Front shifting issues on a half-step setup

I've read most of the postings about half-step here; I respectfully ask
that this not become another question as to if half-step is useful or
outdated. I'd like to give it a try and a newly acquired bike gives me
that chance.

I am having two problems:
1. Shifting to inner chainring almost always causes the chain to miss
the ring and fall inward onto the bottom bracket.
2. When shifting from inner chainring to the middle, the chain often
overshoots and ends up between the middle and outer. It stays on top
and between the rings rings while I pedal. If I am lucky I can just
nudge it one way or the other. When unlucky it wedges between the two
rings.

It is a 1990 Trek 520 with mostly original components. The rear is a
12-28 hyperglide casset and the front is a 28-46-50 with round
chainrings. (the 46 was my worn outer ring of my 21 speed mountain bike
that is now, in its new position as a middle ring, flipped over. The
others are Shimano rings circa 1990) I am using barcon shifters
(indexed rear, friction front) and have a Shimano Deore II half-step
front derailer. The chain is of unknown origin (unmarked) but looks
like an older model (no powerlink) and measurements show it to be
virtually unworn. All components have been cleaned and relubricated.

Is the 46 to 28 jump just too far for reliable shifting or is it an
adjustment issue? It is wost when I am already in my 28 on the rear and
then shift from the 46 to the 28. I've tried making the outer plate of
the derailer more parallel to the chainrings (it was slightly inboard)
but that hasn't helped. I've adjusted the inward stop with no avail (it
happens even when shifting slowly so I didn't expect an improvement.) I
suppose a chain watcher would help but that seems inelegant.

About the chain going between the two larger rings, is this just
because they are so close in size? If I changed the 46 to the 47, would
that be worse? Will a new chain (8 speed PC-48) make it worse? Perhaps
a 9 speed center chainring will solve this.

Thank you.

Joe LoBuglio

Joe LoBuglio wrote:

I've read most of the postings about half-step here; I respectfully ask
that this not become another question as to if half-step is useful or
outdated. I'd like to give it a try and a newly acquired bike gives me
that chance.

I am having two problems:
1. Shifting to inner chainring almost always causes the chain to miss
the ring and fall inward onto the bottom bracket.
2. When shifting from inner chainring to the middle, the chain often
overshoots and ends up between the middle and outer. It stays on top
and between the rings rings while I pedal. If I am lucky I can just
nudge it one way or the other. When unlucky it wedges between the two
rings.

It is a 1990 Trek 520 with mostly original components. The rear is a
12-28 hyperglide casset and the front is a 28-46-50 with round
chainrings. (the 46 was my worn outer ring of my 21 speed mountain bike
that is now, in its new position as a middle ring, flipped over. The
others are Shimano rings circa 1990) I am using barcon shifters
(indexed rear, friction front) and have a Shimano Deore II half-step
front derailer. The chain is of unknown origin (unmarked) but looks
like an older model (no powerlink) and measurements show it to be
virtually unworn. All components have been cleaned and relubricated.

Is the 46 to 28 jump just too far for reliable shifting

No.

or is it an adjustment issue?

Probably not.

It is wost when I am already in my 28 on the rear and
then shift from the 46 to the 28. I've tried making the outer plate of
the derailer more parallel to the chainrings (it was slightly inboard)
but that hasn't helped. I've adjusted the inward stop with no avail (it
happens even when shifting slowly so I didn't expect an improvement.) I
suppose a chain watcher would help but that seems inelegant.

A chain watcher would fix that issue, but I'm pretty sure the root of
both problems is the middle chainring.

About the chain going between the two larger rings, is this just
because they are so close in size?

No.

If I changed the 46 to the 47, would
that be worse? Will a new chain (8 speed PC-48) make it worse? Perhaps
a 9 speed center chainring will solve this.

You need a middle chainring _designed_ for the middle position. This
will put the teeth in the right plane, and you'll have the correct bevel
on the inner surface of the chainring.

http://harriscyclery.com/chainrings.html

Sheldon "Wrong Ring" Brown
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