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Chain Suck Sucks!

What is “chain suck”? When the chain gets stuck on the large chain ring while down shifting, doesn’t release at the bottom of the rotation and hence gets “sucked” up between the chain rings and the chain stay. This is typically nothing to do with the front derailleur adjustment. The FD adjustment can only be part to blame if it is very slow at shifting down, hence

the rider will slow dramatically when easing off the power to shift, so they keep powering hard and then when the shift finally happens, the chain gets stuck while leaving the big ring.

 

So, what causes chain suck?

 

There are four main possible causes:

1. Dirty drivetrain - more common on MTBs but also possible on road bikes. Grit and grime on the chain and cogs/chainrings can clog the spacing between the chain and the teeth, hence not allowing the chain to release efficiently from the teeth during a shift.

 

2. Worn drivetrain - chains wear out, chainrings and cassette cogs wear out, worn chains exaggerate wear on rings/cogs, more commonly used gears wear faster. Uneven wear between chain and chainrings is also a common cause of chain suck. Shimano chains and cassettes typically require replacement around 2000-4000km (or 1% of wear).

 

3. Shifting under load. Please see the images below for a more detailed explanation.

Please note:

a. Red circles highlight teeth that are being torsionally loaded at the end (rear derailleur - RD) or beginning (front derailleur - FD)

of the shift.

 

b. FD shifts take longer than RD shifts. This is because there are more teeth per rotation of the ring/cog, so a longer distance to travel. Also, the distance between chain rings is greater than the distance between cassette cogs. Hence, power needs to be backed off longer during a front shift.

 

c. FD big-> small ring shifts can cause more damage than small-& big ring shifts. FD shifting up under load mostly strains the FD itself. Once the first big ring tooth is engaged, the next follows smoothly and the load is then shared over each successive tooth. FD down shifts however leave the final big ring tooth torsionally loaded for up to a full half rotation of the cranks (depending on when the chain engages with the small ring teeth). Note, even when the chain is engaged with the small ring teeth, there is almost definitely still tension on the chain to the point where it remains engaged with the final big ring tooth. Also, the inner edge of the big ring can be under pressure from the chain due to the sharp angle ti makes toward the small ring.

 

d. During any of the shift period, excessive load through the drivetrain (pedals to cranks to chainrings to chain to cassette) will potentially damage the front derailleur, chain links, cassette teeth, chain ring teeth or chain ring shift ramps.

 

4. Chain suck has happened before (due to #1 or #2), and now either the chain or chainrings are damaged (burred or bent) - it will keep happening unless the damage is repaired.

 

How to prevent chain suck:

1. Clean the drivetrain - regularly. Use a dry wax based lube (Squirtz is awesome) which does not attract dirt or road grime onto the chain.

 

2. Replace parts of the drivetrain when they are worn. Be careful about chain ring wear when replacing chain and cassette.

 

3. Do NOT shift under load. N.B. Di2 shifts are so fast and smooth that it can be tempting to shift under load, because 99% of the time you don’t even feel or hear that destructive crunch as the last link of the chain gets ripped away from that individual tooth of the cassette or chainring, under 500 watts of Strava KOM stealing glory. However, don’t be fooled by this fine engineering, during each shift the power you’re outputting ends up being transferred through one tooth and one link!

 

a. How do I down shift when climbing? Put in a couple of hard pedals before the shift, then as you shift, back off the power momentarily until the shift is complete, then drop another watt- bomb. If the shift is taking a long time, then the derailleur is not adjusted correctly, or the drivetrain is worn out.

 

4. Once the chain or chainring is burred or bent, it is likely to keep chain sucking, even when shifting correctly with reduced load through the pedals. The chain and chainrings should be carefully inspected; any burrs should be filed off, bent parts should be straightened or replaced.


 

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