Originally Posted by Shiznaz
I have tried 3 different biopace rings on fixed gear or SS bikes before and its never worked properly (crazy looseness and binding). Maybe I just suck at the mechanics of it.
I think biopace magnifies several mechanical issues.
I had a little difficulty with a new crank, and I attributed it to an out of round crank.
I discussed it on this forum, but eventually got around to talking to one of my more respected lbs's about it.
They described all bikes as demonstrating symptoms of "something" out of round.
This particular lbs attributed out of round symptoms to out of round cranks, out of plane cranks, out of round chainrings, bent crank spindles, and out of line frames.
In the case of road bikes, the system of derailleurs and chain tensioner masks these out of round conditions, and they don't really show up except on some single-speeds and all fixies.
A biopace ring, by design, has two tight spots and two slack spots on every revolution.
If one adds a normal out of round condition to this equation, then one adds the extra tight and slack of an out of round to the tight and slack of the biopace.
With a square taper spindle, this gives the chain at least eight different tight and slack combinations, one of which involves a reasonably tight slack point for the first 180 degrees of rotation, but a puzzling extreme slackness for the other 180 degrees of rotation.
This extreme slackness defies a logical fix and can drive a person crazy.
I think it involves either a slightly bent spindle, or an out of line bottom bracket, or both.
In the case of this 42t biopace ring, because of having the bevels against the star, I had more movement in the chainring than normal, and Sheldon Brown's centering method
REALLY worked well.
Now that I have two years experience with biopace, I don't recommend it for anyone other than an avid hobbyist.
I really like biopace, but then I like fiddling around and experimenting with my bike, and I have a wall full of chainrings, cogs and chains.
So, short story made long, I don't think Shiznaz has any deficiencies as a mechanic, but simply encountered one of the difficulties of playing with biopace.
If he wants to take another run at it, this 42t ring might work for him, since it has more centering potential.
In any event, I feel very pleased with my 64.9 gear inch experiment, and I may spend the summer at this gear inch.
Thanks to my fellow riders who encouraged me to try it.