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Front derailleur slipping slightly

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Front derailleur slipping slightly

Old 08-30-14, 11:04 AM
  #1  
vespoli
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Front derailleur slipping slightly

Hi there,

I have an issue with the front shifting on my Surly Cross-Check. The bike has an Ultegra FD, paired with an old Suntour MTB triple crankset. I'm using Dura-Ace bar-end shifters. If it matters, I'm running an XT RD on a 9-speed cassette. I have both shifters set in friction mode.

OK, now the issue, and a little backstory:

I just finished a self-supported TransAm, so the bike had a lot of miles (almost 5,300) put on in a short time. When in Boulder, Colorado, I noticed I couldn't reliably serve up the big ring -- it was like I ran out of travel in the shifter body before the FD would knock the chain over. I tried to fix this problem by taking the FD cable and pulling it tighter through the clamp (what I thought would take the slack out). This solved the problem, but also seemed to reduce the travel in the shifter body -- that is, I would use about 2/3 of the shifter travel for all three rings. The cable is too tight to allow me to pull the shifter all the way up to parallel. I can still shift into all three chainrings. So the issue: now after I shift into either the biggest or second biggest ring, the FD seems to slip in toward the BB, causing the chain to rub on the FD. This happens more quickly if I'm out of the saddle pedaling or really cranking on the pedals, but even at smooth, gentle pace, eventually the FD will slide inward, and the chain will rub. It doesn't seem to slip far enough to cause a shift down to the next smallest ring, but I usually get annoyed before that happens and nudge the shifter body to push the derailleur back outward. This solves the problem, but just for a few revoltions before it slips again.

I hope this description is adequate to explain what's going on -- curious what adjustments I should make to start solving the problem. Thank you very much.
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Old 08-30-14, 12:06 PM
  #2  
bradtx
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vespoli, I have the same bar ends and FD (FD-6503) on my distance roadie. The FD's lever isn't straight out, or level when on the big ring, maybe sitting at 60 degrees or thereabouts.

My FD's bar end was touchy when adjusting tension, it seems there is a fine line between holding position and too tight to easily shift. My 8S Ultegra FD's bar end shifter is better in this respect.

The shift cable should run to the frame side of the hold down screw on the FD.

Brad
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Old 08-30-14, 01:15 PM
  #3  
vespoli
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Brad,

Thanks for your insight. I'll just try to loosen the cable slightly, then.

David
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Old 08-30-14, 02:24 PM
  #4  
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This seems to be happening more these days, possibly because crank/centerline distance is greater than it used to be. We had a long thread about a similar problem a few days ago.

Step one, measure the crank centerline, and see if it conforms to standard spec. Based on your description of limited FD travel, I'll venture that it's about 3-4mm beyond ideal. So to compensate, you're using some extra cable tension to force the shift, and paying the consequences, which in your case are movement of the entire FD.

Step, confirm FD travel by backing off the limit screw and seeing if there's enough to shift beyond and dump the chain to the outside. IMO, this is the acid test of adequate (excess) FD travel. If you cannot dump the chain to the outside, then the FD travel is limited, and may or may not be adequate to shifting without excess tension.

In a borderline situation, the FD might just barely have enough travel to reach, shift and trim on the large chainring, but if you need to overcome any resistance to do so, you'll suffer "rebound" downshifts, shorter cable life, FD slippage, and overall poor performance.

The right cure is to move the chainrings inboard 1-4mm so they're within spec, or at least close. Or you can sometimes, with some careful filing, remove some of the material on the limit screw cam, or the FD body where the FD bottoms, and buy yourself the millimeter or two of travel needed to make it work right. Or you can spend dough and replace the FD with one which, hopefully, has a greater travel range.

Of course, if things aren't terrible, you could treat the symptoms by using some coarse lapping compound under the FD clamp to improve traction and prevent slippage.
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