Slipping Chain on Big Ring but not Small!

[gbb]

What I'd do...being a mere mortal of course.
Inspect chain ring...does it appear worn.
Measure chain, is it worn.
If either are worn, replace.

If neither are worn, select a rarely used sprocket on the rear. This will be less worn as it's had less use and if it doesn't slip, move to a sprocket you do use a lot and try again, if it slips, the cassette is worn.

If all those appear ok, dont forget to suspect the freehub...I'm thinking if you're on the big ring, you'd be applying more pressure so it'd be more likely to slip.

 

[grellboy]

This is my chainring. Maybe a bit "pointy", compared to my mountain bike which it is resting next to. Would pointiness be the cause? Probably done about 2k miles on it.

 

[Smokin Joe]

That is not the cause of your chain slip, it looks ok. It will be the sprocket(s).

 

[grellboy]

So....new cassette and chain? I honestly am a complete amateur, but if that's the case, why is it fine on little ring?

 

[Yellow Saddle]

This is my chainring. Maybe a bit "pointy", compared to my mountain bike which it is resting next to. Would pointiness be the cause? Probably done about 2k miles on it.

That's the cause of your slippage. That chainring is now in it's second stage of wear. Compare your photo that that of Rooster's in the 4th post here. You'll notice that in his photo, the teeth (cogs) point forwards and in yours backwards.

This indicates that you've ridden extensively with a chain that was way beyond worn.

The mechanism for this type of wear is as follows.

Stage one, the cogs wear a pocket into their pressure face (forward facing edge) for very obvious reasons - the chain pushes against it and the metal eventually wears away, especially if you do a lot of cross-chaining and the chainring is worn thin.

Stage two: The tips of the teeth wear in such a way that they appear to point backwards wrt rotation. The reason for this is totally unrelated to the reason above. It only happens if the chain is worn and out-of-pitch with the chainring. To visualise this, take note that the chain enters the front sprocket under tension - huge tension. If the chain is longer in pitch than the receiving gear, then the incoming tooth will hit against the incoming roller instead of neatly poking into the hole between rollers. This hitting motion eventually wears the top of the tooth down and eventually at an angle that allows the chain to lift up and slip over the teeth.

The second stage can happen without going through the first. They are not sequential and I suppose I should find another term for "stage". Yours didn't even go through stage one, judging from that photo, meaning that you rode the bike into the ground and destroyed a relatively good chainring.

This happens with considerable noise and vibration in the system and you should have felt this long ago and realised something is amiss. Swap chainrings, make sure the chain is still within spec and go for a ride, focussing on the change in feeling. Keep that in mind for the future. It's called experience.

 

[Yellow Saddle]

So....new cassette and chain? I honestly am a complete amateur, but if that's the case, why is it fine on little ring?

You may not need a new cassette and chain. But if that chain on there is the one that inflicted the damage, then yes.

There's another current thread on here somewhere where I've explained recently when to change the cassette. It's been described here quite often. Do a little research and it will pop up.

 

[grellboy]

Hang on. So in one person's opinion it's the chainring, but another person thinks it's the cassette? Am I right to be confused?

 

[Smokin Joe]

Hang on. So in one person's opinion it's the chainring, but another person thinks it's the cassette? Am I right to be confused?

No, go by what Yellow Saddle tells you as his knowledge is superior to mine. New chainring.

 

[User6179]

Hang on. So in one person's opinion it's the chainring, but another person thinks it's the cassette? Am I right to be confused?

The chainring is well worn.

 

[vickster]

Hang on. So in one person's opinion it's the chainring, but another person thinks it's the cassette? Am I right to be confused?

Go to your LBS, get them to check

 

[S-Express]

Hang on. So in one person's opinion it's the chainring, but another person thinks it's the cassette? Am I right to be confused?

The chainring looks like it should have been thrown in the scrap bin about 6 months ago, if that helps make your mind up...

 

[gbb]

Big ring looks as though it's had it. Specifically, look at the teeth to the left of the F of the FSA logo at the top. One of those teeth is seriously ground out and at least 5 others are showing considerable wear. Then compare them with the teeth directly above and to the right of the F....wildly different.

 

[gbb]

Replace big chain ring. Unless you've replaced it very recently, I'd replace the chain as well.
Given the level of wear on the chain ring, I'd closely inspect the cassette teeth as well.

 

[Yellow Saddle]

Big ring looks as though it's had it. Specifically, look at the teeth to the left of the F of the FSA logo at the top. One of those teeth is seriously ground out and at least 5 others are showing considerable wear. Then compare them with the teeth directly above and to the right of the F....wildly different.

The reason they look different is that they were different to start off with. Those are the shifting teeth and are made shorter to readily accept the chain when moving from the smaller ring to the bigger. They'll be placed slightly back in the rotational direction from shifting pins in the ring just below the cog line.
Have a look at a brand new chainring and you'll see.
Cogs don't wear unevenly - how can that be? And as for inspecting cassette teeth - what do you expect to see that will give you a clue as to their state of health?

 

[gbb]

Oh good lord I know about ramping/shifting teeth.
It's a shame, you are probably THE most knowledgeable mechanic here but that damage goes way beyond ramping/shifting teeth. I'd take my own experience over your analysis this time. It's obvious grellboy is having problems,it's obvious we all agree the chain ring is FUBAR'd...stop trying to wrap it up in exactitudes