Sunday, June 08, 2008

Cluster Headaches

The brawny lad who had his Mavic Cosmic wheel replaced with a snazzy DT on DT we got for him came back to report weird shifting issues on the nine-speed 12-27 cluster with which he had replaced his 12-25.

Most commonly with index-only shifting systems, the derailleur moves sluggishly shifting up the cassette or down, because cable tension is to strong or too weak. If it moves sluggishly both directions, the most common culprit is 4 millimeter housing, further constricted by grit and grime. The next thing to check is whether the linear wires in the housing have pushed through the ferrules or the cable itself has frayed inside some recess, even inside the shifter itself.

Less commonly, shifters of advanced age can get dementia as all those tiny ratchets and springs have simply had enough. Or young shifters can develop amnesia due to manufacturing defects.

There are a few minor repairable issues in the areas you can get into with road brifters, but only a few. They include The Mystery Screw in some generations, and the return spring for the shift levers.

Sorry, I don't have a picture of The Mystery Screw right now. Look on the inside of a brifter for a small, short, Phillips head screw. Early and late models don't have it at all. Somehow it affects the synchronization of the levers.

After exhausting other possibilities, I could only conclude that the shifter was wearing out. I had to scavenge one off a new bike we've been parting out. But when I got it hooked up, it exhibited the same strange pattern of shifting errors.

The rider had no problems with his 12-25. I put on a new one from our stock and the bike shifted perfectly. That meant I had to put his old shifter back on. It still shifted perfectly on the 12-25.

The seven cogs from 12 to 21 are the same on the 12-25 and the 12-27. The errors only occurred below 21. I switched those cog sets and the bike shifted perfectly on the 12-27. Something was a tiny bit off in the spacing of the set that originally came with the 12-27. Tiny discrepancies make a big difference as the spacing gets tighter. It took hours to hunt this crap down, hours we really couldn't bill for. But we cured it.

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