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Old 08-26-06, 02:57 PM   #1
Digital Gee
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RD question

My Trek 3900 RD is acting up a little. When i shift, nothing happens unless I shift TWO gears, not just one. Then it shifts as smooth as butter. This seems to be the case in either direction, up or down.

The RD is a Shimano Acera, which is nothing special, I realize, but it seems to get the job done. Anyway, my question is: what's likely causing this, and is it a simple adjustment even a knucklehead like me can do, or does it probably require a quick trip to the LBS?

BTW, they replaced my RD at 900 miles or so under warranty. This one has about 1,100 miles on it. The old one was doing the same thing only much more annoyingly (and would sometimes shift unexpectedly maybe 300 yards after I shifted -- SURPRISE!). Back then, I didn't ask what the adjustment was; I just took it to Trek.

So...what's going on, and can i fix it?
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Old 08-26-06, 03:48 PM   #2
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I can't answer your question, Gary, but I'd sure look at the cable to make sure it is not frayed, bent, or sticking. If the cable moves smoothly, the derailleur would be my next spot to look - is it clean, lubricated, and able to move without physical restriction? The only other part of the assembly that it could be is the shifter. Shifters usually are reasonably long-lasting and trouble-free, but yours may be worn (probably not).

My usual troubleshooting method (with bikes and computers) is: If a quick visual inspection reveals no specific problem, substitute known good parts for the "suspect" ones and see if the problem goes away. If so, the "suspect" part is now "known-bad," and gets tossed.

My I also recommend Shimano's XTR derailleurs - I've found them BULLETPROOF (although somewhat pricey).
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Old 08-26-06, 03:57 PM   #3
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I have this on the MTB and is just something I get used to. I just click twice on certain changes and No real problem. However I started getting this on the Giant and it was rear derailler adjustment. You have a friendly LBS and get them to show you the trick of adjusting the rear derailler. Onece shown irt is very easy to understand. That is taken that you do not have a tight or frayed cable that is causing the problem.
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Old 08-26-06, 04:01 PM   #4
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Most likely a minor adjustment. I agree with Stapfam that if your friendly LBS will show you how to make adjustments, take it on in.... can't hurt.
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Old 08-26-06, 06:10 PM   #5
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Mine did that and it turned out the cables were sticky and worn and needed replacing - but that was after about 6 years and 12,000 miles.
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Old 08-26-06, 07:07 PM   #6
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Most shifting problems are due to 1. cable tension 2. gruck.

A clean drivetrain is a happy drivetrain. Lube sparingly with a good-quality lube.

Quick way to check cable tension: Shift into the highest gear (the littlest cog). Grab the cable at the point it goes into the rearmost run of housing, the section that goes to the DR. There should be little or no slop in the cable, nor should it be guitar-string tight.
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Old 08-26-06, 09:50 PM   #7
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Since your original rear derailleur did the same thing I suspect the problem is neither the rear derailleur nor it's adjustment. Whenever I encounter a rear shifting problem like that the first thing that I do is to check the hanger alignment. Most of the time that fixes it.
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Old 08-27-06, 11:23 PM   #8
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Is it only double shifting in one direction, or both up and down the gear range? The first thing I do is scrub every part and moving joint of the derailleur with a toothbrush and Simple Green. You also have to brush the plastic guide piece where the cables go under the bottom bracket. Carb and protein drinks leak out of the water bottles and "glue" the cables there.
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Old 08-28-06, 12:22 AM   #9
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Besides sticking cables and bent hangers, Cables may stretch or slip and housings can compress a little, so before you go spending money try turning the barrel adjuster on the derailleur one full turn counter clockwise to see if improves the shifting. If it helps try 1/4 turns till shifting is normal. If it doesn't help, return the barrel adjuster to its original position and take it to the shop

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