For 2-ring suck, adverse rotational alignment of teeth on adjacent rings is more usually the MAIN CAUSE of 2-RING-Suck. This is an alternative source of mismatch in pitch between the chain links and the teeth of chain-rings. This time it is across chain-rings, instead of just on a single ring. It occurs when the distance between teeth on adjacent rings is not well matched to the length of transferring chain. In a manner similar to 1-ring suck, when the cross-over length of chain is shorter than the corresponding distance between dispatching and receiving teeth, then the pressure-face of the dispatching tooth (which leads in the pedalling cycle) never gets unloaded because the receiving tooth isn't able to take up the load and remove it from the dispatching tooth. The pressure-face of the dispatching tooth then remains overloaded until reaching the bottom of the ring where it needs to both disengage from the chain, and allow final dispatch of the chain to the receiving ring. Dispatch is difficult while loaded ; frictional & mechanical resistance to disengagement (ie mud/grit & tooth indentation/hooking) will worsen the problem if present ...and 2-ring suck becomes likely.

The above is a reason why its always advisable to slack off pedalling pressure while changing gears. However, there is a more severe form of 2-ring suck which can occur despite doing this. It is sometimes referred to as the chain-bridge mechanism where the trailing tooth faces interfere with dispatch/disengagement, even with little or no pedalling load on the chain.

Old designs of chain-rings tended to be more vulnerable to 2-ring suck than modern ones. Modern teeth have less steep trailing faces to reduce interference by the trailing faces on the dispatch and disengagement of chain rollers. They also have special teeth shaped to promote chain transfer at specific locations where conditions are made to be favourable. Thus, teeth on adjacent rings are positioned to favour a match for the length of transferring chain, and also to co-incide with low loading in the crank cycle BUT ...
... some makes of crank/chain-rings are more successful at achieving all this than others (for example, Shimano tends to be good at this aspect), and some tooth ratios of adjacent rings tend to be better than others (for example, 22t-32t is usually without this problem, while 24t-36t gives more problems).

This form of mismatch does not necessarily require any significant tooth wear ; wear can improve matters or make it worse. When chain-rings are new, 2-ring suck can affect them if the rotational alignment of teeth is unfavourable, because this factor alone can cause a "chain-bridge" and loading of bottom teeth .

DOES IT OCCUR READILY ?

Tooth wear is the combination of two main contributors :

♦	Metal failure of tooth pressure-faces from over-stressing by high loads
♦	True wear of these faces, by abrasive grit, and by metal-to-metal wear

Plastic deformation failure is the most common cause of indented pockets in tooth pressure faces; a by-product of the indentation is burrs on tooth sides and tooth widening. The by-product, as already stated, is not a significant cause of chain-suck despite popular belief; the main effect (indent in pressure-face) is a key cause. Tooth face failure can arise from a single instance of over-stressing. This has been observed when high pedal forces are used on brand new granny-rings of unsuitably soft metal during a single first outing with steep hills. The chain-ring becomes vulnerable to chain-suck from that moment onward.

Abrasive wear by grit and mud, on the other hand, takes more time, even if tooth loading is high. Also, it does not cause tooth widening or side-burrs on its own (unless accompanied by indentation failure of softer tooth material). Instead the tooth face becomes eroded to the familiar shape of concave / hooked teeth. This hooking is also a key cause of chain-suck.

Material used for many chain-rings is not strong, hard, or tough enough for the smaller rings. Even special heat-treated alloys of aluminium, die-pressed powder-alloys, and many steels used for chain-rings are notoriously soft for the high stress environment of granny-ring teeth. Only special types of hard tough alloyed or stainless steel, or titanium, are really adequate for these small rings.

Manufacture of chain-rings by common die-stamping methods is inaccurate. This gives rise to increased stress concentrations where chain-rollers contact the teeth, and increases the opportunity for localised metal failure & wear, and indentations.

Total wear does not need to be very great for chain-suck to start when high friction circumstances are present ; the wear may be noticeable only as a slight steepening (or alternatively indentation) of the pressure faces of the teeth.

Chain-suck is also known to occur with new or almost new drive-trains, or components thereof. For 1-ring suck, the problem is usually caused by soft chain-ring material (and sometimes by manufacturer's errors not easily detected by the unaided eye). 2-ring suck can also occur when new where the rotational alignment of adjacent rings is poor (actually more common on new than worn rings).

Any of the above problems should be cause for rejecting components as manufacturer errors, with the components unsuitable for purpose, or faulty !

CAN YOU TEST YOUR BIKE FOR CHAIN-SUCK ?

There is a simple practical test which anyone can do with their bike to see if it is likely to suffer from chain-suck. It needs no special equipment. The test can also be used to identify the causes of the problem. (See Chain-Suck TEST)

HOW DO YOU GET RID OF CHAIN-SUCK ?

There are two basic aims for alleviating chain-suck, namely ensuring that :

a)	Tooth pressure-faces are suitably shaped to disengage even if they are loaded
b)	Disengaging bottom teeth are subjected to little or no chain load

1-ring suck : replace the worn chain-ring (usually the granny, but sometimes the middle-ring) with a ring of strong and tough material to achieve the above aims; if it is also accurately machined rather than die-stamped, this will improve things even more. This is the fundamental solution which gets rid of the problem at its source. For the granny, avoid aluminium, even special alloys ; die-pressed powder-alloys also result in weak rings ; use an especially tough hard steel (eg an appropriate type of stainless steel), or use titanium. If the problem item is a larger chain-ring (say 30 or more teeth), a very high strength aluminium alloy may provide an acceptable component life. The above invariably resolves 1-ring suck, and fairly often but not always 2-ring suck, for which the middle ring is sometimes the culprit.

2-ring suck : follow these procedures and measures to achieve the same aim of unloading bottom teeth ; this time it is across chain-rings instead of a single ring.

Damaged components (teeth:bent, rings:bent, chain:twisted or tight-links) can cause chain-suck. Obviously, these must always be investigated first and resolved. Chain-suck itself can cause such damage - so even if other underlying causes are resolved, parts thus damaged must also be dealt with to prevent recurrence.

Other ways of avoiding chain-suck and even methods for alleviating it on-the-trail (eg filing tooth profiles), are also described in "the OVERALL picture".

Anti chain-suck plates in a wide variety of commercial and home-made designs claim to prevent the consequences of chain-suck. These plates can be effective when they work, but often don't work well, then causing worse damage to chain-stays than if they were absent - therefore not recommended as a general solution.

Extra spring tensioning devices for the rear derailleur, mostly of home-made design, are claimed to work but at the expense of more sluggish gear changes. For strong chain-suck conditions, high spring tension would be required which is not advisable for the sensitive mechanism of rear derailleurs.

Maintenance : Clean and Lube your chain regularly ; generally this will not alleviate chain-suck directly (see below), but it does help to prevent abrasive wear to teeth and chain over the longer term, which helps indirectly. Not to mention preventing sonic ear pain. Replace your chain at or before the stretch limit ; this will prevent undue wear to the chain-rings (causing chain-suck), and to the rear sprockets.

MYTHS AND FACTS ABOUT LUBRICATION

When the CHAIN-SUCK-TRIANGLE grips your bike during extended muddy or wet/gritty conditions, even generous lubrication won't help for long, because it is stripped out rapidly by the mud and wet conditions.

Manufacturers make untenable claims about their lubricants' abilities in the presence of mud. Actual field testing and the slightest understanding of both the flow dynamics and the scouring abilities of a mud slurry within a chain's interior, debunk such claims for any of the lubricants commonly used. In fact, a good way to clean a chain really thoroughly is to go for a muddy ride and head for all mud and water puddles ; thereafter use detergent and a hose ; the chain will be clean and free of lubricant both inside and out ; even the old caked gunk will be gone.

Chain-rings must have both design and materials which are suitable for resisting wear and providing a long component life free of chain-suck, even at those times when lubrication is absent and mud scours everything.

Even without mud, lubrication will not necessarily prevent chain-suck. This depends on how badly the teeth are worn, and whether they are subject to the additional resistance to disengagement arising from a new unstretched chain.

Cleaning and lubrication always remain essential to reduce tooth and chain wear.


Cross-country racers or those planning long rides with mud should weigh the modest cost of a suitable new chain-ring against the risk of having their ride spoiled by chain-suck.

	Copyright © Jonathan Levy, 2000. All rights reserved.