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?