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:
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?
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:
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?)
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?)
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?