Go Back  Bike Forums > Bike Forums > Bicycle Mechanics
Reload this Page >

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

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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

Old 06-09-14, 02:59 PM
Junior Member
Thread Starter
Join Date: Mar 2013
Posts: 7
Likes: 0
Liked 0 Times in 0 Posts
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.
Attached Images
zendot is offline  
Old 06-09-14, 03:21 PM
Senior Member
Join Date: Apr 2013
Location: SoCal
Posts: 6,496
Likes: 0
Liked 6 Times in 3 Posts
If you are running a 1x5 set up , you may need to install a chain keeper up front to prevent that from happening
rms13 is offline  
Old 06-09-14, 03:55 PM
Senior Member
bikeman715's Avatar
Join Date: Mar 2008
Location: Salinas , Ca.
Posts: 2,646

Bikes: Bike Nashbar AL-1 ,Raligh M50 , Schwinn Traveler , and others

Liked 2 Times in 2 Posts
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 .
bikeman715 is offline  
Old 06-09-14, 04:02 PM
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 39,142

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

Liked 2,962 Times in 1,642 Posts
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.
Chain-L site

An ounce of diagnosis is worth a pound of cure.

Just because I'm tired of arguing, doesn't mean you're right.

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FBinNY is offline  
Old 06-10-14, 02:49 AM
Junior Member
Thread Starter
Join Date: Mar 2013
Posts: 7
Likes: 0
Liked 0 Times in 0 Posts
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.
zendot is offline  
Old 06-10-14, 02:40 PM
Junior Member
Thread Starter
Join Date: Mar 2013
Posts: 7
Likes: 0
Liked 0 Times in 0 Posts
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.
zendot is offline  
Related Topics
Thread Starter
Last Post
Singlespeed & Fixed Gear
10-04-13 09:21 AM
Singlespeed & Fixed Gear
04-11-13 08:15 AM
the accountant
Classic & Vintage
09-21-11 10:44 AM
Singlespeed & Fixed Gear
05-20-10 12:05 PM

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Your Privacy Choices -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.