lols, check out this guy...

I wrote quickly and I must have been unclear: If you stand to the side of the bike and look at the drivetrain, the chain sort of moves in a triangle. Not true, I know, but the principle is the same that each leg of the chain has to lengthen as the wide part of the elliptical ring is vertical.

Check out the pics of BioPace fg bikes that Sheldon used on his website. In every case, it was a small ring. I would guess below thirty teeth. Notice also that the squish of the ellipse gets much more pronounced in the larger rings. I think that the way he happened to set up his BP bikes made it not be an issue. All I am saying is that his conclusion might not cover the larger rings.

jim

First, buy a new chain, then get it all set up and rotate the Biopace chainring to the "tight spot" then making sure it stays on the tight spot loosen your rear wheel and make the chain adjustment just like you would any other chainring and your done. Your chain "popped" because you built a "frankenchain" not because your running a Biopace ring. Well that might be wrong, if you were running a Biopace ring that was to tight on the "tight spot" along with the crappy "frankenchain" then thats why it popped, but dont blame the chainring as it will run just fine with the Biopace and a new chain as long as you set it up right, which takes all of five minutes

They aren't ellipses, btw.

I know. Shimano made a point of saying that unlike earlier oddball rings (I think Sugino tried one out before them, and it was a simple ellipse), theirs were something more complicated. But still, the point is the same: they were flatter in one dimension than they were in the other.

jim

igedwa,
i didn't mean to imply that you were wrong; in fact, i agree with what you said. i meant to say that i believe that your use of the exaggerated example of a true triangle (or rectangle, more accurately) is inappropriate in that it's too extreme.
the big portions and small portions aren't disparaged enough to create the same problems that a triangle would, even in large rings.
though, i can't say this from experience, as i only rode a 42. but from what i penciled out, things would still be safe at larger ring sizes.

I believe the relevant point about biopace rings (and it is not intuitive at all, at least not to me), is that since the ring is symmetrical, the same number of teeth, and hence the same amount of chain, are always engaged by the ring. that implies that the amount of chain between the cog and the ring is always the same. The amount of chain between the cog and the ring does NOT go up and down as the chain ring turns. the only variable is the "apparent diameter" of the ring; as that changes, distance from the engagement point of the cog and the ring varies ever so slightly, as if the chainstay length were varying a tiny amount with each revolution of the crank.

In a back-of-the-envelope calculation I think this might result in a change in cog-to-chainring distance of about 1.5 millimeters... which is probably with in the range of acceptable chain tension that most of us maintain.

Does any one happen to know the high and low radii of a biopace chain ring (yes, I know, that will depend on the size)? My winter bike has them... but its at home and I am not.

not exact, but perhaps you can print this out?

https://www.sudibe.de/fotos/technik/biopace48.jpg

Has anyone posted this link yet ?

Sheldon Brown on Biopace

yep.
unfortunately, sheldon isn't an oldschool track cat, and therefore shouldn't be considered an expert on the matter.
if only he'd trained with Chris Williams in '93...

OK, That worked. I measure a short axis of about 180 millimeters, and a long axis about 189 millimeters. (I may have scaled the picture wrong... so I'm open to corrections here or anywhere in my calculations)

Assuming a 16 tooh cog has a radius of about 32 mm, and a chain stay length of 405 mm (from a Kilo TT schematic at bikesdirect.com), the length of chain between the top of the cog and the top of the chain ring varies from about 411.7mm to about 412.5 mm or, about 0.8 millimeter.

I think I could live with that.

nice.

sfcrossrider, please see that chris williams and the oldschool track cat/mechanic community gets the memo; the mystery has been solved.

If you insist on being an @sshole at least get the facts correct. Chris William was the EDS team mechanic/ bike guru. There is a whole history of people riding track bikes long before you jumped on the fad. We had different opinions about biopace (some liked it, some thought it was a bad idea). I never claimed I was all knowing, or Sheldon was incorrect. I just said the people I worked with (some were pros) didn't think it was a good idea.

If you spend more time on your bike, and less time being a d!ckhead to strangers on the internet you would be a much better rider.

if i could ride at work, i would. trust me.

anyway, glad the issue could be resolved.
OP, you should be fine-- have a blast.