Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  

Go Back   > >

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.

User Tag List

Thread Tools Search this Thread
Old 07-21-10, 06:17 AM   #1
Junior Member
Thread Starter
Join Date: Jul 2009
Posts: 6
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Unintentionally chainring shifting

Hey guys,

I ran into a problem where I noticed that when in the big chainring (53t ultegra), and shifting to the lower end of the cassette (19t or 21t for example), my chain has a tendency to drop down to the smaller chainring . I know that cross chaining isn't advisable however this is still a few cogs from the lowest gear. Is this possibly a chainline issue or are there any other things I can try?

gzhelnin is offline   Reply With Quote
Old 07-21-10, 07:25 AM   #2
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
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
Posts: 35,304
Mentioned: 105 Post(s)
Tagged: 1 Thread(s)
Quoted: 3646 Post(s)
It might be a chainline issue, so start by measuring that. If you're not sure how here's a guide to help you.

One other thing to check is how the chain feeds onto the chainring. The chain should be able to stay on the chainring coming from any cassette position except maybe the most inboard one or two (max.). If the chainline is per spec, trim the FD so it doesn't touch the chain at all, and watch to see how the chain is feeding onto the chainring. Odds are it's catching on the first tooth after the shift gate, but it could be catching on any tooth. that;s because when the chain feeds from an angle the upcoming teeth touch the inner plates rather than finding the gap between them.

There are fixes. One is to find a chain with more bellmouth formed into the inner plates, forming a wider funnel to meet the teeth and guide them in. Another is to move the points of the teeth, or at least the one after the shift gate more inboard. You do this by filing the outer tip at the same taper angle as now, so the point is a bit more inboard. For all the teeth you can lay the file against them and spin the crank, for just one, use a small file and do a light trim.

Before you try a radical solution, make sure you've exhausted all others. If it's just marginal for example (your's seems worse) sometimes better chain lube helps. One thing LBS mechanics do is trim the FD so the cage pushes the chain over and guides it on. I consider this unacceptable since chain tension will strain the FD when climbing, and eventually the chain will saw it's way through the cage.

Good luck.
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“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   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

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

All times are GMT -6. The time now is 06:43 PM.

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to and affiliated sites.
  • Ask a Question
    get answers from real people!
Click to start entering your question.