This summary provides a glimpse into the substantial investigation done to unravel this frustrating problem.

Numerous field and workshop tests were involved. Causes and mechanisms were confirmed and practical solutions were found.

WHAT IS IT ?

Chain-suck is THE major issue when you experience it. Pedal action comes to an abrupt binding stop, and you fall over into the dirt. It can be tenaciously difficult to eliminate during a ride and keeps recurring once initiated.

It usually occurs when the chain and cogs get muddy or wet and gritty (but can also occur in clean conditions), and is worse on steep hills. You can end up walking up the hills, pushing your bike.

The chain fails to disengage from the bottom teeth of a front chain-ring ; instead the teeth carry it up and around the full circumference of the ring, to wind back onto itself, and to jam between the chain-ring and chain-stay. Because it usually occurs during forceful pedalling, the chain can become permanently twisted, teeth can be damaged, and chain-stays of aluminium or carbon-fibre also damaged by the chain being wedged hard against them.
It affects mountain bikes more frequently, but does also affect road bikes.

There are two main forms of chain suck :

• 1-ring suck : the chain hangs-up on a single ring during normal pedalling (usually the “granny-ring”, but sometimes the middle-ring)

• 2-ring suck : the chain hangs-up across two rings when changing gears (usually from the middle-ring to the “granny”)

WHAT CAUSES IT ?

For a chain to disengage easily from a chain-ring :

• Links and rollers must be completely free of pedalling load when they reach the bottom teeth ; this allows the chain to float freely away from these teeth ; and it is the desired situation.

• Pressure-faces of the teeth must be properly shaped ; this allows the rollers to disengage even if load does remain on the bottom teeth ; in this case the link continuously sheds the chain’s pedalling tension as its roller moves down off the tooth.

But if tooth pressure-faces become loaded at the bottom of the chain-ring at the same time as becoming adversely shaped and possibly also subjected to friction, then disengagement becomes more difficult, and can fail.

Tooth-Wear, and Low-Stretch (New) Chains, and High Friction (eg Mud), when present in various combinations, cause the chain-ring’s bottom teeth to become loaded or overloaded by the chain’s pedalling tension, and also cause resistance to disengagement of the chain. The rear-derailleur spring cannot provide enough tension, and the chain does not have enough weight to disengage itself from the bottom teeth of the chain-ring. The links are carried up the rear of the chain-ring under continuing load …the chain “sucks” !!
Wear on the tooth’s pressure-faces (not burrs on tooth sides or thickened teeth) is the MAIN CAUSE of 1-RING-Suck, and sometimes 2-ring suck, which is however a bit more complex.

Adverse rotational alignment of teeth across adjacent rings is more usually the MAIN CAUSE of 2-RING-Suck. It results in the tooth which needs to dispatch the transferring chain to the adjacent ring, remaining under load even when it reaches the bottom of the chain-ring (just a different means by which the bottom tooth becomes loaded) ; thereafter the mechanism of sucking is similar to 1-ring.

To understand chain-suck in more detail, read about :

• Methods for RESOLVING CHAIN-SUCK PROBLEMS

• Mechanisms of the CHAIN-SUCK-TRIANGLE in detail

• Field testing, and simulating chain-suck in the workshop

• Popular myths contrasted with real causes of chain-suck

• Why 1-ring and 2-ring suck can occur on new drive-trains

• Why lubrication does not help when mud triggers chain-suck

• Many other interesting and useful details about the phenomenon…. by following the link below.

Copyright © Jonathan Levy, Cape Town, 2000. All rights reserved.

 Chain-Suck : in a NUTSHELL …comprehensive summary and solutions