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I have a Dahon Speed P8 that I use for touring Japan, which has about 3000km on it. The bicycle has 8 gears on the back, and 1 gear on the front.

Note: I love riding my bike, but honestly (and shamefully) I'm not very proficient at mechanics, so I'm not very knowledgeable either in bicycle part terminology or actual repairing. Please have some patience with me if I don't properly use or understand specialized terminology

Recently, while riding it, I noticed sometimes, that the gears don't shift as perfectly as they used to, and when shifting to a high gear (7 or 8), sometimes the chain falls off the chainring to the outside of the bicycle, to be caught in between the chainring and the bashgard. There is just enough room for the chain to get stuck in there as I show in this image:

Gap

I noticed that I can temporarily fix this problem by either stopping and manually engaging the chain back, or by shifting to a lower gear and keep on pedaling for a few spins. The chain will then go back into gear, only to fall off again after a few kilometers.

My guess is that the back shifting mechanism is not properly aligned, and may pull the chain way too much to the outside. I think this is a very precise piece of machinery, so I took the bicycle to my local shop where I explained the problem.

They put the bicycle on a stand, and tried shifting, but the gears worked perfectly. I even tried shifting it myself, and it worked perfectly. I took the bicycle back, only for the chain to fall back off on a ride I did last Saturday.

I have no idea what to do. The chain falls off, and I'm worried that this may hurt the chain and the bicycle itself. I want to have my bicycle in perfect condition for a week long ride I have planned in about a month, but I can't get the problem to show up in the shop, so they don't know what to do to solve the problem.

I once changed the chain about 6 months ago, and apart from other minor parts (pedals, standing foot and so on), I have not changed a single other part of the actual mechanisms.

I would like to know:

  • Is the chain falling off harmful to the chain/bicycle?

  • Am I damaging the bicycle by reengaging the chain by switching to a lower gear as I described above?

  • What could be the cause of a problem like this? Is this something I can easily fix myself?

  • Is there anything I can do to make it more likely for this problem to happen, so I can reproduce the problem in front of my bicycle technician, and for me to know that the problem is actually fixed before leaving the shop?

Update:

I took the bicycle to a different shop, and they told me that both the front crank, and the arm in the rear gear mechanism had developed bends, and that was causing the problem.

So I ordered two brand new parts for both the rear shifting mechanism and the front crank. (I was surprised that the front crank was 4 times as expensive as the rear shifting mechanism, even though it has no moving parts)

The bicycle initially worked perfectly, as if it was brand new. However, a few days later, the same problem started to recur. Shifting is not perfect anymore, and the chain falls to the right just as before.

Today I took it to the same store, and they are now claiming that:

  1. This model has an imperfection which makes this problem happen (if so, then why was I able to ride the bike perfectly for three years before this started to happen?)

  2. I am somehow shifting it wrong, and causing this problem to happen (once again, why was I able to ride 3000km with no problems, while doing mountain passes, which require much more shifting, yet now I'm having this problem again just by riding 800m from my house to the train station on pavement?)

Update:

I got my bicycle back, with yet another hypothesis of what could be happening.

The technician told me that he contacted Dahon support, and they recommended to flip the front cog. It seems like the cog itself is not symmetrical on the sagittal plane, and is slightly slanted towards the inside of the bicycle.

Flipping the cog turns these slants to the outside, make it considerably more difficult for the chain to fall to the outside, although it makes it easier for the chain to fall to the inside.

However, the back cogs are mostly biased to the outside, which means that only in the lowest gear setting the chain will be pulled to the inside, making it less likely for the chain to actually fall to the inside. Additionally, there is a built in plastic stopper on the inside (which can be seen in the picture above, like a shark fin), which helps prevent the chain from falling to the inside.

The technician also explained to me that in folding bicycles with small (20 inch) wheels like these, the back cogs have to be at a lower height than the front cogs, and that this makes the chain move more when shifting, so he recommended me to try to pedal a bit slower when shifting, especially into the higher gears, where this is most likely to happen.

Does this make any sense?

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    It's actually quite common that you can't reproduce certain problems with the bike on the stand. The should should take the bike out for a test ride or put it on a trainer (might be difficult with a folding bike) in order to get a better idea of what's happening with the bike. The frame flex (especially on folding bikes), and other parts can move in ways which don't happen when there is no load on the components – Kibbee Jun 22 '15 at 14:33
  • "to be caught in between the cog and a circular plate" Your chain should not be able to get caught in there. That circular plate is called a Bash Guard (it may be called something else but bash guard is pretty common vernacular). A properly positioned bashguard will be outboard far enough to not rub the chain when in the smallest cog but no further. It sounds like it's further. One way to produce the problem - you need to pedal with the bike on its side - drive side down. Pedal and shift like this and the chain should fall off. A picture of that space can help, are there spacers in there? – jqning Jun 23 '15 at 0:52
  • @jqning I posted a picture – Panda Pajama Jun 23 '15 at 11:59
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    @jqning I would guess the purpose of the guard on the outside of the front sprockets is to keep trouser legs clean rather than keep the chain from falling off. Anyway, the primary line of protection against is the limit screw settings. A had a Dahon in Japan - I think they are good bikes. Rode up Fuji Subaru line from Kawaguchi-ko, hiked Fuji, then rode back down again. It was lot of fun. – Craig Hicks Jun 26 '15 at 18:27
  • @CraigHicks there is no front derailleur on this bike. – jqning Jun 26 '15 at 18:34
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Carel misses the point that there is no front derailleur so it's definitely not a limit screw issue on the front derailleur! It's also highly unlikely the chainring is worn from just 3,000km. It's also unlikely to be the chain given that you changed it not that long ago. Finally, limit screws very rarely come out of adjustment on their own.

I think people are failing to recognize that this is a folding bike and that you've been touring on it. I would unhook the chain from the front chainring and carefully inspect the teeth on the chainring. It's very possible that one of them has been bent and is throwing the chain off the chainring. This is more common on folding/travel bikes. It's fine in most of your gears because the chain running at a fairly straight angle (when you look down at it). When you hit your outer cogs you'll see the chain is running at more of an angle. If one of the chainring teeth are bent even a little they'll start to catch the chain and keep it from fully seating on the chainring and instead will derail the chain.

Here's a good way to check this: Next time this happens try to make note of where the crank arm is pointing when the chain comes off. Is it in the 3 o'clock position? 9 o'clock position? Now put the chain on and start riding again. Try to replicate the problem. Was the crank arm in roughly the same position as before? If so it's almost certainly a bent tooth on the chain ring. The fix is to carefully file the burr on the tooth so it no longer catches the chain. You'll probably want to let a competent, trained mechanic handle that.

Finally, check the cable tension. If the shifting is consistently off through all 8 gears than the cable that runs from the shift lever on the handlebars to the rear derailleur (the thing at the back of the bike that actually moves the chain) has developed some slack or stretch (though technically this isn't true). This is a really easy, tool free fix. Where the cable loops around and enters the rear derailleur you should see a small knob. Turn it 1/4 turn counter clockwise. No see if the bike shifts more smoothly. If it's a little better then you've found your problem. Give the knob (called a barrel adjuster) another quarter turn and then try shifting through all 8 gears again. Repeat this process until the gears are shifting smoothly through all 8 gears in both directions. Key here is don't turn the barrel adjuster more than 1/4 turn at a time.

The final likely issue is the small metal tab on the frame that the rear derailleur bolts to. This is called the derailleur hanger and it's very easy for it to get bent inwards (never lay your bike on it's right side!) If it's bent even a tiny bit it can make it impossible for the derailleur to properly line up with each cog on the rear wheel. Only real way to check and fix this is with a derailleur hanger alignment gauge which is a special purpose tool. Every bike shop will have one and it only takes a few minutes for them to bolt it on your bike and check if the alignment is correct.

  • I bought brand new front and rear cogs. Except for the chain, the entire shifting mechanism is completely new, yet now I'm having the same problem. I posted an update in the original question. – Panda Pajama Jul 28 '15 at 4:37
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Your chain or, less likely, your chainring (what you call front cog), could be worn. That would be a little early and could indicate less than optimal maintenance (keeping things clean and lubricated), but wear depends highly on circumstances (rain, mud, velocity).

It's simple to measure chain wear: they lengthen in use. Just measure the length of 10 complete links, they are nominally 254mm. A common rule is to replace it once they reach 256 mm.

It's not so simple to measure chainring or cog wear, but very worn ones resemble shark fins - if yours get in that direction, you want to change them. But you can sure ask your mechanic what s/he thinks specifically.

  • There are simple cheap tools to measure chain lengthening. Tools to check chainrings exist but not on the cheap side. If your chain keeps falling off check the limiting screws on the front derailleur. If the rear derailleur doesn't keep the proper tension (weak spring) the chain may fall off when shifting at the front on a bumpy surface. – Carel Jun 22 '15 at 14:37
  • search for "chain wear tool"- online from $5, in store from $10. Every bicycle store has these. Very simple to use. – Craig Hicks Jun 26 '15 at 17:46
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So to first answer the direct questions you asked:

  1. The chain falling off the front chainring will harm the chainring and/or the chain only if you are pedaling hard at the time it falls off.
  2. Getting the chain back on the ways you are describing shouldn't harm any part of the bicycle as long as you aren't pedaling hard as you shift, which I'm sure you're not.
  3. See below, as I think this is the main question.
  4. The mechanic would need to take the bike for a ride to prove to himself that the chain comes off as you describe. I'm not sure this is necessary though, as your description should be enough to start the fix attempt.

Some background that you might already understand is that 20" wheel bikes usually have shorter chainstays (the frame tube that connects where the crank bearings are to where the rear wheel attaches) than more conventional bicycles. This is true on your Dahon. This means that as you shift the gears in back, the angle that the chain makes as it engages the front chainring is sharper than if the chainstays were longer. This is a bit hard to visualize maybe without a picture. This is the best I could find.

enter image description here

Maybe you can imagine that if the rear cogs were further from the front chainring that the angle the chain has to make to engage the front chainring would be less. I doubt this has changed any since you got the bike so it is not the direct problem, but it could be making it worse.

Another issue that is evident from your description is that your bike has bad chainline. Ideally the front chainring should line up in between cogs 4 and 5 in the back (for an 8sp), but it sounds like the front chainring is too close to the centerline of the bicycle and probably lines up with cogs 2 or 3 or somewhere in there (with the largest cog being cog 1). This can be fixed by changing the spindle for the cranks to one that is longer and places the crank (and therefore the chainring) farther from the centerline of the bicycle. That might fix your problem, but sometimes getting the right spindle takes a bit of work.

The mystery of course is why it worked for so long then stopped working. My guess is that one of the teeth on your front chainring is misshaped, maybe by hitting a rock. Careful inspection and gentle filing might fix this. The mechanic's suggestion to reverse the chainring could work as well, as it puts the more ramped side of the teeth where it needs to be to make up for the bad chainline.

Another possibility is that the first time this happened it was just bad luck, but with the chain getting in between the chainring and the chainguard (bashguard), the chainguard was bent outward enough to allow the chain to come off more easily. If you look at the chainguard and see any bending near where it bolts to the crank, I would suspect that this is the case. Certainly the chainguard would be more effective if it were closer to the chainring, so if you can think of a way to bend it a little bit without bending the chainring, you might try that.

If you find a mechanic who will work with you, you could also put the chainring in the outer position of the crank and use spacers to get the chainguard the correct distance from the chainring. This would improve the chainline and get the chainguard closer to the chainring where it would be more useful.

One less likely possibility would be a tight chain link, especially if the chain comes off more when you are pedaling softly than when you are pedaling hard. I think the mechanic might have already checked this, but you could check it yourself by getting where you can watch the chain as it snakes though the rear derailleur. Rotate the pedals backward slowly, then more quickly, with your hand and you should see the chain go smoothly through the derailleur. If there is a tight link you will see it. If you find a tight link, inspect it to make sure the metal looks okay, then (if it's okay) gently bend the chain sideways to loosen the link.

I don't think the rear derailleur is to blame for any of this as long as it is shifting correctly. The rear derailleur spring tension has very little to do with the top part of the chain loop when you are pedaling.

Good luck!

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Why not just move the chainring outward a bit with a bottom bracket spacer (comes between bottom bracket cup and bicycle frame, effectively moving pedal and ring outwards). Won't address the cause of the issue but may solve the symptom.

  • Maybe it could help, but when the bike's in a low gear (largest cog on the block) the cross chaining effectively will pull the chain off the inside. 20" folders have short chainstays so that exacerbates chainline issues. – Criggie Sep 6 '18 at 10:56
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I had a vaguely similar issue, and mitigated it by filing a couple of millimetres off the plastic bash guard.

I unscrewed it from the chain ring, either 4 or 6 small screws, and then applied a flat file to the flat surface that was pressing against the chainring. Could have filed more off, but I didn't want the screws to bottom out in the screw holes.

You could get the bash guard as close as just clearing the chain when its in the smallest cog.

Also, if your bash guard is a bit flexible or sagged with age, fitting a new replacement one might be easier than finangling the existing one.

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As Carel says - there are limit screws to prevent the chain from going out of range. It is important to set those limits correctly or the chain will fall off. Did the LBS not check the limit settings?

Even with a new chain and new correctly set derailleur, the chain can and will jump off the sprocket if the limit screws are not set properly. In the high gears (7 and 8 as you say) the chain will inevitably have more slack. If the limit screw is not set, the front derailleur will momentarily go too far out when you change the front gear from lo to hi (inside to outside).

It is also true that any of chain, cassette, sprocket, derailleur, etc., could be involved - when any component is not in good condition the margin of error for the other components may be exceeded. So yes you should check your chain as the chain is usually the first component to wear out, especially if it is not cleaned and lubricated thoroughly every N miles (or every M months whichever comes first). If your chain is already worn out, then you will need to replace the cassette as well. Front sprockets can usually survive longer being used with a worn chain, but the cassette is usually a goner.

  • Thank you very much for your answer. I am not familiar with the term "LBS". Could you please clarify its meaning? – Panda Pajama Jun 25 '15 at 8:34
  • LBS = Local Bike Store ! – Carel Jun 25 '15 at 10:07
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    There is no front der on this bike. – jqning Jun 26 '15 at 18:32

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