CHAINS that SUCK.

Chain-suck becomes THE major issue when you experience it. Your pedals lock solid, you come 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.

So it can bring your trip or race to a complete standstill, and you walk up the hills pushing your bike. It can also damage both frame and drive-train components.

There are two situations where it occurs most commonly :

-          With mud or wet grit on your chain and cogs – simply washing the chain and cogs often won’t get rid of the problem

-          During gear changes on the front chain-rings

Its always worse on steep uphill slopes or when otherwise pedalling with force. Surprisingly, it can occur with new equipment, and also in clean conditions.

The chain fails to disengage from the bottom teeth of a front chain-ring ; instead the teeth snag the chain and carry it up and around the rear circumference of the ring, winding it back onto itself, and jamming it between the chain-rings and chain-stay.

Because it usually occurs unexpectedly during forceful pedalling, the chain can be permanently twisted, teeth can be damaged, and chain-stays of aluminium or carbon-fibre can also be damaged by the chain being wedged hard against them.

It occurs most frequently with small chain-rings (granny-rings) but does also occur with larger rings. There are two main forms of chain suck :

A substantial investigation was carried out to understand the causes and mechanisms of chain-suck, and to test solutions for it. The investigation involved field and workshop tests, consideration of issues raised and insights provided by fellow bikers and respondents to my initial chain-suck articles, and understanding why various possible solutions which were tested, worked, or didn't work.

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

Links and rollers perform best if completely free of pedalling load when they reach the bottom teeth, this allows the chain to float freely.

Pressure faces of the teeth must be properly shaped to allow the rollers to disengage even if load does remain on the bottom teeth, and in this case the link continuously sheds the chains pedalling tension as it’s roller moves down and finally 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 perhaps subjected to the increased friction of grit or mud, then disengagement can be impaired.

Small chain-rings have high chain loads due to the pedal arm's high leverage ratio. They also have fewer teeth to carry the load ; thus each individual tooth carries a high load and there is a tendency for some load to be carried even by the bottom teeth. High tooth loading also leads to higher tooth wear. For these reasons, small granny-rings are more vulnerable to chain-suck than larger rings.

There are some obvious issues which can cause chain-suck such as damage to chain-rings, teeth, or the chain. Tight chain links caused by damage, or by poorly maintained chains packed with dried mud, can also cause chain-suck. Clearly, these problems can directly cause snagging of the chain. But common chain-suck detailed below is the most frequent, and can affect any bike without obvious cause.

When Tooth-Wear, and Low-Stretch (New) Chains, and High Friction (eg Mud) are present in various combinations, they cause the chain-ring's bottom teeth to be loaded or overloaded, and also resist disengagement of the chain. The chain does not have enough weight to disengage itself from the bottom teeth of the chain-ring in these circumstances, nor can the rear-derailleur spring provide enough tension after the chain starts being dragged up. The links are carried around and up the rear of the chain-ring under continuing load ...the chain sucks !!