Why a worn cassette cannot damage a new chain

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.
 
Thanks I learned something new. Always worth hanging out here! :okay:
 

Arjimlad

Tights of Cydonia
Location
South Glos
Is there a ready explanation for my current predicament...

Higher grade KMC shiny chain & cassette bought new in Summer 2018 & fitted to my Giant Revolt, used for family rides, wet & winter commuting.

My chain wore more than the 0.75 marker before I noticed (mea culpa!). Ordered a new chain in hope that the gears weren't too badly worn.

I bought a new lower grade KMC 8-speed chain which then did the dreaded skipping on the rear cogs I use the most. After about 30 miles commuting I hung the bike up, & rode the other one until a new Sunrace 8 spd cassette was fitted.

Now, I am still getting occasional slipping on the rear cogs.

Any tips appreciated !
 
OP
Yellow Saddle
Location
Loch side.
Is there a ready explanation for my current predicament...

Higher grade KMC shiny chain & cassette bought new in Summer 2018 & fitted to my Giant Revolt, used for family rides, wet & winter commuting.

My chain wore more than the 0.75 marker before I noticed (mea culpa!). Ordered a new chain in hope that the gears weren't too badly worn.

I bought a new lower grade KMC 8-speed chain which then did the dreaded skipping on the rear cogs I use the most. After about 30 miles commuting I hung the bike up, & rode the other one until a new Sunrace 8 spd cassette was fitted.

Now, I am still getting occasional slipping on the rear cogs.

Any tips appreciated !
OK, let's simplify the question and hone in before answering.

You rode your bike to a point where the chain was worn too much. You fitted a new chain and the chain skipped. You then fitted a new cassette and there's now some skipping on the new chain and new cassette.

Firstly, you have to distinguish between skating and skipping. Skipping is when a chain vacillates between gears. In other words, it is changing gear rapidly. Skating is when a chain lifts up over the sprocket teeth when you pedal hard and skates over the top. It returns to normal when you pedal gently.

Are you sure you have skating and not skipping? If so, how do you know?
 

Arjimlad

Tights of Cydonia
Location
South Glos
OK, let's simplify the question and hone in before answering.

You rode your bike to a point where the chain was worn too much. You fitted a new chain and the chain skipped. You then fitted a new cassette and there's now some skipping on the new chain and new cassette.

Firstly, you have to distinguish between skating and skipping. Skipping is when a chain vacillates between gears. In other words, it is changing gear rapidly. Skating is when a chain lifts up over the sprocket teeth when you pedal hard and skates over the top. It returns to normal when you pedal gently.

Are you sure you have skating and not skipping? If so, how do you know?
Thanks YS. To me it feels like skating - chiefly when setting off after an unexpected stop in too high a gear.
 

Ajax Bay

Guru
Location
East Devon
Thank you for the description of the chain/chainring/sprocket interface/wear mechanism.
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.
I realise you've used 100N as a 'sample' pedalling force. Leaving aside that it's likely that the maximum force applied to a pedal by the average rider will be rather more than that, the tension which the chain is placed under is much more than the force applied at the pedal. Pedal force = 100N, Length of crank = 170mm. Moment = 17Nm. Radius of a (say) 39t chainring = 76mm. So tension in the chain is 224N.
None of this detail undermines your explanation and premise, but there's a risk that if one calls 'Dick' "dick" they might complain.
 

Ajax Bay

Guru
Location
East Devon
I bought a new . . chain which then did the dreaded [skating] on the rear cogs I use the most.
[time gap] . . .new Sunrace 8 spd cassette fitted . . . I am still getting occasional [skating] on the rear cogs.
Any tips appreciated!
I remember being perplexed by this, in similar circumstances. After swapping chains in and out, and cassettes in and out, I changed my middle chain ring (which had over 30,000km on it (well on probably half the 30,000km given it was the middle ring)). Sorted. (I'd replaced the inner ring the previous year.)
 
OP
Yellow Saddle
Location
Loch side.
Thanks YS. To me it feels like skating - chiefly when setting off after an unexpected stop in too high a gear.
OK, but setting off in too high a gear sometimes causes frame flex and then the chain skips. To be absolutely sure, find a steep hill. Change to the suspect gear and pedal harder and harder whilst remaining seated. If it then skates, you have a cassette problem.

A new chain (why not measure the chain to make sure this is the case?) on a new cassette will only skate if there is insufficient wrap on the chain. In other words, the B-tension screw is in a position where too little wrap causes the chain to lift off. Show us a photo of the cassette and derailer, dead-on. The chain should wrap beyond the 6 o' clock position.
 

Arjimlad

Tights of Cydonia
Location
South Glos
I've looked at this today and adjusted the derailleur, will see how I get on tomorrow.

Here are before and after shots. I've never adjusted a B screw before but this derailleur had clearly got out of alignment.
496696
 

silva

Senior Member
Location
Belgium
if pedaling force is a minor than articulation factor then that would also imply that a slow cadence (low gear) inflicts less wear on a chain?
 
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