"Robert Strickland" <
[email protected]> wrote in message
news:<
[email protected]>...
> > 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
> >
> > Sheldon "It Ain't Broke" Brown
>
> 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?
>
> Rob "Just Curious" Strickland
Dear Rob,
Just guessing, but the initial problem is the chain is angled between the inside front chain-ring
and the outside rear cog.
The sideways pull presumably increases wear on any size cog.
The poor 11-tooth is the smallest widely available cog, so each of its teeth wears at a rate of 1/11
or 9.1% of the wear, noticeably more rapidly than the teeth on larger gears--a 12-tooth cog
distributes its wear over a dozen teeth at a rate of 1/12 or 8.33% per tooth, a 13-tooth at 1/13 or
7.7%, and so on.
This is why my heavily used front 53-tooth chain-ring outlasts my hideously overused 11-tooth rear
cog--the wear is spread out over more teeth (at shallower angles, too). Normal riders use other rear
cogs far more than I do, spreading the wear out over up to 10 rear cogs, so I'm a bit sensitive
about this.
A simpler way to look at such figures is that each tooth on the 11-tooth bears almost 10% more of
the stress than a tooth on a 12-tooth cog--not just when the chain is pulled over at an awkward
angle, but in all operations.
Added to this is problem that the chain enters, wraps around, and exits from the 11-tooth cog more
sharply than from larger cogs.
A final problem is that the load on an 11-tooth is likely to be higher, gear mashing.
A 12-tooth would last longer under such severe cross-chaining, but would still be abused. So, to a
lesser extent, is the opposite large-large cross-chaining. And the chain doesn't like to be bent
sideways under load like that, either.
Plus, there's often an annoying noise.
If nothing else, this may provoke more informed explanations.
Carl Fogel