I prescribed:

You should never, ever, under any circumstances be using the 39/11
combination.

It's generally abusive to ride in the small/small combination, but
especially so when there's an 11 tooth involved. There's nothing wrong
with your bike.

If you do space out the BB to make the useless 39/11 gear usable, you'll
cause deterioration on some of the more useful combinations involving
the big chainring.

See: http://sheldonbrown.com/gears.html

Rob "Just Curious" Strickland asked:

I've heard it said many times that the small/small combination is a no-no
but what makes it worse when an 11 tooth is involved?

The large/large and small/small combinations both suffer from poor chain
alignment, but the small/small combination has more severe problems:

The smaller chainring in front creates a stronger pull on the chain for
a given amount of pedal force.

The smallest rear sprocket shares that increased pull among fewer teeth
and chain rollers, leading to a high concentration of the load.

With an 11 tooth sprocket, you're lucky to have 5 teeth actually
engaging the chain at any time, 6 at the very most.

The smaller both sprockets are, the more damaging the small/small gear
combination is both to the sprockets and the chain.

Generally, with double chainring setups, it is best to avoid the
small/small gear combination, but when there's an 11 or 12 tooth
involved (or a triple chainwheel in front) riding in that combination
constitutes actual abuse of the equpiment.

Sheldon "Straight And Narrow" Brown
+---------------------------------------------------------------+
| The poet Henry O'Meara (1848-1904) was my great-grandfather |
| I've put his book "Ballads of America and Other Poems" |
| on the Web at: http://sheldonbrown.com/omeara |
+---------------------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
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