Hi there..A friend has purchased a second hand San Francisco Globe and the chain keeps coming off the front chain ring every time the bike's in highest gear...any ideas?
He's played about with rear derailleur but still the same problem.
Thanks in advance.

The H limit screw probably needs adjusting.

Just to confirm,this is a Specialized brand bike and does not have a front derailleur?

Hi ..that;s right..It's a single chain ring on the crank set..with no front derailleur.
The chain just keeps missing the teeth in highest rear gear..so I adjusted the rear derailleur to avoid the highest gear, as a stop gap measure, but when my friend attempted to ride it..off it came again.
Crank set looks like it could accommodate an inner chain ring so thought I might swap the chain ring to the inner position to move it towards the frame 5mm or so and thereby straighten up the chain angle from front to back..but it's odd that it's doing it in the first place.

Wouldn't moving the ring to the inner position actually increase the angle to the high gear? When you say "high gear", are you talking about the small cog?

Is there slack in the chain when in the small cog? Is the chain the stock chain? Does the freehub backpedal freely? I've had instances where backpedaling would cause a chain to drop, and it's surprising how often one moves one's feet in the opposite direction w/o thinking about it.

You need an narrow/wide chain ring if the front derailleur has been been removed.

To me it sounds like the derailleur was removed and inner the chainring was removed from the inner position on the crank...ghetto 1x conversion.

https://mbaction.com/a-guide-to-narrow-wide-chainrings/

It looks like this is some kind of Urban bike with no FD from factory.

I would check the rear derailleur hanger if it's been bent. From there I'd check the RD set screws.

Can you post pictures or at least what your RD is?