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Old 01-12-10, 09:28 AM   #1
daey
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5th gear is malfunctioning

I rode my bike (Trek 720 Multitrack) to a doctor's appointment four days ago and left it outside for about 45 minutes. The temperature was approximately 20F, and there were 20-30mph wind gusts. I got back on the bike to ride to my office, and when I got up from the seat for more power on a hill, my gear shifted. I thought maybe I accidentally shifted while I was pedaling, but then it happened five more times in a mile, both standing and sitting.

The bike was in my apartment all weekend (I always keep it indoors at home and at work), and it seemed fine yesterday. But this morning the gear shifted by itself when I was about 40 feet from my office. It seems to happen 99% of the time when it's in fifth gear, and if it happens in another gear, it's usually 6th or 4th (my big gear is at 2). I had the bike fully tuned up and a tire replaced a few months ago.

I'm pretty bike-dumb and mainly just use this bike to go the mile to and from my office five days a week and maybe run a couple errands on the weekends. Any ideas as to what could be causing this? Your help is greatly appreciated.
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Old 01-12-10, 09:58 AM   #2
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If that's the only gear you normally use, it could be that just that specific cog is worn out. Unfortunately, you'll end up having to replace the entire cassette cog, and the chain.

But before you take such drastic action, it could be something is mis-adjusted or has gone out of adjustment. It could be a bunch of things- frozen cable, dirty cable path, cassette lockring, rear derailleur to frame connecting bolt, derailleur hanger bolt, etc. The rear wheel axle nuts or QR may be loose too. Or the cassette body could be backing out of the hub.
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Old 01-12-10, 10:13 AM   #3
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I would think everything would be tight/aligned/etc. because I had the annual tune-up so recently, and I only put about 10-20 miles on it a week. I think the frozen cable idea makes the most sense at the moment, since it's happening in very cold weather. I use gears 3-7 pretty evenly, because the commute to work is a big downhill and then a flat, and the way back is flat and then steeply uphill.

For now, I'll just try to avoid that gear and see if the warm spell we're supposed to get at the end of the week changes anything.

Thanks!
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Old 01-12-10, 10:37 AM   #4
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Freewheels will also freeze and feel like the are slipping.
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Old 01-12-10, 11:37 AM   #5
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I would think everything would be tight/aligned/etc. because I had the annual tune-up so recently, and I only put about 10-20 miles on it a week. I think the frozen cable idea makes the most sense at the moment, since it's happening in very cold weather. I use gears 3-7 pretty evenly, because the commute to work is a big downhill and then a flat, and the way back is flat and then steeply uphill.

For now, I'll just try to avoid that gear and see if the warm spell we're supposed to get at the end of the week changes anything.

Thanks!
Irrelevant.

You need to determine if it's freewheeling forward and then catching or if the chain is actually skipping over the cogs. If the former, the grease inside the freehub is either frozen or its being contaminated by something that does freeze.
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Old 01-12-10, 12:58 PM   #6
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Does it shift up to a smaller cog or down to a bigger cog? You could also look for ice building up between the cogs - try cleaning out with popsicle stick
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Old 01-12-10, 12:59 PM   #7
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if it is actually "shifting" by itself, as in dropping to a lower cog of its own accord, it could easily just be that you need to tighten up the rear derailleur cable a bit. i know you say that you just had the bike tuned up, but that could very well be the issue if they replaced the cable, as cables can stretch, usually in the first little while after installing. if this is the case i would think your bike would be making a lot of unnessessary noise in a lot of gears due to the misaligned derailleur.
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Old 01-12-10, 01:25 PM   #8
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Does it shift up to a smaller cog or down to a bigger cog?
No clue. I'll have to make a note of where the chain is, wait for it to shift, and then check.

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if it is actually "shifting" by itself, as in dropping to a lower cog of its own accord, it could easily just be that you need to tighten up the rear derailleur cable a bit.
I could look up where that is on a bike chart and check. Sometimes the gear gets higher, and sometimes it gets lower. But the gears in general don't seem to be making any noise.
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Old 01-12-10, 01:37 PM   #9
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if it is actually "shifting" by itself, as in dropping to a lower cog of its own accord, it could easily just be that you need to tighten up the rear derailleur cable a bit. i know you say that you just had the bike tuned up, but that could very well be the issue if they replaced the cable, as cables can stretch, usually in the first little while after installing. if this is the case i would think your bike would be making a lot of unnessessary noise in a lot of gears due to the misaligned derailleur.
This is exactly it. If your rear derailleur isn't tensioned properly, it can "ghost shift."
Try unscrewing the barrel screw on your rear derailleur a turn or so to increase the cable tension. (It's the screw that the rear derailleur cable goes into.) Keep track of what you do, so you can undo it if you have to take it back in to the shop.
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Old 01-12-10, 01:46 PM   #10
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Sheldon Brown, RIP, has a good explanation of "auto-shifting": http://www.sheldonbrown.com/autoshift.html
Two common causes are 1) poorly adjusted shift cable, this you should learn to fix yourself.
And 2) a bent rear derailleur hanger, if you cannot fix the problem by adjusting the cable at the barrel adjuster then take it back to the dealer and have it adjusted and checked for a bent derailleur hanger. This would also be a good time to lubricate the shift cables.

Al
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Old 01-12-10, 02:06 PM   #11
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Just had this happen to me. After adjusting the cable, everything was fine for about 25miles. Then it started to do it again. The real problem was the cable was starting to fail. Replaced cable and everything has been fine for the last 50miles.
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Old 01-12-10, 03:10 PM   #12
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Great ideas, thank you. I will try tightening the cable when I get home.

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Just had this happen to me. After adjusting the cable, everything was fine for about 25miles. Then it started to do it again. The real problem was the cable was starting to fail. Replaced cable and everything has been fine for the last 50miles.
This is also a possibility. I bought the bike used a couple years ago. It was in great condition, because the owner never used it, even though it's probably 15 years old. But the tires had started to dry rot. So a failing cable isn't inconceivable.
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Old 01-12-10, 03:40 PM   #13
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This is also a possibility. I bought the bike used a couple years ago. It was in great condition, because the owner never used it, even though it's probably 15 years old. But the tires had started to dry rot. So a failing cable isn't inconceivable.
You should probably consider replacing both shift cables and cable housings. Through normal use the condition of the housings is at least as important as the cables. The short housing at the rear derailleur is often subjected to dust, dirt, water, etc. which cause excess friction on the cable.

Al
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Old 01-12-10, 04:43 PM   #14
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You should probably consider replacing both shift cables and cable housings. Through normal use the condition of the housings is at least as important as the cables. The short housing at the rear derailleur is often subjected to dust, dirt, water, etc. which cause excess friction on the cable.
Would that be very expensive?
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Old 01-12-10, 05:16 PM   #15
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Would that be very expensive?
No. Check at your LBS. If you can get the shifting adjusted so that you're happy with it you could do the cables and housings at a later date. If you have a good mechanical aptitude you could release the cable from the rear derailleur, slide the housing forward on the cable exposing the part of the cable that runs through the housing, and lube the cable. Getting the cable reattached to the FD and then adjusted takes some patience. If you try this, start with the chain on the smallest cog.

Al
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Old 01-12-10, 08:13 PM   #16
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New cables and housings for both brakes and shifters will make the bike feel like new again... well NEAR to new again We all respond to how our bikes feel when riding and there's not many items that'll make as big a difference to how they feel as new cables and housings all around.

If you have any sort of working area at all it's not a big deal to do this yourself. You'll save about $15 to $20 in labour and learn something at the same time. Cables and housing will run you roughly $20 to $25 depending on what sort of deals you can get in your area. To do the work you'll need a set of linesman pliers or big side cutters to cut the cable and housing and a metal file to dress the ends of the housing.

Measure how much shifter and brake housing you have on the bike now and buy a long length that is a little longer than you need. And be sure to buy shifter AND brake housing since they are different. For the cables you'll need two brake and two shifter cables. You might also need the little metal ferrules for the ends of the housing segments if your old ones don't come off without damage. If you do get new ones there's no need to crimp them onto the ends. Just leave them loose.

You can find out more about it at http://www.parktool.com/repair/ . Just hover your mouse over the brake handles and rear derrailleur to click to the page about those parts.
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