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Old 07-11-10, 05:49 PM   #1
Phil_gretz
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Help With Rear Derailleur Jumping

Thanks in advance for the help. I thought that I had this one solved, but it is a recurring problem on my wife's touring bike, and I need a lifeline here... Long description, but I want you experts to know what we're working with...

The bike is a Trek 620, which I have set up with a new SRAM 9-speed 11-32 wide range cassette, a Shimano Deore XT reverse action 9-speed rear derailleur, the original Sugino AT triple crankset with new rings (48/38/26), Shimano 9-speed bar end shifters, and new Shimano cables and housings (the housings are silver, and very firmly seated). The only barrel adjuster is on the derailleur, as I had to add clamp-on cable stops (no downtube braze ons) so they're just plain stops. The chain is a Shimano 9-speed. All parts are new, except the shifters, but they're in excellent shape and shift perfectly when the bike's on the stand.

I set the B-screw according to the derailleur instructions and the gap between the top pulley and the largest cog appears to be correct.

There is little slack in the chain. So with the largest chainring and the largest cog, the derailleur cage is on a nearly straight line toward the front bottom of the chain ring. Taking out one more link pair doesn't appear to be an option, unless we want to risk an accidental big-big shift that would torque the derailleur...

Here is the symptoms, and I've had hours of riding behind my wife to observe this and can't solve it.

When she's climbing in the smallest chainring, and the climb is leveling out, she shifts from say the third largest cog (third position) to the fourth position. Or from the fourth position to the fifth position. In either case, the derailleur cage is nearly horizontal backwards taking up the slack that this creates. But the problem is that the chain wants to jump up to the larger neighbor, and does so periodically. Remember that this is a reverse action derailleur, so it indicates a cable that's too loose, right. So, I back out the barrel adjuster a quarter turn (and the derailleur cage moves imperceptably outward toward the smaller cog).

The jumping appears to be solved briefly, then later in a subsequent steep climb, she'll go to the first cog with the inner chain ring. As she shifts to the second position, it jumps to the third (cable too tight?).

The jumping between the fifth and fourth cog also happens from the middle chain ring... When I adjust the barrel at the rear derailleur to prevent one symptom, it causes another. The main problem is around the fourth and fifth cogs, which are from a wide range setup (11-12-14-16-18-21-24-28-32). Maybe the jump from 26(front)-24(rear), which is okay, to the 26(front)-21 or 18 (rear) isn't okay from a slack standpoint? Or is my problem really cable tension adjustment?

I'm about to pull my hair out. Maybe the cable housings are changing length somehow, by not being seated correctly in their stops? H E L P ! ?

I know you guys are the best and someone will have come across this before.

Last edited by Phil_gretz; 07-11-10 at 05:56 PM. Reason: correct an error in the cassette cog sizes
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Old 07-11-10, 05:54 PM   #2
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Two questions...

Have you checked the RD hanger alignment?
Is the cable routed properly around the RD anchor screw?
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Old 07-11-10, 06:03 PM   #3
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did you check for friction in cable/housing, also? housing ends square cut?

Last edited by roberth33tiger; 07-11-10 at 06:07 PM.
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Old 07-11-10, 06:37 PM   #4
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Quote:
Originally Posted by Ingleside View Post
Two questions...

Have you checked the RD hanger alignment?
Is the cable routed properly around the RD anchor screw?

Thanks for asking. The RD hanger is part of the forged dropout, and yes, it appears square. There hasn't been any frame damage on this bike from what I can tell... The derailleur cage looks to be in the correct up and down plane, same as the cogs. That's eyeballing it, though. I've been wondering about the hanger, but it doesn't seem bent to me. I have a mountain bike, too, and know from experience what bent looks like. Fortunately, they're replaceable aluminum hooks on my mountain bike, so the fix is inexpensive and quick...

As for the cable routing, it is smoothly in an arc from the braze-on stop on the chain stay looping around the rear hub, RD anchor, and entering the barrel adjuster straight on. It looks to be text book from my experience.

I'm pretty sure it isn't one of these two issues. But thanks, though.
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Old 07-11-10, 07:22 PM   #5
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Originally Posted by roberth33tiger View Post
did you check for friction in cable/housing, also? housing ends square cut?
Thanks. Yes, the housings are uncut, used in the lengths supplied by Shimano in its shifter kit. The bar end to the cable stop is one piece, then the cable stop on the chain stay to the rear derailleur is the other piece. Both came with Shimano ferrule ends molded into the housing. I would guess that friction in the housing is minimal, but you could be right about friction underneath the bottom bracket where Trek has the bolt-on plastic cable guide thing. It's original, and I haven't checked it recently to see that it's clean. Could be...
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Old 07-11-10, 07:49 PM   #6
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Is it possible that the cable is slipping at the fixing bolt on the derailleur? (Happened to me)
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Old 07-11-10, 08:45 PM   #7
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Here's what I do when things like this start happening: Start over by turning the barrel adjuster in all the way, shift to the smallest cog and check the cable tension. Also check the stop screws as this really starts the process. I also agree that you should check the der. hanger alignment. (It only takes a couple of minutes and can make a huge difference.)
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