Had a really scary experience today. I was practising sprints and my chain fell off to the outside during the sprint causing me to nearly crash. I’ve scratched one of my new s works shoes and ruined two new cleats but just got away with it.
Now i have no idea why it happened.
First ride on new Chain, chainrings and cassette after a long winter.
It’s an Ultegra 8100 groupset with minimal millage. I set the gear before starting the sprint and didn’t shift gear or hit a bump in the road. Front Shifting has been perfect and I’ve not dropped a chain over bumps. The only strange part of my set up is that im using alugear 54/40 chainrings. They feel flexible but surely that can’t be the reason?
Any advice would be great because I’m a bit scared to ride my bike now. The bike performed fine for the rest of the 60 mile ride
- Why not? Flex under high loads seems like a very plausible reason for a chain to eject.
I just thought they would have designed them to not be dangerous. They weren’t paticulary cheap either.
Any way of checking if its ring flex without having to sprint again? 
Where’s the video of @Nate_Pearson dropping his chain during sprint reps? Between bike flex, chainring flex, chain stretch…it can happen.
You didn’t crash, sounds like, and for that I salute you!
I would normally agree that its just one of those things, its just this is the first time I’ve sprinted on this chainring so at the moment it’s 100% failure rate which is making me nervous.
Full Ultegra groupset, so Ultegra chain as well?
@Alexander_Hardy try setting up a gopro on a little mini-tripod to film a trainer sprint from under the bottom bracket but a little in front…use a frame rate so you can get some slo-mo. Remote trigger the gopro and record some sprints. That will give you a better read of what is flexing and how much.
It could be that the additional torque you’re getting from a bigger chainring is causing some additional flex in the chainstay, too. What’s the bike frame?
I don’t blame you for being a little freaked out. Whenever I watch that vid of Nate dropping his chain it’s terrifying.
Have you checked your chain length? Maybe it’s too long and your rear derailleur isn’t adjusting tension appropriately?
@Alexander_Hardy here is the footage I was thinking of…at about the 7min 10sec mark:
Kind of interesting to have that video of the incident. If you watch it in slow mo, he never really got a large power reading before the chain slipped. Also, it came off when Nate was in the power stroke of the non-drive side pedal. But in any event…scary.
I’m always terrified seeing Nate on a bike. 
That is a terrifying video but is basically what happened to me.
So I chucked the bike on the turbo, the chain is jumping obviously now. 800 watts can get it to jump indoors. Here are two videos from below and the side.
Checked the front mech and it isn’t touching the chain at all when still. Bike is an orbea orca aero.
Chain was sized with park tools big 2 big method. I could probably shorten it 1 more set of links but it is already pulling the rear mech a lot when crosschaining. Checked b limit screw that looks in spec. Really confused now.
Excuse the baggy work clothes and messy floor
As your right foot hits 12:00 at around 0:23, you can see the chain doesn’t properly engage the ring. I’d guess it is the chainring flex, though without video of the RD it could be something there too.
(This is at about 0:11 in the video from below, though it’s not until a few frames later you see the “bump” in that video)
(edit: you can actually see the first link fail to engage the ring about 45 degrees earlier in the rotation, during second 0:22.)
Wow. I don’t know what to say about that! Except that it would make me never be able to sprint with a clear conscience.
Maybe you could show that footage to the chainring manufacturer and see what they say? I’m curious to see what other forum contributors have to say…
Couldnit be an issue with the derailleur capacity? Ultegra 8100 is rated up to 52T, while Dura Ace is 54T.
I thought that but these rings are listed for this crank and it doesnt say I need to use a dura ace front mech. Heres a picture of the front mech in the gear combo used, chain is clear of the cage.
I’ve set up loads of weird chainrings in my time (ossymetric, q rings, 58 tooth 1x, 54/39 11 speed) and never had this problem.
Heres flex applied via thumb pressure, does this seem more than normal chain rings? I can get it to touch the mech fairly easily.
That is certainly terrifying. How on earth does he stay upright?
Yeah, I wouldn’t ride that chainring, that is crazy flex compared to stock rings. It looks like they have prioritized bling over stiffness. I don’t know anything about the company who made it, never heard of them. My rule for anything safety-related on the bike - buy from an established company with deep pockets who has engineering/quality focused on not getting sued from product shortcomings. No off-brand bars, wheels, frame, stems, cranks, chainrings, etc. I’d throw that chainring in the trash or put it on a coffee ride “show” bike if they won’t let you return it.
Yep, the spider’s there for a reason. I had similar with a ‘bling’ fixie chainset where the ring pretty much folded in half when applying power suddenly
Probably one of his grandparents was a Weeble.
I appreciate the videos. Oftentimes people will ask a question and then can’t really provide more info!
I’ve never seen that exact issue before but I’d agree with the others who think it’s probably the chainring flexing too much. I’m running the SRAM AXS 1x aero chainring and just tried pushing on the edge, and it feels very, very solid, without play. The only time I’ve had a chain jump is when I put on a new chain and the cassette is too worn (I saw you put on new parts so that’s likely not your issue). It may be expensive to test but I’d try a different chainring, probably from a different manufacturer, and then try to return the one from the video.