original minus two length.

This allowed me to back out the B-screw significantly without riding against the rear cassette in 30/28. By backing out the B-screw the RD rolls slightly more forward, wrapping more of the cassette, and staying closer to the cassette, particularly in the higher gears. I believed this would improve shifting. And I was right. Most of my indexing / shifting woes went away. But there was still a little pain between 7-8 cogs in both directions. Having ruled out alignment and cable friction, and RD adjustment (aside from indexing), that left the shifter, and the rear cassette's torque. I checked the torque because that's low hanging fruit that I should have checked in the first place. It was fine, so that leaves the shifter.

I cleaned the shifter by hanging the bike from its rear wheel so the shifter was pointing at the ground. First I blew it out with compressed air, then washed it out with WD40, then with degreaser (to chase away the WD40), and finally blew it out again. A few grains of sand came out, along with some general gunk. Where did the sand come from? Eight years ago I was riding in Santa Monica CA and went off the bike path when someone stepped in front of me. I thought I had gotten all of it, but there's always another grain or two, I guess.

The outcome: Reliable shifts between all rear cogs, regardless of which front chainring I'm in. And with the shortened chain I have less slap when I hit bumps, and because it allowed me to back out the B-screw a bit, faster shifts. Feels like a new bike when I'm shifting.
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