Transmission Wear
Posted: 7 Nov 2012, 4:02pm
When a chain is brand new it has a pitch of exactly half an inch, and it will sit snugly at the base of the teeth, but as a chain wears it gets longer, and the chain has to ride higher on the teeth in order that the tooth pitch always matches the ever increasing pitch of the chain. Eventually, the chain gets replaced before it becomes worn enough to slip over the tops of the teeth. That’s the familiar cycle of chain wear, but let’s consider it in more detail starting with a couple of hypothetical chains: one an everlasting chain that never wears out and always remains exactly half an inch pitch, and the other a poor quality chain that wears out in no time (a few miles, say).
Consider the everlasting chain first. If it never wears out then it will always remain snugly seated at the base of the teeth, so the chainring wear all occurs at the base of the teeth too, and the familiar hooked tooth wear pattern develops. Eventually the teeth become sufficiently undermined that the chain can’t separate, gets drawn up the back of the chainring, and the ring is scrapped.
Now compare this with the poor quality chain that wears rapidly. As we’ve seen above, the chain rises up the tooth as it wears so although the teeth still wear, the chain would be wearing at a much faster rate, and never remain in the same place on the tooth for very long. So the fast wearing chain would wear a thin skim off the whole height of the tooth before being replaced by one chain after another, each of which would do the same. Many chains later, the tooth would have accumulated the same amount of wear as with the everlasting chain, but instead of all the wear being concentrated at the base it would be evenly distributed up the whole height of the tooth. A pattern of wear like this preserves the profile of the tooth, so although it may become very worn there is no hook to snag the chain and prevent it from separating from the ring. Indeed, because the wear is spread over a larger area it will take many more miles to erode the tooth to the same depth. It’s a win-win situation, on one hand there is less depth of wear for any given mileage, and on the other hand this pattern of wear will tolerate a greater depth worn away before the ring fails.
Look at the rings below, the one at the bottom looks in much worse condition, and indeed it has done many more miles, but it is the one at the top that is unusable because the teeth are hooked and drag the chain up the back of the ring!
The moral of this tale is that the wear pattern depends on the relative rates of wear of the ring and the chain. If your chain is hard wearing compared with the ring you will get a hooked wear pattern and a short chainring life, if you have a chain that wears quickly relative to the ring the more even wear pattern will prolong the ring life. Short lived chains are not bad for your chainring life, quite the contrary.
Now consider some chains from the real world, with an ordinary lifespan. Let’s compare what happens if you wear out one chain at a time, with what happens if you buy a set of three and rotate them regularly. The diagram below shows how chain wear accumulates with mileage:
The green line shows the wear of a single chain, the red shows the wear for the continually rotated set of three chains, and the orange is a smoothed out average of the red line. As you can see from the graph, the three chains collectively behave in much the same way as a single chain with three times the lifespan, but we’ve just seen above that long lasting chains create a tooth profile that’s more hooked and force you to scrap the ring prematurely. Hence the second moral of this tale is if you rotate your chains, it will reduce the life of your chainring not increase it.
So, to sum up it is the rate that the chain wears relative to the chainring that determines the wear pattern, and a hooked pattern is less tolerant of wear because it prevents the chain from separating. Continually fitting new chains, buying hardwearing chains, and rotating a set of three chains all reduce the chain wear rate relative to that of the chainring, and thus shorten the life of your rings by concentrating all the wear at the base of the tooth rather than distributing it more evenly.
Consider the everlasting chain first. If it never wears out then it will always remain snugly seated at the base of the teeth, so the chainring wear all occurs at the base of the teeth too, and the familiar hooked tooth wear pattern develops. Eventually the teeth become sufficiently undermined that the chain can’t separate, gets drawn up the back of the chainring, and the ring is scrapped.
Now compare this with the poor quality chain that wears rapidly. As we’ve seen above, the chain rises up the tooth as it wears so although the teeth still wear, the chain would be wearing at a much faster rate, and never remain in the same place on the tooth for very long. So the fast wearing chain would wear a thin skim off the whole height of the tooth before being replaced by one chain after another, each of which would do the same. Many chains later, the tooth would have accumulated the same amount of wear as with the everlasting chain, but instead of all the wear being concentrated at the base it would be evenly distributed up the whole height of the tooth. A pattern of wear like this preserves the profile of the tooth, so although it may become very worn there is no hook to snag the chain and prevent it from separating from the ring. Indeed, because the wear is spread over a larger area it will take many more miles to erode the tooth to the same depth. It’s a win-win situation, on one hand there is less depth of wear for any given mileage, and on the other hand this pattern of wear will tolerate a greater depth worn away before the ring fails.
Look at the rings below, the one at the bottom looks in much worse condition, and indeed it has done many more miles, but it is the one at the top that is unusable because the teeth are hooked and drag the chain up the back of the ring!
The moral of this tale is that the wear pattern depends on the relative rates of wear of the ring and the chain. If your chain is hard wearing compared with the ring you will get a hooked wear pattern and a short chainring life, if you have a chain that wears quickly relative to the ring the more even wear pattern will prolong the ring life. Short lived chains are not bad for your chainring life, quite the contrary.
Now consider some chains from the real world, with an ordinary lifespan. Let’s compare what happens if you wear out one chain at a time, with what happens if you buy a set of three and rotate them regularly. The diagram below shows how chain wear accumulates with mileage:
The green line shows the wear of a single chain, the red shows the wear for the continually rotated set of three chains, and the orange is a smoothed out average of the red line. As you can see from the graph, the three chains collectively behave in much the same way as a single chain with three times the lifespan, but we’ve just seen above that long lasting chains create a tooth profile that’s more hooked and force you to scrap the ring prematurely. Hence the second moral of this tale is if you rotate your chains, it will reduce the life of your chainring not increase it.
So, to sum up it is the rate that the chain wears relative to the chainring that determines the wear pattern, and a hooked pattern is less tolerant of wear because it prevents the chain from separating. Continually fitting new chains, buying hardwearing chains, and rotating a set of three chains all reduce the chain wear rate relative to that of the chainring, and thus shorten the life of your rings by concentrating all the wear at the base of the tooth rather than distributing it more evenly.