Chain-Suck :
see Main Article : Chain-Suck in a NUTSHELL
Restoring Tooth Pressure-Faces

by Jonathan Levy


WHERE & HOW TO FILE WORN CHAIN-RING TEETH

If you are suffering from chain-suck on your bicycle, it may suite you to restore the profile of worn teeth as best you can, rather than replacing the chain-ring. This can be a practical way to extend the life of the chain-ring.

This filing method might be adopted for a number of reasons such as cost, time constraints, intention to continue using a partially worn chain, lack of suitable chain-rings at the bike shop, being in a remote area, etc. A further advantage of the method is that neither chain, chain-rings, nor crank need be removed from the bike; the work can even be done on-the-trail, if necessary.

A reminder : This filing process is not about removing burrs from the sides of thickened teeth, although there is no harm in doing this at the same time ; such side-burrs are not normally a major contributor to chain-suck. The process is about reshaping the curved pressure-faces of the teeth.

For 1-ring suck the teeth of the single problem ring will need filing. For 2-ring suck, worn teeth may or may not be the problem ; if wear is the problem here, then the teeth on the ring from which the chain is being dispatched are the primary concern (usually the middle-ring), but it is also advisable to check the teeth on the receiving ring (usually the "granny").

While this method can re-shape the teeth so that they no longer interfere with disengagement of the chain, it restores neither a truly correct shape to them, nor a correct contact pitch. The previous mismatch in tooth/chain pitch will continue to result in fewer teeth carrying the load. Thus, teeth will continue to be overstressed and their pressure-faces will continue to wear at a greater rate (mainly by metal failure). The overloading can also affect the wear rate of chain-rollers and their bearing surfaces.

If you intend replacing the chain, don't use this method; rather replace or flip the chain-ring, so that the whole drive-train does not wear unduly fast.

Anyone who understands the issues and is reasonably handy, will be able to file the pressure-faces of the teeth to a profile which avoids chain-suck.

The key to understanding where and how to file is that, after re-shaping is complete, the indentations in the pressure-faces must not remain, and these faces must not be too steep. For these reasons, it is mostly the "upper" portions of the teeth which must be filed, ie farthest away from the root of each tooth.

Thus, remove material from the pressure-face of each tooth above the point where the middle of the chain-roller contacts it, and increasingly more material towards the tip of the tooth to eliminate any "overhang" or "hook".

With the crank horizontal, the bike upright, and viewed from the side of the bike where the chain-rings are mounted, the teeth that need the most filing are those located from 4-to-7 o'clock and from 10-to-1 o'clock. These teeth are usually the most worn (or indented) because they were stressed (when at the top of the chain-ring) by the highest pedalling loads in the crank cycle, and correspondingly also need to release the chain (when at the bottom of the chain-ring) under the same high crank-cycle loads. Nonetheless, it is still important to examine all the teeth to identify others with significant wear. For 2-ring suck, a wider range of teeth can cause the problem, especially on older design chain-rings. It is best to mark all the teeth identified as worn, with a marker pen or other means.

If the crank and chain-rings remain on the bike, the filing process is awkward. It is easier to do from the side of the bike opposite the chain-rings, and with the bike upside down. Teeth on all chain-rings can then be accessed with less interference from the crank-spider and other chain-rings.

When filing is done on-the-bike, and where the file cannot fit through the gaps between rings and spider, you will need to use very short filing strokes with the tip of the file, which can be time consuming. To make it a bit easier to file through the gaps, use a square or very narrow file. If a broader file is used, it may be desirable to improve access to the "granny-ring" on triple-ring bikes by undoing the middle ring (if removable), then manouvre it past the "granny" and allow it to rest on the BB out of the way. If the middle ring needs filing, removal of the large ring would help, but this is sometimes riveted in place. On double-ring road bikes, it is usually easy to remove the large chain-ring and gain free access to the small ring.

It is best to use a small, fine, narrow, flat file with its thin edges smooth. File with the smooth edge nearest the tooth root (to prevent yourself filing in this area). Use your your thumb or other means to guide the filing. Make sure that you file "square" to the tooth cross-section, so that subsequent pressure by the chain rollers is not angled across the teeth.

To even out tooth wear, further advantage can be gained by rotating the chain-ring by one bolt (i.e. 1/4 or 1/5 turn) on the spider. If the chain-ring has special short teeth or other attachments to ease gear changes, then rotating the ring may result in less crisp gear changes ; it can also worsen (or alleviate) 2-ring suck. If either of these problems occur, it is easy to rotate the ring back to its original position.



[thanks to Chris Juden of the CTC (Cycle Touring Club - UK) for bringing this method to my attention ; the above description is based on my experiences with it]


Copyright Jonathan Levy, 2000. All rights reserved.


For the related articles, follow the links below ...
Chain-Suck : in a NUTSHELL ...comprehensive summary and solutions
Chain-Suck : the OVERALL picture ...details of mechanisms & solutions
Chain-Suck : the DETAILED Investigations ...field & workshop testing
Chain-Suck : the FIELD TEST ...a field testing method for identifying causes
Chain-Suck : Restoring Worn Tooth Pressure-Faces ...by filing them
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