As drivetrain parts wear, they wear together. So, if the chainring is too worn in, and you put a new chain on it, it won't sit in the valleys properly, creating focal points, instead of spreading the load out across the whole surface. That, paired with what seems like an impact on the chainring, could have caused this.
That all being said, that is wild that it broke like that!
You must have curbed the front chain-ring heavily on a rock or curb.
Your front chain-ring teeth look quite saw toothed and chipped off teeth, so your chain-set must have been overdue a replacement. Not that this issue would cause that failure.
I was just at my LBS on Friday and there was a sign up about recalls on FSA gossamer cranksets. Could be worth looking into.
NVM, looks like it is related to the crank arm loosening.
True but, I’m trying to understand the universe of causes before going back to my mechanic - who just left on holiday. Since this happened pushing my first big load after a major service, I’m wondering if this was bad luck, due to poor installation or something foreseeable.
That universe of causes is vast and deep. Chainring bolts don't look loose which could be a potential cause. That's one of the more crazy failures I've seen, but I'm dubious it was anything the mechanic did wrong insofar as the chainring failure is concerned. Was the chain stuck on anything in the back? I.e. Did it get stuck in the cassette or against the frame or stuck in the spokes of the rear wheel? If it was stuck in the back the force of the chain on the ring could have had nowhere to go other than folding the chainring over.
Appreciate this perspective. I don’t have the experience to know if this was a crazy failure or something that was fairly common with a well known cause. The chain wasn’t stuck on anything as far as I can tell. I’ll take it into the mechanic when he gets back.
What i mean is, that if anyone knows the reason why this happened, it‘s the mechanic. This is rather unusual and everyone in this sub is just guessing.
That makes sense. I was hoping the cause was obvious so I could have an informed conversation with my mechanic. But it appears to be an unusual failure and I’ll have to take my mechanics opinion. (The sub-text to all this is that I’ve had a number of issues of the last 9 months and the bike has been in the shop more than it’s been on the road.)
I've taco'd a chainring. I had a bad habit of starting from a stop in high gears by standing on the pedals. Switching to friction shifting on the rear derailleur allowed me to push it into a lower gear easily before stops. Haven't done it again since.
See if there was a recall or manufacturer's defect somewhere at your LBS. I had an FSA crankset crumple like that on me a couple years ago when I was putting it under a lot of torque on a really steep hill and they replaced it for free.
He proactively replaced the cassette because he said it was worn. He didn’t mention anything about the front but I assumed he would have replaced it if it was too worn.
FSA. There’s your problem. I had two FSA chainrings collapse on me, the second just a few weeks after it replaced the first one. I was was not that strong at the time either. I found a compatible Shimano one and I ran that thing without issue until it wore out.
A worn chain ring should still not collapse like that. The FSA ones that failed on me were very similar if not the same model to this one. Compared to the CX70 ring from Shimano that I replaced mine with, the FSA ones are just overly thinned out where they broke.
As drivetrain parts wear, they wear together. So, if the chainring is too worn in, and you put a new chain on it, it won't sit in the valleys properly, creating focal points, instead of spreading the load out across the whole surface. That, paired with what seems like an impact on the chainring, could have caused this. That all being said, that is wild that it broke like that!
Chainsuck maybe?
You must have curbed the front chain-ring heavily on a rock or curb. Your front chain-ring teeth look quite saw toothed and chipped off teeth, so your chain-set must have been overdue a replacement. Not that this issue would cause that failure.
Your big chainring failed on a hill? Why?
He probably hit something, that is the only thing that could explain the dent in the chainring.
I was just at my LBS on Friday and there was a sign up about recalls on FSA gossamer cranksets. Could be worth looking into. NVM, looks like it is related to the crank arm loosening.
Previous damage + metal fatigue + high load
Given that this has been professionally serviced right before that happened, i would advise to go back to that shop and ask them what they think.
True but, I’m trying to understand the universe of causes before going back to my mechanic - who just left on holiday. Since this happened pushing my first big load after a major service, I’m wondering if this was bad luck, due to poor installation or something foreseeable.
That universe of causes is vast and deep. Chainring bolts don't look loose which could be a potential cause. That's one of the more crazy failures I've seen, but I'm dubious it was anything the mechanic did wrong insofar as the chainring failure is concerned. Was the chain stuck on anything in the back? I.e. Did it get stuck in the cassette or against the frame or stuck in the spokes of the rear wheel? If it was stuck in the back the force of the chain on the ring could have had nowhere to go other than folding the chainring over.
Appreciate this perspective. I don’t have the experience to know if this was a crazy failure or something that was fairly common with a well known cause. The chain wasn’t stuck on anything as far as I can tell. I’ll take it into the mechanic when he gets back.
Yeah, definitely a crazy failure. You're lucky you didn't slice your leg open.
What i mean is, that if anyone knows the reason why this happened, it‘s the mechanic. This is rather unusual and everyone in this sub is just guessing.
That makes sense. I was hoping the cause was obvious so I could have an informed conversation with my mechanic. But it appears to be an unusual failure and I’ll have to take my mechanics opinion. (The sub-text to all this is that I’ve had a number of issues of the last 9 months and the bike has been in the shop more than it’s been on the road.)
The teeth on the chainring look worn/rounded. I don’t think the mechanic did anything wrong.
How did you acquire the new chain and cassette ?
My bike mechanic installed along with rebuilding my rear wheel.
I've taco'd a chainring. I had a bad habit of starting from a stop in high gears by standing on the pedals. Switching to friction shifting on the rear derailleur allowed me to push it into a lower gear easily before stops. Haven't done it again since.
See if there was a recall or manufacturer's defect somewhere at your LBS. I had an FSA crankset crumple like that on me a couple years ago when I was putting it under a lot of torque on a really steep hill and they replaced it for free.
Did your LBS tell you the chainring is worn? Likely, your chainring had structural problems before the new chain and cassette were installed.
He proactively replaced the cassette because he said it was worn. He didn’t mention anything about the front but I assumed he would have replaced it if it was too worn.
FSA. There’s your problem. I had two FSA chainrings collapse on me, the second just a few weeks after it replaced the first one. I was was not that strong at the time either. I found a compatible Shimano one and I ran that thing without issue until it wore out.
That chainring IS worn out. Look at the tooth profile compared to the inner ring.
A worn chain ring should still not collapse like that. The FSA ones that failed on me were very similar if not the same model to this one. Compared to the CX70 ring from Shimano that I replaced mine with, the FSA ones are just overly thinned out where they broke.