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I have a Faulkner folding bike and I notice often the 6th gear, high gear falls off switching to high gear. Someone mentioned that the cog might be a bad choice for the front of folding bike and may be why it has the plastic guard around it - not just for keeping it off your pant leg.

It's generally been falling off and pulling the guard off, which it sticks in now... the deraileur high setting seems right as I moved it a bit and had other problems... it stays in the right gears adjusting it back.

Should I take the cheap plastic guard halves and the little screws screwing them together and replace with screw+bolts to make it more secure, maybe insert a metal shim? or replace the whole front crank? I found this but it doesn't list a place to size and purchase replacement front cog. https://www.downtube.com/pages/convert-to-triple-chain-ring-should-you-do-it/

EDIT - Here's a picture of the problem crank: Front crank

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  • I'm not sure what you mean. Are you saying that, when you switch to sixth gear (the smallest cog on the back), the chain falls off at the front? The chain guard is only to keep your pants clean and has no mechanical purpose. – David Richerby May 25 at 10:22
  • Yes. Many times it shifts just fine though. I just noted that even on a much more expensive folder it lists in features "Plastic chain ring cover to keep dirt off your chain (It also keeps the chain from falling off.)" - so I suppose this is a common problem in folding bikes. downtube.com/9s-front-suspension-9sp-commuter-folding-bike – NoBugs May 25 at 14:26
  • I'd guess that the chain is too long or the spring tension of the RD is too weak. – Carel May 25 at 18:54
  • I suspect that the chain is worn out. – Daniel R Hicks May 25 at 23:03
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    Then why hasn't it been returned to the place where you bought it? – Daniel R Hicks May 25 at 23:42
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If a new chairing is the solution, then the best chainring to solve your issue is a 1× specific narrow-wide chainring. I'm not able to identify your crankset/chainring interface from the Amazon link. Once you know this, you can see if there's such a chainring available or if you'd need to replace your crankset entirely (probably too expensive to be worth doing on this bike).

I would also be tempted to check if you can shorten your chain by one link. This would mean that your rear derailleur would hold the chain on the chainring more tightly.

  • If its like mine, the chainring will be a cheap stamped steel 38 to 42 tooth, and it will be welded to the right side crankarm. Your idea is great but will be pricey. – Criggie May 26 at 0:23
  • 1 link shorter, along with some adjustment seems to have done it. It seems a little noisier though with the 6th gear adjusted to be exactly below the 6th gear, adjusting "high" screw on derailleur. Not seeing anything rubbing bad and it shifts into all gears, though sometimes a little slowly on the 6th, highest gear. I guess cheapie folder bikes may come with too long a chain from some random full size or youth full size bike by whoever puts them together? – NoBugs Jun 7 at 14:31
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I believe folding bikes are more susceptible to having the chain come off the chainring because the seat stays are relatively short. When on the largest or smallest sprocket in the rear, the chain arrives at the chainring at a greater angle than on a regular bicycle, making the chain more likely to climb the chainring teeth. If there is no front derailleur cage guiding the chain the chain can come off the chainring.

A very dirty or worn chain may be more susceptible to coming off. I'd also check the chain tension when on the smallest rear sprocket. The chain could be too long. Is the derailleur cage at the limit of its movement leaving the chain slack? The derailleur springs could also be broken or weak. You can check this by again shifting to the smallest rear sprocket and pulling the cage forward by hand. There are two sprung pivots, one at the derailleur attachment bolt and the other at the cage. Does the spring tension at either of these feel weak?

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It looks like a 20" wheel so the stays are incredibly short compared to a 26" MTB or a 700c road bike. I have a very similar-looking folder that started as a 1x6 with 14-28 tooth freewheel, and group parts topped out at tourney level.

To improve the ratios I put on a triple front crank which was rescued from a donor MTB. I built an 8 speed freehub into the original rim, and fitted an 11-32 tooth 8 speed cassette, and upgraded to deore V brakes/levers. I had to cut the freewheel with a grinder to get it off, cos they hadn't used a standard fitting or any grease.

Since the frame doesn't allow for a front derailleur, I shift at the front by hand. As a consequence, the bike can drop the chain pretty easily.

FIRST - before you get stuck in, try modifying your pedal stroke. Try pedalling in circles and keeping an even tension on the chain not riding like an 8 year old all choppy. This will help by keeping the incremental change per tooth smaller as it spins the chain onto the teeth.

SECOND I suggest that you start by cleaning the chain and cogs and chainring. Check the chain for wear, and eyeball the chainring for wear (should be fine given the limited mileage).

You should also eyeball the chainline - look down from above and the chainring should be in line with about the middle of the cogs. It could be biased a little to the outside, but should not be outside the lines of the cogs.

Also, ensure your chainring is flat too - if the chain always drops at one point on the rotation then it could have "a divergence from the plane" Without the chain on the teeth you should be able to spin the cranks and see the teeth all in line from above, with no/minimal wobbling side to side. Small areas of deflection could throw the chain.

THIRD put the chain on the biggest cog, and look at the bottom chain run - it should be pretty forward. This is testing whether you can remove one or more links to help add tension to the chain in the smaller cogs.

FOURTH take that broken chainguard off and try to fix it. It should not have a glaring gap as per your photo so maybe the chain coming off has broken it. The chainguard does help somewhat keeping the chain on the chainring, but its more for reducing the trouser-cuff contact with the oily chain. Given this bike is a 1x you could fit a chainguard ring that stands a little taller on the outside.

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