At the outset the term "chain stretch" is misleading and wrong. Chains do not stretch, in the dictionary sense, by elongating the metal through tension. They lengthen because their hinge pins and sleeves wear. This wear is caused almost exclusively by road grit that enters the chain when it is oiled. Grit sticks to the outside of a chain in the ugly black stuff that can get on one's leg, but external grime has little functional effect because it is on the outside where it does the chain no damage. Only when a dirty chain is oiled, or has excessive oil on it, can this grit move inside where it causes damage.
Commercial abrasive grinding paste is made of oil and silicon dioxide (sand) and silicon carbide (sand). You couldn't do a better job if you tried to destroy a chain, than to oil a dirty chain.Primitive rule #1: Therefore, never oil a chain on the bike.
This means the chain should be cleaned of grit before oiling it and because this is practically impossible without submerging the chain in a solvent bath (kerosene or commercial solvent), it must be taken off the bicycle. Devices with rotating brushes, that can be clamped on the chain on the bicycle, do a fair job but are messy and do not prevent fine grit from becoming suspended in the solvent. External brushing or wiping moves grit out of sight, but mainly into the openings in the chain where subsequent oiling will carry it inside.
Do not use gasoline because it is explosive and contains toxic light petroleum fractions that penetrate the skin.
Removing the solvent from the chain after rinsing is important. Compressed air is not readily available in the household nor is a centrifuge. Manually slinging the chain around outdoors works best.
The other way is to evaporate it. Accelerated drying methods by heating should be avoided, because they can be explosive.
Lubricating the chain with hot 90W gear lube works but it is also efficient fly paper, collecting plenty of hardpack between sprockets and on the outside of the chain. Motor oil is far better, but motorcycle chain and chainsaw lubricants are better yet, because they have volatile solvents that allow good penetration for their relatively viscous lubricant. Paraffin (canning wax), although clean, works poorly because it is not mobile and cannot replenish the bearing surfaces once it has been displaced. This becomes apparent with any water that gets on the chain. It immediately sqeaks.
Chain life is almost entirely a cleanliness and lubrication question rather than a load problem. For bicycles the effect of load variations is insignificant compared to the lubricant and grit effects. For example, motorcycle primary chains, operated under oil in clean conditions, last years while the exposed rear chains must be replaced often.
The best way to determine whether a chain is worn is by measuring its length. A new chain has a half inch pitch with a pin at exactly every half inch. As the pins and sleeves wear, this spacing increases and this concentrates more load on the last tooth of engagement, changing the tooth profile. When the chain pitch grows over one half percent, it is time for a new chain. At one percent, sprocket wear progresses rapidly because this length change occurs only between pin and sleeve so that it is concentrated on every second pitch; the pitch of the inner link containing the rollers remaining constant. By holding a ruler along the chain on the bicycle, align an inch mark with a pin and see how far off the mark the pin is at twelve inches. An eighth of an inch (0.125) is a little over the one percent limit while more than a sixteenth is a prudent time to get a new chain.
Sprockets do not change pitch when they wear, only their tooth form changes. The number of teeth and base circle remain unchanged by normal sprocket wear.
A new chain often will not freely engage a worn rear sprocket under load, even though it has the same pitch as the chain. This occurs because the previous (worn and elongated) chain formed pockets in the teeth by exiting under load. A chain with correct pitch cannot enter the pockets when its previous roller bears the previous tooth, because the pocket has an overhang that prevents entry.
Without a strong chain tensioner or a non derailleur bicycle, the chain has insufficient force on its slack run to engage a driven sprocket. In contrast, engagement of a driving sprocket, the crank sprocket, generally succeeds even with substantial tooth wear, because the drive tension forces engagement.
However, worn teeth on a driving sprocket cause "chainsuck", the failure of the chain to disengage the chainwheel. This occurs more easily with a long arm derailleur, common to most MTB's, that is one reason this occurs less with road racing bicycles, that experience a noisy disengagement instead.
In contrast a worn chain will not run on a new driving sprocket. This is less apparent because new chainwheels are not often used with an old chain. In contrast to a driven sprocket (rear) the chain enters the driving sprocket under tension, where the previous chain links pull it into engagement. However, because a used chain has a longer pitch than the sprocket, previous rollers bear almost no load and allow the incoming chain link to climb the ramp of the tooth, each successive link riding higher than the previous until the chain jumps.
The pockets in a used sprocket are small but they change the pressure angle of the teeth enough to overcoming this problem.
Chain Care
