Hi all.
I have an issue with my new bike(around 1000km) on it. This problem has been occurring for the last 200km or so.
When I'm in the small chainring (39) and the largest two cogs (25,23) and am putting some force, but not crazy, into the pedals the chain falls off the front small chainring. This seems to happen when going up hill (hence the force).
The bike shifts fine. It does have a carbon hollow crank (FSA Slk) and I'm starting to think if this is a problem.
The chain is in good condition.
Any ideas on what could be causing this? I'm lost with this one

First, you need to have the shop where you bought the bike check the limit screws on the front der. You can do it yourself if so inclined but I assume that the bike must be under some kind of warranty with the shop.

If you want to do it yourself have a look here.

Also, I would suggest buying a chainfall device to protect your from from damage by the chain in the future.

Do you think you're twisting/torqueing the frame, cranks or BB when you climb? Is there any noise or weird feeling? I'd find this quite unlikely, and would be checking the FD's limit screws as previously mentioned.

But check the tightness of everything else down there, and alignment of the rear wheel. Take it all apart, grease (or anti-seize) where you need to, and put it back together again, etc.

Yes, check the limit screws. It is altogether likely that the front derailleur was adjusted by cable tension, not by the limit screw. As the cable has "worn in" (some folks say stretched, but that isn't so well accepted anymore), it may have allowed the front derailleur to move to the left and that is unshipping the chain under some riding conditions. First you set the screw, then you tighten the cable.

Agreed. The "L" limit screw needs to be adjusted.

Best to have the shop where you bought the bike adjust everything.

Also a good idea to learn how to do it yourself. It is not difficult.

I sort of get taking it into the shop since it is under warranty. But adjusting the limit screw is only going to take a couple minutes to do yourself.

I guess if the shop is close and will fix it while you wait it would be ok. But this is the busy time for a lot of shops so even for small jobs you need to drop it off

Thanks for the replies. I was out for a cycle on it this morning, nice sunny day, and was planning to work on it after. Near home going up a hill the chain fell off again while in the 3rd largest cog, with no gear change. A chain link was damaged in the process. I was near the shop at the time so free wheeled down and they have it now. Thanks again!

Chain shouldn't just fall off while riding. Very odd. Bent link ?
Tightening the limit may help the symptom, but not the cause.

As the OP describes the problem, the chain falls off under load when on the inner rear 2 sprockets.

So there are two elements, the chain angle coming from the inner rear, and possible deflections of the chainring under load. Since it's easy enough to test the latter, that's where to start.

Shift to the problem gears and apply the brakes or put the front wheel against a wall so the bike can't roll. Set the right crank in the forward horizon position and step into it as if climbing. Watch the chain inside the RD cage. Normally you'll see some movement outward. The question is whether it's moving far enough to touch the outer plate and initiate a shift. If so, some outward trim of the inner limit will probably help.

Also check for anything which might be allowing excess flex, usually a loose bottom bracket, but could be elsewhere.

I've also had a broken chairing bolt cause this same problem. Interested to see what the shop finds.

Perhaps a very slightly bent chain ring could do it, but they usually cause problems all the time, not just under high stress.

Yes, there are multiple possible causes, which is why doing a proper diagnosis to find what is actually happening is the right approach.

Simply retrimming the FD may solve the problem in the short term, but may mask something which can get worse, ie. a loose bolt that will get looser and possibly fall off and get lost, or lead to the ring folding under load.

Popping off under load is most likely wear or failure of a chainring bolt. Chain is/was most likely pretty worn. If it was new and this is happening then it's because your ring is too worn.

My understanding is that this is a very new bike with only 2k of riding on it.

It looks like the bike has 1,000k on it and the problem started 200k ago.

I have no idea how far that is in American but I can google it later.

it's 621 miles total, and the problem started about 120 miles ago.

So it's too soon to be blaming wear, plus if there was no problem before, and is now, er can rule out everything that didn't change, ie. chain line, etc.

A metric century (100k) is 62.5 miles.

I had a problem with my chain popping off the small chain ring up front with barely any load. Turns out the chain ring was installed backwards. Reinstalled correctly and never had the problem again