Emergency single-speed forcing chain to larger sprocket

[Johan Bornman]

After suffering a broken jockey, an emergency single-speed was created
by bypassing the jockey and running the chain from a MTB's centre
front chainring to the centre (9-speed) rear sprocket. In spite of the
chainline appearing to be straight, the chain would still force itself
onto the larger sprockets up to a point where it breaks.

What causes this phenomenum?

JB

[Leo Lichtman]

"Johan Bornman" wrote: (clip) In spite of the
> chainline appearing to be straight, the chain would still force itself
> onto the larger sprockets (clip)
^^^^^^^^^^^^^^^^^^^^^
It can't be caused by the chainline, because each auto-shift makes the
chainline favor downshifting more. Is it possible that the excess tension
brought on by each shift is tilting the sprockets or axle in some way to
cause this?