Someone wrote:
>
>>Counterintuitively, it isn't a problem. The chainring is lopsided, but
>>symmetrical, since a full half of it is covered by the chain at a time, it all
>>averages out.
>
John Thompson wrote:
>
> Ah, but it *IS* a problem. The issue isn't that half the ring isn't always
> wrapped by chain, but rather that the effective radius of the ring changes
> as the pedals move. This in turn changes the angular distance between the
> last tooth on the rear cog and the first tooth on the ring, which means
> the chain continually slackens and tightens as you pedal. This isn't too
> much of an issue provided you can keep contant tension on the chain by
> pedalling, but becomes problematic at high rpms -- such as experienced on
> high-speed downhill runs, and precisely the situation where you DON'T want
> to worry about your chain coming off!
Actually the radius has nothing to do with it, the issue is concentricity.
A properly centered chainring, whether round, biopace or elliptical will
always have half of the teeth on either side of its "equator."
A 42 tooth chainring (what I mostly use on my fixed gears) will engage
21 rollers with 21 teeth.
There is a _slight_ tension variance due to the fact that the angle
between two straight chain sections varies slightly with the orientation
of the chainring.
I can assure you, John, that this _does_ work. I have Biopace
chainrings on several of my fixed gear bikes, including my fixed gear
tandem (which has three of them!) and also on a couple of bikes with
internal gear hubs. No chain tensioner is needed, and my chains do not
fall off.
> I have an ostensibly round ring (but in fact about 2mm out of round)
> I've used on a fixed gear and had considerable trouble setting the
> chain tension such that it is loose enough to freely move but tight
> enough to prevent derailment. Biopace rings are MUCH further out of
> round than that.
They may not be round, but they are well made and tend to be pretty
concentric.
For your eccentric setup, there is a technique that often helps to
correct this problem. I explain it at:
http://sheldonbrown.com/fixed-conversion.html#tension
Sheldon "Biopace" Brown
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