Emergency single-speed forcing chain to larger sprocket

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


PS I inadvertently posted this on wreck.bike.misc first, but this is
it's right home.

Johan Bornman wrote:
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


PS I inadvertently posted this on wreck.bike.misc first, but this is
it's right home.

I had this happen too the one time I tried it after trashing my RD in
the woods. I assumed it was because I didn't get the chain line right.
It's hard to do this with vertical dropouts. I have done it successfully
at other times with older freewheel bike & horizontal dropouts.

When my chain climbed up it didn't break but it was so tightly bound I
ruined a chain tool removing it.