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Chain Falls Off Front Cog Pedal-Side When 5-Speed Freewheel on Small Cog

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Chain Falls Off Front Cog Pedal-Side When 5-Speed Freewheel on Small Cog

Old 06-09-14, 02:59 PM
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Chain Falls Off Front Cog Pedal-Side When 5-Speed Freewheel on Small Cog

Just finished putting an old bike together with a mix of new and NOS components. As per the title of this post, after shifting down and pedalling once or twice around, the chain falls off the front cog pedal-side when the 5-speed freewheel is on the small cog. All the other gears work OK. To my eye, the middle rear cog is aligned reasonably well with the front cog. The front cog is new and cheap, but the closest thing to the original I could find. The freewheel is NOS. The chain is an 8-speed KMC. I read (not on this forum) that I could use up to an 8-speed on this 5-speed without a problem, and it was a nice silver colour . Could the front cog be too close to the bottom bracket? Could the freewheel be too far away from the wheel? Could the chain be too narrow, too short or too long? I have a week or so to sort this out before a "vintage" bike race. I'd be happy to spend money to replace any or all of the components, but I don't know where to start. Any help very gratefully received. Photos attached. Thanks.
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Old 06-09-14, 03:21 PM
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If you are running a 1x5 set up , you may need to install a chain keeper up front to prevent that from happening
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Old 06-09-14, 03:55 PM
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Yes it could be the chainring ( front cog ) is to close to the bottom bracket . You need a longer bottom bracket axle . You need to check the chainline . If the chain is on the 3rd cog in the back and a straight line to the front cog you are go to go . Park Tool Co. » ParkTool Blog » Chain Line this link might help you out with the chain line .
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Old 06-09-14, 04:02 PM
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Look down at the top of the chainring right where the chain is engaging (behind the top center). Odds are that the tips of the teeth are just under the inner (toward the bike) inner plates. (outer plates always clear).

There are 2 fixes for this.

1- look for a chain with inner plates bent outward between the outers or with the inner edges beveled, so as to makesort of a funnel to pick up and guide the teeth in as the chain engages.

2- move the points of the teeth outward with respect to the meshing chain. The sides of the teeth hold the chain, so you move the points out by filing at a shallow angle on the inner faces. You don't have to do much, and should do this by degrees testing until the problem resolves. Note that going too far, can worsen chain pickup when the chain comes from the opposite side (but you'd have go way too far).

Since you already have a chain, and time is critical, it's time to man up and grab the file.
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Old 06-10-14, 02:49 AM
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Thanks everyone. I'm going to see if I can fix it without resorting to a chain keeper. Although I don't have anything to measure it accurately, it looks to me like the chain line is pretty much spot on when the chain is on the third of the five rear cogs, so I'll not change the BB axle for now. I bought a new chain this morning (SRAM PC 850) which looks like it has inner plates bent outward. I'll put that on tonight and if that doesn't fix it I'll file the teeth. I'll aim to let you know how I get on tomorrow.
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Old 06-10-14, 02:40 PM
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Success! I swapped my 8-speed KMC X8 99 chain for the 7/8-speed SRAM PC 850 and the problem went away. Thanks again for your help everyone.
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