Worn granny ring

[Trickedem]

I'm servicing my friend's bike.
How did this happen?

 

[si_c]

My first thought is that that's a shifting aid - if you look at the tooth at 3'o'clock it has a similar profile.

Which chainset is it? Might be worth looking at pictures of a new one online.

 

[screenman]

Nothing worn there in the small ring.

 

[Trickedem]

My first thought is that that's a shifting aid - if you look at the tooth at 3'o'clock it has a similar profile.

Which chainset is it? Might be worth looking at pictures of a new one online.

Thanks. I thought that might be a possibility. It seems to be a bit random though

 

[CanucksTraveller]

It's not as random as it looks, it's all designed to feed the chain from the teeth of the middle ring more smoothly onto the smaller ring, especially under load, and it does work. It's normal, nothing wrong with the chainring and it's good to go.

 

[Lovacott]

My first thought is that that's a shifting aid - if you look at the tooth at 3'o'clock it has a similar profile.

Some are profiled to drive, some are profiled to catch the chain when you shift. Look at any Shimano crank set or freewheel in close up detail and it looks like a dogs dinner but it's all by design.

The main suspects when a chain slips are the chainrings but nobody seems to point the finger at the chain itself?

Chains have many hundreds of moving parts and they are made of very thin metal. They take on all of the load when you stand up and pump your legs to get up the very last bit of that bloody great hill. They get covered in grit, they get rained on.

As a result of all of this abuse, chains wear and stretch.

When a chain becomes a few millimetres longer that it should be, not all of the chain is in contact with all of the chainring teeth. Less contact leads to chain slip which can then damage the chainrings.

If you put a 1000 mile chain alongside a brand new one, you will see a very noticeable stretch in the used chain. At the first sign of slip, I pop on a new chain (I have spare in my toolbox).

Chains are cheap in comparison to freewheels and cassettes. I will go through three chains before I even consider changing a freewheel or crankset.

 

[Mr Celine]

All of the granny rings on the bikes at Celine Towers have even sized teeth - 30 tooth rings on 9 speed sora, 9 speed 105, 10 speed 105 and a 28 tooth Stronglight. Which is not surprising as the low height teeth on chainrings are mainly to allow the chain to drop down on to a smaller ring.

On the other hand none of the teeth on any of the rings are showing the wear pattern caused by a worn chain and the shorter teeth don't look broken.

Apologies for a not very helpful post.

 

[Ajax Bay]

When a chain becomes a few millimetres longer that it should be, not all of the chain is in contact with all of the chainring teeth. Less contact leads to chain slip which can then damage the chainrings. If you put a 1000 mile chain alongside a brand new one, you will see a very noticeable stretch in the used chain. At the first sign of slip, I pop on a new chain (I have spare in my toolbox).

A new 57 link (1" links) chain is 1448mm long. If you're going to change a chain and allow the cassette to carry on, this is best done when the chain is 0.5% elongated (NB chains do not 'stretch') - so max 7mm more than when new.
"not all of the chain is in contact with all of the chainring teeth"
All the time very little of the chain is in contact with the chainring teeth. Whatever the number of chainring teeth, the number of teeth in contact is 'half'. But the load (full on contact) is taken by the top few teeth and this number reduces to one as the chain elongates (same as on the sprocket).
Have a read of this thread: https://www.cyclechat.net/threads/why-a-worn-cassette-cannot-damage-a-new-chain.255700/ where we had great fun looking at the detail.
"chain slip which can then damage the chainrings"
"Chain slip" occurs when the chain is so elongated that the chain skates over/off a sprocket.
Please explain how the chain skating over a sprocket can 'damage the chainring'. It's the elongated chain that damages the chainring, not the chain skating/slipping per se (which will happen earlier).
I find that I experience very little elongation in 1000 miles (but chain wear/elongation rate varies with riding style and terrain/soil nature) but at 1500 miles it needs replacement if I'm to do so without chainging the cassette as well.
"At the first sign of slip, I pop on a new chain"
And then I presume you replace the cassette pronto. If you've let the chain/cassette combo go as far as the chain skating, then the chain is more than 0.75% elongated, the cassette is worn and a new chain on it will not be 'happy'. YMMV.

 

[Lovacott]

Please explain how the chain skating over a sprocket can 'damage the chainring'.

Friction when one metal part brushes over another metal part.

 

[Ajax Bay]

There's friction between the chain links and the chainring's teeth ALL the time. Is the chainring 'damaged' by this interaction? There will be wear, but we're talking a ring needing changing every (maybe) 15,000 miles, so negligible, compared to the wear on the sprockets (maybe 4,000 miles with careful chain management (3 chains)). The chain skating on a (rear) sprocket doesn't damage the chainring per se. an elongated chain does wear the chainring more that one which is within 0.5% of its original length.
If a rider has experienced skating, and changed both chain and cassette, and that doesn't sort it, then it's worth testing if the skating is happening on one particular chainring; and change that.

 

[Lovacott]

There's friction between the chain links and the chainring's teeth ALL the time.

Yes there is and I 100% agree.

But metal meeting with metal causes a lot less friction than metal rubbing over metal.

When a chain slips, it's akin to a very fine file hitting every tooth on the chainring. Multiply that by 1000 slips and you've got rounded spikes which can no longer drive the bike up a bloody great hill.