Do these chainrings need replacing?

Question

I know this has been asked several times but I'm curious if anyone can weigh in on whether these specific chainrings need replacing. They're from a triple chainring setup.

I'm replacing my chain and casette right now so it's a good time to replace the rings too if needed. But they're also pricey so don't want to chuck them prematurely.

Middle chainring was replaced about 6,000km ago. I think I ride about 70% middle ring, 25% largest ring, and 5% or less smallest ring (flat city). So I'd say the middle has had about 4,200km on it.

Largest chainring is original to bike, which has ~20,000km on it, so I'd estimate 5,000km on that ring.

Middle ring, outside (drive) side:

Middle ring, inside (non-drive) side:

Large ring, outside (drive) side:

Large ring, inside (non-drive) side:

Note: the middle chainring has a subset of teeth where the leading edge has been flattened a little bit and the dull surface finish of the metal has been worn off on the sides of the leading edge so it looks like a pronounced shark fin pattern but in fact is not that worn.

I.e. the teeth below look very worn but in fact their profile still follows the red lines. It's just hard to capture them properly on camera.

Answer 1

Are you suffering from chain slip? That would be a clear sign that something needs replacing.

Another thing to do is compare by touch the un-worn granny ring's teeth with the other rings. The larger chainrings should not feel "wider" in each valley.

Lastly, fit up your new chain, and while its clean use one hand to press the chain into the chainring in one spot.
Then "sweep around" the chainring with the other hand. There should be a nice close fit between the new chain and the old chainring. If there's significant space between the chain and chainring by the time you get to a point 180 degrees opposite, then the chainring is worn.

Visually its hard to tell - the wear pattern of the paint can make the teeth look more worn than they are. From your photos I'd say yes replace both bigger chainrings, but the red lines would show an unworn chainring. I suspect the photos aren't telling the full story.
One trick is to clean the chainrings and then blacken them with spray paint or permanent marker. It won't last at all, but will help show the structure vs the background.

20,000 km is not a lot for a chainring - I've done double that without issue in the past, but I've also heard of some bikes/conditions that eat chainrings in a few thousand km. So total mileage is an indicator but not a hard-and-fast measure.

Good luck!

Answer 2

As mentioned in Criggie's answer, if the new chain is slipping on either of the used chainrings then you have your answer as to excessive wear of the rings and they'll need replacing.

From my perspective it appears there is likely more life in both chainrings, more so the large ring than middle, which appears more worn. My suggestion is to replace the middle ring only (given no new chain slip is evident when on the large ring). My reasoning is based on that 1) you obviously put on significant km's per unit of time 2) you state the middle ring gets the lion's share of the work 3) as far as cost, middle and granny rings are significantly less expensive than the large ring. In fact, it may be reasonable to get a new crankset that comes with a warranty as opposed to purchasing a new large and middle ring for most cranksets. At any rate, replacing the middle ring should be financially favorable as well as offering some peace of mind knowing your front drive is definitely ready for the next few thousand kilometers.