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
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.