BioWheels MTB Tech: Anatomy of Shiftsuck

Everyone with a mountain bike has heard of chainsuck…. how about shiftsuck?

Chainsuck can potentially stop you cold in your tracks. Excessively worn chainrings develop small burrs that effectively thicken the chainring - causing the chain to stick to theses areas and not release. The accumulation of mud on the chain and rings accentuate this condition. The suck part is your chain getting “sucked” up into your chainstay, either jamming or causing you to have to backpedal to get the chainring to release the chain.

A lesser known condition, Shiftsuck is chainsuck that occurs while downshifting from middle ring to granny, and can occur on brand-new drivetrains. Without sufficient clearance between chainrings and chainstays, shiftsuck can lead to your chain jamming into your bike’s chainstay - eventually leading to chainstay failure. Below are four photos showing the progression of shiftsuck. The first two show the underneath of the chainrings: in the photo the chainrings would be rotating backwards, and the chain is transitioning from the middle ring to granny gear.

Photo A: chain attempting to release middle ring and complete shift onto granny gear

Photo B: as chain comes off middle ring and heads back towards rear derailleur, it rotates clockwise.

Photo C: chain is snagged by tab in XT big chainring before being carried into chainstay of bike.

Photo D: closeup of XT large chainring with tabs that are catching chain during middle ring downshift

Fix: if you are experiencing shiftsuck on your bike, there are two fixes. First, minimize your pedaling pressure during downshifts, and try and anticipate shifts as early as possible on the trails. Mechanically, a strategic filing down of the edges of the outer chainring tabs will reduce or eliminate the occurence of shiftsuck.

Mitch Graham
BioWheels
Cincinnati