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I've looked at most of the answers for related questions and they all speak about replacing parts that are old.

I just bought a new bike from a friend who rode the bike for about a month before leaving it in their garage for about 3 years only riding it on occasion.

When riding it home I came round a corner and was about to head up a slight hill. I shifted gears and the chain came off.

Yesterday I rode it to work and it was fine, but the chain keeps slipping whenever it is under reasonable load (such as going up hills).

I did some research and watched a few youtube videos. I also have a housemate who has been riding for years and we adjusted the derailleur this morning. But on my way to work this morning it still slips regularly. It is not slipping into another gear just crunching for a split second. It also changes gears without problem.

I looked at teeth on the gears and everything looks good as new. It is a 21 speed with Shimano gears.

Although everything is quite new could the chain be stretched just sitting in storage? What else should I be checking?


Update to cover some of the points in the answers:

  • The chain only came off once on the day I was riding it home after I purchased it. The chain is now just slipping. I think it came off due to the way I changed gears.

  • I'm not sure if the slipping is happening on the front or rear gear or as it goes through the derailleur

  • It's not my gear changing technique. At first I thought this may be the issue so I tried going easy on it yesterday and it still sleeps even if I have been in the same gear for 2 minutes before going up hill.

  • I have noticed it slips if I can't maintain the same pressure on the pedals. So if I slightly weaken the pressure for a second or pedal slightly slower it will slip.

  • There is no visible rust on the gears or the chain, but I will look at some of the other posts and see how to clean and lube the chain and will check this weekend to see if there are any stiff links. There is also no visible damage to any of the gear teeth, front or back. However the derailleur does look dirty.

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My first guess would be that the rear derailer is gummed up and not maintaining tension. Hosing it down with a little WD40 (try to keep it off the tires) will help immensely. Use an old toothbrush if there's a lot of gunk. Otherwise it's possible that the derailer was defective from the factory, which is why the guy never rode it much. –  Daniel R Hicks Nov 10 '11 at 12:39
    
What you're describing is usually due to something amiss with the rear derailer. It could be gummed up, could be that the derailer or the "hanger" is bent, could be that the tensioning spring is broken or has popped off its peg. Another possibility is that the chain is too long or too short. Also, you should make sure you're not habitually using the "extreme" gear combos -- big front and big rear or small front and small rear. These either stretch the chain tight or leave it quite slack, in addition to causing the chain to be extremely diagonal. –  Daniel R Hicks Nov 11 '11 at 1:59
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7 Answers 7

up vote 5 down vote accepted

Learning some better shifting technique may solve this. It also could be that, despite your best efforts, the derailers are out of alignment.

Two questions:

  • Is the chain falling off on the front chainring or on the rear cogs? If so, is it happening on the inside or outside? (If it's happening in the rear and the chain is hitting the spokes, this can be an extremely serious problem that can be a danger to the rider. Other scenarios are not as serious.)
  • Can you tell us more about the bike? You mention a Shimano drivetrain; Shimano makes differing levels of components, and even top-level components won't work very well if a bike is assembled poorly. Is this a reputable brand of bike purchased at a bike shop, or is this a big box bike?

Shifting technique

This is particularly a problem with bikes that have twist-grip, bar-end, downtube, or other shifters that allow the user to shift through many gears all at once--or at least allow them to try; this will often pull the chain off the gears. The solution to this problem--aside from designing the bike to not do this in the first place, or having it adjusted more regularly--is to shift gears slowly, preferably one at a time. If you must shift quickly, at least slow down near the end of the shift!

You mentioned that the chain is coming off when shifting under load. On derailer bikes, you really are meant to shift while continuing to pedal but letting up a bit on the pedals. Shifting under load is not advised. I can't stress this enough.

A big part of proper climbing technique is getting into the habit of shifting in advance to avoid having to shift under load. (If you're stuck in a difficult gear, sometimes you can coast for just a second while you shift. You'll lose a little momentum, but you'll avoid walking the hill.)

Mechanical causes

If none of these solve the problem, you may need to face that your derailer adjustments may not have done the trick. I'd bring the bike into a shop and have a pro wrencher adjust the derailers.

Cable stretch tends to come about with use more than time, but it could be a contributing factor. A derailer service will include adjusting the cables.

There are other possible causes, although they're less likely than shifting technique problems or derailer maladjustment:

Chain stretch is actually the wearing away of the insides of the metal links, and it only happens with use. While a stretched chain can increase problems like this, I suspect that chain stretch isn't an issue. However, it's at least possible that the bike came with an older chain, and it's so easy to check for chain stretch that it can't hurt to do so.

Similarly, worn gear teeth can also cause this, but this is extremely unlikely to be the cause. (Checking for those is a little more involved than checking the chain.)

It's also possible that the bike has the wrong size of chain on it, or that the chain is too tight (or too loose).

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Of course, most of those issues don't apply to a bike that probably has less than 500 miles on it. –  Daniel R Hicks Nov 10 '11 at 12:40
    
@Neil - I have updated my answer to cover some of your points. –  xiaohouzi79 Nov 10 '11 at 21:56
    
@xiaohouzi79 - By "slipping" do you mean that the chain isn't actually coming off the cogs? Because that's a different problem than the chain actually coming off the gears. –  Neil Fein Nov 10 '11 at 23:03
    
@NeilFein - Apologies for my lack of knowledge. It doesn't go up or down a gear it just crunches for a second like the feeling of the chain not sitting right on the gear (just my perception). It might do that once every 10 seconds while going up an incline, even if it is a slight incline. Every time I look down I can't spot it because I can't keep peddling hard while watching my feet. –  xiaohouzi79 Nov 10 '11 at 23:08
    
Just FYI for others reading this. I finally got my bike properly serviced a couple of days ago and they adjusted everything and mentioned that the rear derailleur was bent. It now works like a dream. –  xiaohouzi79 Jan 1 '12 at 11:18
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Assuming it's not been used it probably needs a touch of lube and some adjustment.

If your friend only rode it for a month he probably didn't even get the bedding in service done on it. New cables on a bike normally bed in after a little while (time depends on use) and so things go out of line fairly quickly initially which is why bike shops normally do a free first service, to tighten things back up.

The lube on the chain may well be fairly stiff by now so I'd also give the chain a clean and a lube to hopefully loosen it up. There are quite a few questions on this site about chain care which should hopefully help with that.

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Cable stretch... good point, +1. –  Neil Fein Nov 10 '11 at 9:06
    
Chain stiffening, good point! I wrote a whole answer about it. +1 –  heltonbiker Nov 10 '11 at 12:30
    
@heltonbiker a stiff link as you say is a definite possibility too. –  Colin Newell Nov 10 '11 at 13:26
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I would bet for the chain having rusted, leaving some links "locked" and not turning free between each other. Also, if rust built over the sprocket's teeth surface, it can cause the chain to skip under pedal load, bacause the chain doesn't engage properly.

I am telling you this because my damn lovely(#@%) cat pissed the cog cluster of one of my bikes, and when I noticed it, it was covered with rust. I cleaned the chain and the sprockets, but they were a mess, and there were some "frozen" links in the chain that turned very hard, even after oiling.

I solved the problem riding the bike. In the beginning, it skipped a lot in almost every gear, but mostly on the smaller sprockets. With frequent use, the problem is almost gone, because the rust over the sprockets' teeth is wearing off, and the chain is becoming loose again.

I am not saying for sure this is your problem, but the symptoms are very suggestive, and I think it is worth checking out. You can do this HANDPEDALLING BACKWARDS while observing the bike from the side, to see if there are sections of the chain which do not flex properly and tend to go awkwardly around the derailleur pulleys.

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If there is a stiff link you can normally free them by flexing the chain and using a spray lube. In the worst case you can use a chain tool to nudge the pin about to separate the plates too. Assuming the plates aren't physically damaged and you've cleaned off the rust with a wire brush that should be all you need to do. –  Colin Newell Nov 10 '11 at 13:21
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Besides what is discussed here it could also be that the back derailleur dropout or the derailleur itself has been bent. If that's the case you won't be able to set up gears properly until it has been aligned. Even it's a new bike it only takes once hitting the wrong place to bend it :(. You can find a number of videos on the net on how to align the derailleur hanger aka dropout.

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Another possibility (though unlikely) is that your jockey wheels are loose. I had a lot of problems adjusting my rear derailleur and couldn't work out why until it split open and folded itself up one ride :-)

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Just one more thing to check if all else fails. I recently experienced the feeling of chain skipping whilst riding, not necessarily applying a lot of power and with a new chain and rear sprocket. Every so often I would feel it skip and heard a CLUNG sound.

Turns out it was the freehub. One of the pawls would occasionally skip over a tooth inside the freehub. I tested it by spinning the sprocket backwards by hand and then engaging it forwards suddenly. I alleviated the problem by disassembling it and removing an excessive amount of lubricant inside.

I say this assuming you've only felt the chain skipping and haven't seen it, so apologies if you know it's definitely the chain skipping.

Good luck!

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My friend and I have identical bikes, bought as a double ride to work scheme. He leaves his outside and mine is kept inside. He started having this problem and replaced everything except the derailleur, which I think is over lubed and dirty and does not allow the tension to be constant on the chain causing it to slip. Every time mine starts slipping I degrease the derailleur, slightly oil it, and it is fine until it gets a bit dirty again.

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Welcome to Bicycles SE. We prefer posts on this site to be formatted into proper sentences and paragraphs. Please edit your answer to meet this standard. You can read more about writing a good answer here: bicycles.stackexchange.com/help/how-to-answer –  jimirings Apr 9 at 14:48
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