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Impressive Upshifts, Impossible Downshifts

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Impressive Upshifts, Impossible Downshifts

Old 07-06-19, 06:07 PM
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CyclesWithDogs
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Impressive Upshifts, Impossible Downshifts

I’m working on a Scott mountain bike owned by a very avid rider who has been riding many, many years. The drivetrain is a 3x9 Shimano Altus M370 setup.

My problem is with the downshifting out back. The upshifting (moving from a larger cog to a smaller one) is perfect — one click, one shift. But the downshifting (moving from a smaller cog to a larger one) is problematic. Specifically, shifting from the smallest cog to the next one is good, however after that each subsequent shift to the next larger cog begins with a click from the shifter followed by the chain trying to make the requested move — it makes a racket and jumps around — but never quite lands on the next larger cog to complete the downshift. The only way to make a downshift is to press the downshift thumb lever twice (ask for two downshifts) then, as the chain struggles to complete the second downshift — which will never happen anyway — click the upshift trigger once, thus canceling the request for the second downshift. (Whew!)

At first I thought it might be a bent derailleur hanger or derailleur cage, but the idler pulley and the guide pulley both appear to line up nicely with whatever cog they’re being asked to service. And remember, when they’re asked move from any cog to a smaller one, the indexed shift is completed flawlessly.

I’m now wondering if the problem is that the Shimano Altus SL-M370, having many miles on it, is just becoming worn out. I’m thinking that during the consistently perfect upshifts I’m witnessing, all the shifter has to do is relax and release one index-worth of shift cable tension and let the spring loaded RD do the rest. But conversely, during a downshift, the shifter has to increase shift cable tension in a tug-of-war effort to overcome the RD spring — a task I would argue is a more demanding one on the internal shifter components than an upshift.

I recently opened the cover of a Shimano ST-EF51 shifter (the only way to replace the shift cable on that model) and noticed just how much of the internal components were plastic. I’m thinking if the M370 shifter of focus here is similarly constructed, and has as many thousands upon thousands of shifts on it as it probably has, that some of these components are becoming fatigued and allowing enough flex and play as to make downshifting impossible.

Thoughts?
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Old 07-06-19, 06:37 PM
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I'm thinking you need to turn the barrel adjuster counter clockwise a "few" clicks.
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Old 07-06-19, 07:28 PM
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How rapidly the symptom onset would be a valuable clue. Maybe check for a frayed cable or excess cable friction.
Those shifters can last many hundreds of thousands of shifts.
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Old 07-06-19, 07:55 PM
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I’d start with new cables.
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Old 07-06-19, 08:07 PM
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I agree with AnkleWork's suggestions and would additionally suggest that you disconnect the wire from the derailleur, keep tension in it and work the shifter to determine if it dependably and consistently feeds out and takes up the wire. Maybe clean and lube the shifter too.
It would be easy and not too expensive to just replace the wire and housings, which are wear items and could also be dirty, might make a huge improvement and would give you a chance to set up the shifting ab initio.
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Old 07-06-19, 08:13 PM
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Yes, cables are the most likely thing. Much more likely than shifter or derailleur. A common problem.
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Old 07-06-19, 08:27 PM
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I'll add that a worn guide pulley bushing has some of the same effects of cable issues. The pulley should have side play with no significant rocking of the pulley off axis. Andy
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Old 07-06-19, 10:56 PM
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The first thing that I'd do would be to twist the barrel adjuster 1/4 turn. If that doesn't solve it, I'd check the derailleur hanger alignment with a gauge. Alignment doesn't have to be very far off to cause all manner of wonky shifting symptoms.
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Old 07-07-19, 05:39 AM
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The impressive upshifts would tell me that I'm slightly overshifting in that direction and I'd add a little cable tension to improve shifting in the other direction.
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Old 07-07-19, 05:46 AM
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How old/cracked/crappy is the cable housing? Like others, my first guess would be excess friction.
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Old 07-07-19, 06:57 AM
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If I interpret your description correctly, you have replaced the shifter cable. I'd start with the simplest first: check the cable routing for any problem and then adjust cable tension with the barrel adjuster. The cable is pulling the derailleur to the the next larger cog and so more tension is needed. Turn the barrel adjuster out/counter clockwise as Bill Kapaun recommended. If that doesn't work you can look at the possibilities that require more work.
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Old 07-08-19, 04:50 AM
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Also check the derailleur itself for play. If it has been used much, it might have developed a significant play in its bushings which also could cause the need to overshift to get to a larger cog.
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Old 07-08-19, 05:59 AM
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Are brake cables being used for brakes and shifter cables used for the shifters? The windings are different. Just one more possibility.
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Old 07-08-19, 07:24 AM
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How old is the chain and cassette?
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