Yellow Saddle
Guru
- Location
- Loch side.
In the light of constant misrepresentation of the effects of a worn cassette on a new chain, I thought I'd explain why a new chain cannot be damaged by a worn cassette. We've seen this stated here as gospel often, and again, this week.
Firstly, we need to understand why a chain wears and what happens when it wears. It does not wear from the tension induced into the chain by pedaling forces. This is important to understand even if by now, most of you will know that a chain does not stretch, but elongates because of wear in the links where the pins become smaller, their load-bearing mating surfaces bigger and hence, the chain becomes longer. The length of the plates in the links never changes because there is no stretch.
Then, why does the chain wear? It wears because of articulation. Each time the chain bends whilst under tension, the link, articulates (folds). This happens on only two places in the chain's run. One of these two places are at the 12 o'clock position on the rear sprocket when the chain exits the sprocket. There it articulates from folded to straight and does so under huge tension. The other place is where it enters the circle of the front sprocket, also at approximately the 12 o' clock position.
Note that although it again articulates at the bottom of these two sprockets, at the 6 'o clock position, wear doesn't occur there because the chain is completely slack on that run. Imagine two hot-spots of wear in the system - front and back, 12 'o clock each side. Nothing else matters in terms of wear. The slack run is essentially inert, as is the entire tension run where the chain remains straight. From the point where a link exits the rear sprocket to the point where it enters the front sprocket, nothing happens to the chain even though it is under tension. The amount of wear it receives at these two points varies and the two factors are dirt (grit in the interface) and pedal force. Dirt is the biggest factor. Force plays a smaller role.
OK, now let's look at the chain when it is wrapped around a rear sprocket. Imagine a chain pulling hard on the sprocket (let's keep the chain stationary for now to simplify the visualisation. Where the chain enters the sprocket (at the bottom), there is no tension in the chain and where it has just exited the sprocket, there's the full tension of the chain on that link and also in every single link of the chain all the way to the front. If you are stepping on the pedal with a force of 100N, then every single link in the area described above, feels 100N of tension.
However, the grey, foggy area of magnitude of tension lies within the sprocket. If tooth number 12 'o clock feels 100N from the chain, what does tooth 11 'o clock feel? And 10, 9 and 8? If the chain and sprocket pitch match perfectly, we could say that these all share the 100N load. But, if the sprocket is worn and the chain is new, then what? Then only the first entry tooth - somewhere at the 20-to position on the clock, feels the load. All the other teeth are unloaded. Thus, articulation under force now takes place early on in the rotation instead of at the top 12 'o clock position. Yet is still the same articulation load - 100N and the same angle of 360/number of teeth degrees of articulation.
Thus, an old sprocket cannot damage a new chain.
There is a caveat though, but don't let this little "if" detract from the argument above. It happens to be that a worn sprocket will not allow a chain to run on it and make the chain skip. Constant skipping of course prevent force in the chain and that prevents articulation under force and hence, wear. A worn sprocket on a cassette will therefore not allow a chain to reach conditions of wear even though, should it allow it, it will still make no difference.
The situation changes for single sprocket bikes with no spring-loaded chain tensioner, but that's another story altogether.
Firstly, we need to understand why a chain wears and what happens when it wears. It does not wear from the tension induced into the chain by pedaling forces. This is important to understand even if by now, most of you will know that a chain does not stretch, but elongates because of wear in the links where the pins become smaller, their load-bearing mating surfaces bigger and hence, the chain becomes longer. The length of the plates in the links never changes because there is no stretch.
Then, why does the chain wear? It wears because of articulation. Each time the chain bends whilst under tension, the link, articulates (folds). This happens on only two places in the chain's run. One of these two places are at the 12 o'clock position on the rear sprocket when the chain exits the sprocket. There it articulates from folded to straight and does so under huge tension. The other place is where it enters the circle of the front sprocket, also at approximately the 12 o' clock position.
Note that although it again articulates at the bottom of these two sprockets, at the 6 'o clock position, wear doesn't occur there because the chain is completely slack on that run. Imagine two hot-spots of wear in the system - front and back, 12 'o clock each side. Nothing else matters in terms of wear. The slack run is essentially inert, as is the entire tension run where the chain remains straight. From the point where a link exits the rear sprocket to the point where it enters the front sprocket, nothing happens to the chain even though it is under tension. The amount of wear it receives at these two points varies and the two factors are dirt (grit in the interface) and pedal force. Dirt is the biggest factor. Force plays a smaller role.
OK, now let's look at the chain when it is wrapped around a rear sprocket. Imagine a chain pulling hard on the sprocket (let's keep the chain stationary for now to simplify the visualisation. Where the chain enters the sprocket (at the bottom), there is no tension in the chain and where it has just exited the sprocket, there's the full tension of the chain on that link and also in every single link of the chain all the way to the front. If you are stepping on the pedal with a force of 100N, then every single link in the area described above, feels 100N of tension.
However, the grey, foggy area of magnitude of tension lies within the sprocket. If tooth number 12 'o clock feels 100N from the chain, what does tooth 11 'o clock feel? And 10, 9 and 8? If the chain and sprocket pitch match perfectly, we could say that these all share the 100N load. But, if the sprocket is worn and the chain is new, then what? Then only the first entry tooth - somewhere at the 20-to position on the clock, feels the load. All the other teeth are unloaded. Thus, articulation under force now takes place early on in the rotation instead of at the top 12 'o clock position. Yet is still the same articulation load - 100N and the same angle of 360/number of teeth degrees of articulation.
Thus, an old sprocket cannot damage a new chain.
There is a caveat though, but don't let this little "if" detract from the argument above. It happens to be that a worn sprocket will not allow a chain to run on it and make the chain skip. Constant skipping of course prevent force in the chain and that prevents articulation under force and hence, wear. A worn sprocket on a cassette will therefore not allow a chain to reach conditions of wear even though, should it allow it, it will still make no difference.
The situation changes for single sprocket bikes with no spring-loaded chain tensioner, but that's another story altogether.
