This chainring does not have (or I can't find) an engraved model-number. Can you identify its brand and model-number? (See attachment: IMG_0975.JPG)


What would cause the worn-down teeth in this location? (See attachment: IMG_0977.jpg)

Wriding the bike. that took off some paint.






....

ONLY in this location?

Nope. The shortened teeth were formed that way at the factory for easier shifting.
[MENTION=469410]gwc[/MENTION], if you could look for a model number on the crank itself (maybe on the back sides of the arms), that would help to figure out replacements.

that too ..... they made it like that.... TDC is where engineers expect power input to slack off.

2 posibilities -

1 - it was made that way, search for an image of a new one of these (may be hard, as the crank is a mid 90's Shimano, and is pre-internet (as we know it today)).

2 - it got ground down during riding - i.e. it hit rocks/a kerb which was stronger than the metal.

If you look at the rear cogs you'll likely find they have different shaped teeth on the same cog too. Likely as intended by the manufacturer.

If you are not having issues with chain skip or shifting, then it's probably the way they were made. Look at others online and you'll see that brand new they have machining that makes them appear worn, but is intended to improve shifting as other have said.

If you are having problems in the drive train, there are other issues to look at first. DR adjusment and/or new chain usually.

See new photos attached

this is just another post by some one just discovering the engineering changes already done to make shifting simpler..

so you can ignore learning the techniques we older riders used to make bike riding easier.. before the engineering de skilled that.

note the difference in the smallest chainring which has no smaller chain ring to shift back and forth between.

Are you having drive train issues?

Not a good picture in this link, but I think you can see similar teeth as you are showing.

MTB Triple chainring - Chainrings - Type

Two teeth out of 40-50 got worn down by repeated strikes with rocks or curbs, but none of the other teeth were affected?

Nope, not a possibility if you think about it for very long. $20 says that there's another pair of shortened teeth behind the crankarm, and I'd love to know how those would get worn down with the crank in the way.

Tell that to my XT chainrings, ground 2 down at a trail center like that a few years back

Iride01: On the photo at the link you provided, many teeth are shorter than the others (but all are on the largest sprocket).

BTW: Shimano recalled all FC-MC12, FC-CT90 and FC-M290 cranks. (I wonder if they will still replace mine?)

those are shifting aids, designed into the rings... it's fine... and in fact, those chain rings look to be in great condition.

go ride your bike... quit worrying.

I'll believe it if you post pictures of the chainring and the destroyed chain. Because there's no grinding the chainring teeth down to that level without getting halfway through the chain.

The OP's shortened teeth are subtly rounded at the tips, and I spy one more shortened tooth right next to the crankarm (circled in green), bolstering my case that these are the typical SG shifting aids that were on XT chainrings for a long time.

There is no conceivable way that the OP could have worn these teeth down in this pattern and not been fully aware of it happening at the time.

Every pic I look at for many older mtb chainrings look similar. There are shorter teeth. Here is one closer to yours...

If you zoom on it, you'll see it has the same short teeth in the same position.

Here is the 1997 Shimano recall notice...
Voluntary recall - FC-CT90, FC-M290, FC-MC12 Cranks

Post #6 in this 2008 thread might give some hope...
recalled crank: fc-mc12, what to do? - Bike Forums

And how would the chain be destroyed as I was on the inner/middle?

I will stick with what I said at the time (I gave a couple of possibilities, including the manufacturering one, and how to check this), the OP has update the photos since my original answer, so what I originally said is possibly inaccurate now the new info (I am loosing interest in re-reading to justify answering you), but was at the time, that is the joy of posting something, and then having new info given.

Again, sorry for not taking photos approx 5 years ago, I wasn't anticipating that I would need them them as crucial evidence to prove what happened to me and my crank getting some minor trail damage in the future. Guess I need a time machine. I will remember to take photos or all my future bike damage and store them away just for you.

I will say that I have some short teeth on my OLD chainring that once had long teeth. I'm not quite sure how many miles... LOTS. 10,000? 20,000? 50,000?

Your chainring wear won't be even. You produce maximum power with the cranks at horizontal, and minimum power with the cranks at vertical. So, with the cranks at horizontal, the chain pulls most on the top few teeth, or if you draw a line perpendicular to the cranks, then go with a tangent off of that, you'll have the main pulling teeth.

It isn't surprising that Shimano has placed the shift gates precisely under the crank, and opposite to that, or the location where power is naturally at the minimum (not yanking on the chain as much), and wear would also be naturally at the minimum.

I just contacted my local bike store. They said for me to bring my bike in to the store and they will contact Shimano for the parts. I will get new cranks, a front derailleur, bottom bracket and chain but I may need a couple other parts depending on what my bike came with.

After my LBS (local bike shop) has replaced the cranks, front derailleur, bottom bracket and chain, I will post a photo showing the worn-down teeth

Chainring teeth wear from the sides, not the tips, but I'm glad your LBS is taking care of you. Broken cranks are no fun: Voluntary recall - FC-CT90, FC-M290, FC-MC12 Cranks

If "Chainring teeth wear from the sides, not the tips", why are the tips of my teeth worn down?

For Pete's sake. Like I and a couple others have been saying throughout this thread, they're not worn down, they're shaped like that on purpose from the factory.

Here's an article about the practice if you don't believe me: https://www.purecycles.com/blogs/bic...hainrings-work

And here are some pictures of chainrings from other companies with shortened, flattened teeth for shifting. Rest assured that if they were not as designed, they would not post such clear pictures of the teeth online.

The teeth on my sprocket look more worn-down than the teeth with the "flat peaks" in this photo:

Single speed is what they now call the older full height tooth designs that won the big pro races in the 70's ..

then the engineers started in on trying to satisfy a Perceived Market demand for faster shifting..

you may not have been born yet ...