Fettling

There are some pretty in depth articles on chain suck around such as this one so I won't go into too much detail here.

Chain suck is when the chain doesn't drop off the bottom of a chain ring (usually middle or granny) when you are pedalling. This usually results in you stopping abruptly and swearing! It can also damage your frame and / or drivetrain.

In my experience it is nearly always caused by a combination of worn drivechain and wet muddy conditions.If this is the case read on. However if your drivetrain is new this article will not be of any use.

A Shimano XT chainset with worn middle ring is used in this example. The best solution to a worn chainring is obviously to replace it. If you do replace it you might also want to replace the chain and cassette if the chain has stretched.

However the following procedure will keep things going for a good while if funds are short, the shops are shut or you want to wait 'til summer before replacing bits.

1. Clean the drivetrain especially the chain. Remove the drive side crank bolt. In this example the bolt is self extracting so the crank comes off with the bolt. Often a crank extractor will have to be used to remove the crank after the bolt has been removed.

 

2. Undo the 4 bolts holding the granny ring on and remove it. If the granny ring is causing chain suck it can be filed as below or it can be reversed as it doesn't have any shifting ramps.

3. Undo the four bolts holding the middle and outer rings to the crank. You might need to stop the chainring nuts from turning using either the proper tool or a flat bladed screwdriver.

 

4. Examine the chainring for wear. Some of the teeth will probably be hooked. They will be shaped like waves about to break. These are the main cause of the problem. Sometimes only a slight hooking combined with the right conditions can be enough to cause chain suck.

5. File the hooks off the teeth. Go around the chainring a tooth at a time. Not all will necessarily need filing. As the chainring is alloy it does not take much filing to take the hooks off but this job is a little tedious. A vice is handy to hold the chainring but don't clamp it up too tight or else you may damage the shifting ramps or mis-shape the chainring.

 

6. Re-assemble the chainrings. Use copper grease on all the bolts. Make sure you line up the pin on the outside of the largest ring with the crank (the pin stops the chain wedging between the crank and rings should it come off). The other two rings also have a marker (a small "tooth" inside the ring) to line up with the crank.

7. Grease the end of the bottom bracket axle if it is splined or octalink (like this one). Leave square tapered axles dry. Replace the crank and tighten the crank bolt fully.

8. Lube the chain thoroughly and test it out on a wet muddy ride!