Hey everybody. This week I was doing some sprints seated and standing. When I am on my big ring and I shift to a heavier gear, my chain just flies off the big chainring to between the big chainring and the crank arm. I also tried to shift to a easier gear( heavy load on the pedals) and the chain flies off to either the inner ring or between the BB and the inner ring.

I've been to a bike shop 4 times, we changed the chain, the cassette and the chainrings and still, same problem. The front derailleur is aligned by-the-book. The rear derailleur looks to be aligned. No matter how spot on we get the gears to shift on the stand, every time I put load on the pedals, the gears are shifted too aggressively to the next gear, up and down.

I run a complete Ultegra 6800 groupset (53/39; 11-25t; short cage rear derailleur) excluding the chain which is KMC X11SL. Frame is Ridley Noah SL size xs.

If you guys have an idea what causes this please let me know,

Cheers.

All the variables but one have been changed. That would be the one on the saddle, pushing the pedals. You are indeed shifting too aggressively. The equipment is not capable of smooth shifts under load, and one's gear ratio is generally chosen before "sprinting." Certainly a sprint should never involve shifting between chainrings, and the small chaining should be selected before a hard climb.

+1. Andy.

Are you by chance a larger rider? It might be that the frame is flexing from the force you're putting on it. Having said that, front shifts under load are a pretty dicey proposition in general. If you can't start your sprint in 53-25, you should probably be running a compact crank or larger cassette. In general, you want your front shifting sorted out BEFORE you drope the hamer.

As I read this, the OP isn't talking about shifting the front under load, since he's saying that while on the larger ring and shifting to a higher gear. So I read it as the chain dumping when he shifts the rear.

If I'm reading him right, it sounds more like a question of chain line, FD trim, or shift gates on the ring, so that when the sine wave that shifting causes comes up the chain, it dumps. I've seen this before, and it's hard to diagnose and cure.

One thing that might help is to look for a chain with more bellmouth, or inside chamfer on the inner plates, so they do a better job picking up the points of the teeth. This is the same advice I give people riding 1x systems, and it works. Chains keep changing and I don't keep up so I can't suggest any offhand, but the OP can eyeball the offerings and look for a likely candidate.

Another factor that might help is better lubrication. Obviously I have a bias, but lower friction between the chain plates and sides of the teeth will reduce the chance of the chain climbing up when there's more side pressure.

If the OP lives in the NY area, I'd be willing to take a look, or recommend a shop who might diagnose the issue.

Of course, if I'm reading the OP wrong and it's about shifting the front under load, that's a nono as the others posted. Even shifting the rear under load is risky, and the problem might go away with some refinement of shift technique.