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Newer Rear Derailleur Suggestions?

Old 06-07-22, 04:04 PM
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sortieavelo
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Newer Rear Derailleur Suggestions?

Hi, my old 1996 Trek 6500ZX is slipping to lower rear cassette cog under torque. Not every cog, just like 3D and 4th from the frame. It’s a 7-speed cassette. Bike has original Shimano STX rear derailleur. It has newish cassette, chain seems ok, chain line is good, and RD hanger looks straight. Its been an ongoing problem. I’ve fiddled with it, barrel adjusted, checked tebsion—nothing helps. It skips downward under load.

Looking for recommendations for a good replacement RD. I haven’t done gear calculator, but maybe I could get new cassette, RD, and convert to 1 X up front? Thank you!!


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Old 06-07-22, 05:50 PM
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Originally Posted by sortieavelo View Post
chain seems ok
Measure it.

Originally Posted by sortieavelo View Post
RD hanger looks straight
Check it with an alignment tool; you really can't eyeball it with any degree of accuracy.

Originally Posted by sortieavelo View Post
maybe I could get new cassette, RD, and convert to 1 X up front?
You can certainly do all that if you want to, but that's what my dad used to call "a hundred-dollar solution to a ten-cent problem."
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Old 06-07-22, 06:06 PM
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Rolla covered it well. If your derailer hanger is bent, then even after replacing everything you listed, your shifting still won't work well. The amount a derailer hanger needs to be off by to cause issues is smaller than many can easily perceive by eye. I'm always amazed at how far off they are when I go to measure them.
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Old 06-08-22, 05:34 AM
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Thanks for the advice. I ordered the Park Tool alignment tool. I have a few other bikes that will benefit from that.
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Old 06-08-22, 05:43 AM
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Originally Posted by Rolla View Post
Check it with an alignment tool; you really can't eyeball it with any degree of accuracy."

Honestly, 7 speed you can pretty much eyeball it.
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Old 06-08-22, 07:03 AM
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"Newish" cassette, chain "looks good"? How new is the cassette, measure what the distance is between 12 links? This sounds like a worn (aka "stretched") chain that's worn the two middle gears of the cassette you use most frequently.

If it were my bike, I'd start by measuring the chain. If it's less than 12 1/16", see if you can tweak the derailer adjustment, and if 1/4 turn in each direction doesn't fix it, put a new cassette on. If the chain measures more than 12 1/8", it's probably time to replace chain and cassette. That's still a whole lot cheaper and easier than wholesale drive train replacement.
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Old 06-08-22, 10:19 AM
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In the event that you do need a new rear derailleur, any 7/8/9-speed Shimano rear derailleur that isn't a 74xx Dura Ace should be compatible as long as it can handle your cog size and chain wrap. Pretty much any modern 8/9-speed Shimano MTB derailleur would be a safe bet.
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Old 06-08-22, 07:45 PM
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I got so frustrated with my shimano Altus rear deraileur jumping between gears, i've been adjusting it on the fly at the shifter. I tried eyeballing it first with poor results. While its not 100% now, Its no longer an annoince. Its the same 7 speed freewheel as your bike. Your deraileur looks a lot better (sturdier) the mine.
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Old 06-09-22, 06:35 AM
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This probably isn't the problem...however...the cable routing at the cinch bolt (nut, really) on the derailer is not how Shimano intended it. The cable should run straight...from where it exits the cable stop at the back of the derailer, straight through next to the cinch stud-and-nut, and then straight out the front. I'd think this would primarly impact shift indexing, if it impacts anything at all, but I thought I'd mention it anyway.

So it's basically ghost-shifting under load, right? Ghost-shifting to the next smaller sprocket? Things like this that happen only in certain sprockets are usually attributed to something like uneven wear on the cassette, or improper alignment of the derailer. That said, 7-speed drivetrains are generally pretty easy to adjust and forgiving of stuff like this. I might tighten the barrel adjuster just a little bit to bias the derailer just slightly inward on each sprocket...almost enough to where it wants to downshift to a larger sprocket on its own, but not quite. This may prevent the chain trying to walk down to the next smaller sprocket on its own.

I suppose a left-field answer might be to check the frame carefully for cracks or a weak weld. It's possible, though unlikely, that there's enough twist imparted into the frame while under load that geometry of the rear triangle is tweaked just enough to perhaps loosen cable tension of the shift cable momentarily. It doesn't appear that the plastic cable end of the rear derailer cable loop is sitting squarely in that seat stay cable stop. I wonder if there's any unwanted movement there. Again, probably grasping at straws with these last few ideas...!
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