Chain falls between the chainrings of the front derailleur when shifting from the big ring to the small one

Question

The issue I have is that at the new bike I'm looking to purchase I'm faced with the followings:

• the chain falls between the 2 chainrings of the front derailleur when shifting from the big ring to the small one (see also attached image).
  • this happens almost all the time when having the rear derailleur in the gears 9 to 6 (the ones farther away from the wheel).
  • in the gears 5 to 1 (the ones closer to the wheel), it works fine most of the time.
  • also this happens mainly when shifting the gears while riding the bike, not that much when the bike is on the bike stand.
• last but not least, the chain never falls in between the chainrings when shifting from the small ring to the big one.

Some of the sales people told me that this happens because of using the 9-6 gears on the rear derailleur, and that one should not shift to the smaller chainring when in those gears.

Is this a normal behavior for a brand new bike? What can be the root cause of this, and how can it be fixed?

From my knowledge this should never happen no matter what gear combination is being used. And I suspect that the issue is either at the crankset, where the distance between the 2 chainrings is a few millimeters bigger than necessary, or with the chain itself.

The model I look into is Trek Zektor 2 with the following specs:

Shift Levers: Shimano Sora
Front Derailleur: Shimano 105
Rear Derailleur: Shimano Sora
Crankset: FSA Vero 50/34T
Rear Cogs: 9-speed

Update (2019-04-16): the store ordered a new bike, same exact model, and this one shifts perfectly fine, the chain never falls in between the 2 chainrings. Everything is the same on the bike, except 2 components: the FD is now a Shimano Sora (instead of Shimano 105), and the chain is a Shimano HG53 9-Speed (instead of Shimano HG54 10-Speed). Therefore it turns out that the problem was caused by the chain being too narrow for the rest of the setup.

Answer 1

This is not normal. Modern bikes should not throw the chain off the rings, either off the outside, inside or between the rings when changing gear.

The front derailleur is probably set up wrong. Don’t let the bike store staff tell you it’s normal. You have a choice here so just buy a bike that works right from the beginning.

It's true that you don't want to be cross-chained: on largest chainring and inboard half of cassette or smallest chainring and outboard half of cassette. That puts extra strain and wear on the chain, but the chain should not come off if you forget to follow this rule.