Read the Sheldon article on chains, and then revise your theory.

https://www.sheldonbrown.com/chains.html

In theory, the oval ring takes up an equal amount of chain all through its revolution. That's all fine, but given its shape, the top (and bottom) point where the chain exits/enters the ring sorta moves up and down. (That's the whole point of biopace, the changing leverage). That means that the chain goes to the cog at a different angle, from a different point vertically. Obviously, it would take a longer bit of chain to connect from the cog to a point higher up on the upper section (and lower down on the lower section) of the chain loop... so it changes tension a bit.

BTW, this is why the "add 4 teeth to the chainring and you will be fine with one more link in the chain" axiom doesn't always work with vertical dropout frames.

Well, come visit me in Bend.
Destination resort and all that.
I'll fix you lunch and you can show me how you easily derailed the chain in the stand, 'cause mine won't do it.
Maybe I don't try hard enough.

(Please read the above as written, in a friendly, respectful voice.)

ken--it could be that because you're running a comparatively large biopace chainring, it has less "differential" in the ovoid than the smaller rings. You might also have a stiffer frame than others (?).

As I posted, on my end, it really was night & day between the 42t biopace and the 43t round ring. My chain was constantly derailing with the biopace, even though I did really like the feel of it, maybe because my frame (185# on a '71 raleigh supercourse conversion) is noticeably flexy, maybe because of the ramped teeth.

Ken Cox writes more clearly and nicely everyone.

nice. the great 'suicide hub' BS even makes it into biopace discussions now.

aeroplane - glad to have a fellow crusader .... where did the school system fail on that one?

EEEEEK.

The chain tension is going to be all over the place and the chain will fall off.

Just get a new ring, they cost what, twenty dollars from QBP?

ok, thanks. So what part of the chain stretch article deals with Bio Pace. I know how and why chains stretch. Live long and prosper Sheldon. Besides that it's just stupid.

Chain "stretch" has nothing at all to do with Biopace, that's why it isn't mentioned.

https://sheldonbrown.com/biopace

https://sheldonbrown.com/chains

Sheldon "Non Sequitur" Brown

Thanks for clarifying that.
It seems to me that you can through chain adjustment in the drop out get a good average of tight to slack ratio. But it if it isn't pretty close the high spot of the CR is going to cause undue fatigue and shorten the life of the chain. Is this accurate?

The chain should never get tight, i.e. taut.

Chains don't fatigue, they wear.

See: https://sheldonbrown.com/chains

Sheldon "My Knees Like Biopace" Brown

Thanks Sheldon. My knees like the proper distance between saddle and extended pedal.

lol sounds like my bike!
except i dont have a biopace. My chainring is just outta wack. Just curious, I dont use clips or straps, is it possible that if i pedal with different strengths on my legs that it can bend the chainring out of round? I dont know how to fix my chainring high and low problem but it is getting worse AND it does correspond to a certain point in my chainring.

Any advice? I'm thinking of just buying a new 44t one because I want to up my gear inches now.

I recently switched my FG over to a biopace II setup (46:16) without any problems so far. I have heard a lot of rumors about them, so I picked one up off ebay for $10. So far its been a good experiment, I understand the problems associated with higher spin rates, but for a city bike its great!

Have been running biopace for over a year. See link to FGG in my sig. Works great. OP: When you tighten the chain, get it very snug with the cranks vertical. They'll be just right when the cranks are horizontal.

I have NO problems with it. Ive run round and biopace (depending on what I have when I build the bike) and I can't tell the difference. I have 3 biopace cranksets (one in use) and my next bike will use them to. I just have no reason NOT to.

In response to ISDH's comment "...with the chain tight enough to pop I could easily derail it in the stand," I went out into the garage and put my bike in the stand.
With the chain in its most slack position, with no tension on it, I could in fact derail it by pulling sideways and pushing the chain forward while turning the crank at the same time.

On the other hand, pressure on the crank kept the chain straight and secure.
In two years of riding with a biopace, I have had no trouble with the chain.
In order to derail the chain while riding, I would need to exactly match the speed of my rear wheel with my crank, so as to not put any tension on the chain, and then someone would need to pull sideways on it.

Or so it seems to me.

This biopace thing brings out as much controversy as daylight savings time.

You missed the point. Your assertation was that the varying forces on the chain due to the eccentric ring would stretch the chain faster. The Sheldon article points out that chain wear occurrs because of running a dirty chain, not the amount of force on it.

Sorry to disagree but it is a combination of both. The dirty and less cared for chain obviously will ruin faster, but to believe that the forces acting on a chain doesn't cause the pins pulling against the plates to actually "bite" into them resulting in the chain appearing stretched is silly.

The average tension on the chain is no different with a Biopace chainring than it is with a round or elliptical ring of the same size.

If that's you main concern, go for big rings and sprockets. Better yet, make sure they are even numbered and keep the chain/sprocket phase relationship constant.

See: https://sheldonbrown.com/chain-life.html for details on this.

Sheldon "Live Long And Prosper" Brown

Now we're getting closer to the OP. So you're saying that the force on the chain is equal regardless of when it is on the high or low point of the BP CR?

No, I said the average tension is the same.

There will be slight variations due to the non-roundness, but the magnitude of these variations is much smaller than, the variations that normally occur duing the pedaling cycle.

The maximum tension will be slightly higher, probably about the equivalent of using a chainring 2 or 3 teeth smaller, but the average will be the same.

Bottom line, I do not believe that there is any difference in chain wear with Biopace chainrings compared with round or even traditional ellipticals.

Sheldon "It's An Imaginary Problem" Brown