Question about Flat Teeth on Front Chainring

December 9, 2018 4:18 PM

I just had a long-overdue drivetrain replacement done on my Trek bike: cassette, derailleur pulleys, chain, crankset/chainring combo. Service was reasonably priced, friendly, and fast, and the bike now rides wonderfully (with very smooth shifting). When I got back from my first short ride after servicing, however, I took a closer look at the chainring and noticed that many of the teeth were stubby, some almost flat, at least on the outer and middle rings. Is this common or typical in new chainrings?

I went online and found a picture of the crankset/chainring installed on my bike, which you can see here: SunRace fcm300

If you look at the teeth on the middle ring at (roughly) the 5, 6, and 11 pm positions, you will see what I am talking about. When I first enquired about pricing for the job at this shop, they told me that they did not have the Shimano 48-38-28 crankset (that is installed on my Trek at the factory) in stock, but they had a SunRace 48-38-28 that was basically the same, just a little heavier. As I needed to have the job done as soon as possible, I agreed to that. I am now having doubts, however, that that was a wise decision. If I hadn't found that picture of what I assume is a new and unused SunRace crankset with flattened teeth, I would have guessed that my chainring was either defective or not new (the black paint has also come off some of the outer ring teeth: how likely is it that happened after a 30 minute ride where I spent no more than 5 minutes on the outer ring?); but maybe that's just how some companies make them and they do not really affect the smoothness or longevity of the chainring...

I wish now that I insisted on the Shimano, even if it meant a delay of a week or more (when I probably won't be riding anyway due to heavy rain). What is the usual way of proceeding in this situation? Am I ultimately fully responsible for making a bad choice in chainrings, or is it typical for bike shops to offer to replace the crankset to the one I originally wanted/that came stock with the bike, if I have doubts about the one they installed?

All feedback appreciated!

posted by tenderly to Sports, Hobbies, & Recreation (5 answers total)

Flat teeth are a thing. They help the chain jump up to that ring. Don't sweat it.
posted by notsnot at 4:30 PM on December 9, 2018 [3 favorites]

If it all shifts fine, there's no problem with the chainrings, as notsnot indicates above.

Just remember that the cables will stretch after a few rides, which may necessitate a minor adjustment. /pedant
posted by computech_apolloniajames at 5:45 PM on December 9, 2018 [1 favorite]

Chain rings wear into pointier shapes, like a cartoon shark fin. Modern rings have some really goofy looking profiles to help them pick up and drop the chain when intended, so I’d call this perfectly normal.
posted by advicepig at 5:57 PM on December 9, 2018 [2 favorites]

Response by poster: Thank you for the replies; i am relieved, and happy to return to enjoying my feels-like-new bike! I should mention that reading the following after noticing the flat teeth did raise my anxiety level: "With just two tasks to perform – carrying the chain, and, to a far lesser degree, aiding its transfer to a second ring – the chainring need only maintain a healthy profile to its teeth. Those that become narrowed, flattened, bent over, or curved in the opposite direction of the chain travel, can become ineffective." signs of a worn drivetrain
posted by tenderly at 9:03 PM on December 9, 2018

Here is a photo of what a worn chainring looks like. They first wear into that "shark tooth" shape and then--if things continue to go south--the tip will eventually break or wear off.

In the photo you can see shark-tooth shaped teeth more to the left and "flattened" teeth more to the right.

That is all very, very different from the pattern & shape your brand new and perfectly good chainrain now has.
posted by flug at 7:02 PM on December 10, 2018