Chain-Suck :
in a NUTSHELL
see chain-suck triangle

.... a binding tale about the bad tooth-fairy
by Jonathan Levy

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What is Chain-Suck ?   What causes it ?   How can it be prevented ?

Check whether you might get it, but before that next bicycle race !!

You really want to suffer from ChainSuck ?

Use a NEW chain with your WORN chain-ring !!
    WHY ?  Read on ...
                         ...or go straight to Fixes

DOES IT MATTER ?
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.

WHEN DOES IT HAPPEN ?
There are two situations where it occurs most commonly :
  ♦ 

With mud or wet grit on your chain and cogs (and 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.

And, surprisingly, it can occur with new equipment, and also in clean conditions. The reasons why are also clarified in this article.

WHAT IS IT ?
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) of triple-ring MOUNTAIN BIKES, but does also occur with larger rings, and on ROAD BIKES. There are two main forms of chain suck :

  • 1-ring suck : the chain is snagged by teeth on a single ring during normal
        pedalling (usually the granny-ring)

  • 2-ring suck : the chain snags on teeth across two rings when changing
        gears (usually from the middle ring to the granny)

  • There are other circumstances which might appear to be chain-suck, but are not. There are many myths which are cited as causes of chain-suck, but are not.

    WHAT CAUSES IT, and WHAT IS THE MECHANISM ?
    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 away from these teeth ; and it is the desired situation.

      ♦ 



    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 chain's pedalling tension as its 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 !!

    see chain-suck MECHANISM

    So how does any chain load get transferred to the bottom teeth in the first place ?

    A mismatch in pitch between the chain links and the teeth of a chain-ring occurs when either or both of these components wears, respectively taking the form of tooth indentation and chain
    "stretch".

    Even for unworn teeth, the meshing of a chain as it wraps over the complex shape and spacing of teeth is itself complex. The chain has a definite pitch, but the teeth may be considered to have a variable pitch which depends on the elevation at which the chain effectively contacts them. Also, the elevation at which contact is made, may vary significantly along the wrap of the chain if the chain pitch does not closely match the tooth pitch near their roots ; thus stretched chains will ride up the faces of unworn teeth to some extent, under higher loads.

    When teeth wear, the contact angle at their roots usually changes as well, even though the basic pitch there doesn't ; the chain (whether new or stretched) will then also mesh under higher loads by riding up the teeth to even higher elevations.

    When chains ride up teeth either because of chain stretch and/or because of tooth wear, then the contact elevation with the teeth can vary depending on whether there is wear or not. So chain load may be distributed either :
      ♦ 

    favourably over a number of upper and mid-height teeth,   or

      ♦ 

    excessively on one or two upper teeth, promoting tooth metal failure,   or

      ♦ 

    with a fair amount on the bottom teeth, setting up conditions for chain-suck

    NEW-teeth and NEW-chain : Little/No load on bottom teeth : Chain floats free

    New (or unworn) Teeth and New (or low-wear) Chain : When both chain and teeth are new (or have little wear), the chain pitch, and tooth's pitch at its base, match closely. The geometric consequence of this is that the chain-roller/tooth contact will tend to spread over a number of upper and mid-height teeth ; and rollers are left able to float free of the bottom teeth.

    New Teeth
    NEW Chain
    New-teeth and OLD/'stretched'-chain : No load on bottom teeth : Chain floats free

    [New Teeth and Worn Chain : This is an unlikely combin-ation because the chain is normally replaced if a new ring is installed - to prevent accelerated damage to the new teeth. If chain wear is too great with new teeth, the chain can also skip over chain-ring teeth, when pedalling hard].

    Worn-teeth and OLD/'stretched'-chain : No load on bottom teeth : Chain floats free

    Worn Teeth and Worn Chain : When chain wear / "stretch" makes its pitch greater than the tooth's effective pitch, the geometric consequence is to transfer chain-roller/tooth contact to only one or two top teeth (also increases tooth wear/indentation). Slack in worn chain joints accumulates along the wrap of the chain, allowing it to elongate and to give rollers more play to float clear of the bottom teeth.

    Worn Teeth
    WORN Chain
    WORN-teeth and NEW-chain : Loads bottom teeth : Chain sucks

    Worn Teeth & New (low-wear) Chain : When tooth wear makes its effective pitch greater than the chain pitch, contact is increasingly transferred to the bottom teeth also. Installation of a new chain will, of course, result in a sudden reduction in chain pitch, with contact transferred more strongly towards the bottom teeth, and rollers will tend to bind on any indentations in these teeth - chain-suck.

    Worn Teeth
    NEW Chain

    Tooth wear results in hooking / indentation / pockets in the pressure faces of the teeth. This is the MAIN CAUSE of CHAIN-SUCK. Wear has two consequences both of which cause chain-suck to get worse, namely :
      ♦ 



    the change to a flatter ramped shape at the base of the pressure-faces forces the chain-rollers up the tooth face resulting in an equilibrium contact point at an increased elevation ; this increases the effective tooth pitch, which in turn causes some transfer of load to the bottom teeth

      ♦ 

    the indentations higher up the tooth faces cause mechanical resistance to disengagement of chain-rollers from the bottom teeth

    The first effect re-inforces the second because mechanical resistance to disengagement is increasingly activated as loading on the bottom teeth increases.

    "Wear" can also cause burrs or ridges on the tooth sides and thickened teeth, particularly with softer chain-ring materials ; but despite popular belief this, in itself, does not normally cause chain-suck, although it might contribute to a much lesser degree. As already stated, it is "wear" to the tooth pressure faces which shapes them adversely, then allowing the following factors to initiate chain-suck.

    A New (or low-stretch) chain has its contact against the tooth pressure-faces extended or transferred farther towards the bottom of the chain-ring. As a result chain load is increasingly transferred towards the bottom teeth where it increases both frictional and mechanical resistance to disengagement. And ....chain-suck becomes likely.    By contrast, the links of more highly stretched chains mesh tightly with one or two upper teeth on the chain-ring, and their slack accumulates to move the rollers away from contact with the pressure-faces of both mid-height and bottom teeth; this may avoid chain-suck in the short term, but it overloads the upper teeth causing greater metal failure and abrasion, ultimately increasing the potential for chain-suck especially when a new chain is installed.

    High friction occurs mainly when the chain gets muddy, or wet and gritty, but it can also occur with inadequate lubrication and dry dusty conditions. Frictional resistance to disengagement becomes increasingly activated as the chain loading on the bottom teeth increases, and also if there is an adverse friction angle due to tooth wear/indentation. Load gets transferred to the bottom teeth as described above. Loaded friction then binds the chain both internally at its rotating pins/bushings, and at the contact between its rollers and the tooth pressure-faces. Mud or grit can have an additional effect probably of lesser magnitude ; it can contribute both to decreasing the effective chain pitch and to directly elevating the chain rollers by packing, respectively into the internal spaces within the chain and between the rollers and tooth roots ; both these effects contribute to an adverse mismatch in pitch, and so increase the transfer of load to the bottom teeth. Not all muds are equal ; different mud, grit, sand, clay, etc, can have very different frictional characteristics and packing characteristics, so some will precipitate chain-suck readily while others may not cause problems. In all cases, it is the in-line loading and friction of the chain on tooth pressure-faces which is the main issue, NOT friction on tooth sides or wedging of mud/grit across teeth. Finally, cohesion/adhesion
    can be high for some clays if they remain stiff in consistency, but this is likely to be a rare situation insofar as it influences chain-suck ; wet splashed mud is usually soft.


    Forceful pedalling on steeper gradients is usually the action which finally precipitates chain-suck. It should, perhaps, be called the CHAIN-SUCK-PYRAMID with high-pedalling-force bringing the underlying CHAIN-SUCK-TRIANGLE into play, namely tooth-wear, low-stretch-chain, and high-friction.

    Extreme tooth wear combined with a low stretch chain can be enough for chain-suck to occur without raised friction. On the other hand, even with a much lesser degree of tooth wear, chain-suck can occur, provided that the low stretch chain is accompanied by high friction.

    see LARGER mechanism image

    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.
      email Jonathan Levy - must copy email address by hand


    For the related articles, follow the links below ...
    Chain-Suck : in a NUTSHELL ...comprehensive summary and solutions
    Chain-Suck : the OVERALL picture ...details of mechanisms & solutions
    Chain-Suck : the DETAILED Investigations ...field & workshop testing
    Chain-Suck : the FIELD TEST ...a field testing method for identifying causes
    Chain-Suck : Restoring Worn Tooth Pressure-Faces ...by filing them
    Chain-Stretch : Norms and Measurement ...when to get rid of your chain
    LINKS ...to other interesting websites
    LINK to YOUR SITE ...how to place a link from your website to this site
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