Changing gearing affecting singlespeed chain tension



S

Sheldon Brown

Guest
Anthony DeLorenzo wrote:

"I have a vertical dropout frame I converted to singlespeed. I lucked
into a 40 x 18 combination that yields ideal chain tension, to the
point where I had to tweak the chainring bolts to eliminate those last
few binding sections.

"I want lower gearing for the winter. According to Sheldon, I could add

one link and go to a 36x18 or add two links and go to a 32x18 or maybe
a 36x22. http://sheldonbrown.com/singlespeed.html#vertical "

That's AWFULLY low gearing. How about 39/19, or even 38/20.

"My question: How exact is this formula (1 link = 4 teeth)?"

As others have pointed out, it isn't _perfectly_ accurate because it
doesn't take into account the changing angle of the upper/lower chain
run, but this is a pretty small effect. In any case, going to lower
gears would only make it slightly looser.

Peter Chisholm suggested:

"Why not get a chain tensioner...easy, quick and not expensive or make
one out of 1/2 of an old RDER..single speed...no problem and you can
use any combo you want."

I can't agree. It's true that pulley type tensioners work after a
fashion, but they're really mono-buttocked and kludgy. Also, it is
possible to have chain jump when using sprockets not intended for
derailer use, if you have a spring loaded tensioner.

Sheldon "Dunno Why Google Groups's Quoting Stopped Working" Brown
+----------------------------------------------------+
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On Sat, 23 Sep 2006 17:44:54 -0700, Sheldon Brown wrote:

> Anthony DeLorenzo wrote:
>
> "I have a vertical dropout frame I converted to singlespeed. I lucked
> into a 40 x 18 combination that yields ideal chain tension, to the
> point where I had to tweak the chainring bolts to eliminate those last
> few binding sections.
>
> "I want lower gearing for the winter.


A 40/18 is a pretty low gear. 58" by my calculator. Are you talking
about commuting in snow? For that, yes, maybe you would want something
lower, but depending on where you live that might be a sometime thing.

To minimize the incompatibility, I would switch both sides, say a 42/20
or 44/21, depending on how much smaller you wanted the gear to be, rather
than just downsizing the chainring. There, the 4-teeth/link calculation
will break down, since you are changing the chain angle too much. Of
course, you will need a new chain in that case, but those are the breaks.
You might just add a link, consisting of one inner link plus a second
master link, and that way you can remove it and go back to the 40/18 for
warmer weather.

But my rain bike is a 48/20, bigger than your gear, and I live in hilly
country. Anything smaller would be really slow, IMO. My dry-weather
commuter has a 48/19 fixed (or an 18; it's a double-sided hub).

--

David L. Johnson

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-- J. R. R. Tolkein