I was cleaning my chain this morning and I noticed that my large chainring looked like it was wonking around. It just didn't look round.
With a fixed reference point, I estimate a difference of about 3mm. It doesn't seem like a lot but it is noticeable.
I'm wondering if this is a big problem? I have about 80 kms on this bike so far and I haven't felt anything weird. Will it affect any functioning? The ring seems to be perfectly flat, just oval.
Would this be a manufacturing defect or caused by an accident? The bike is 14 years old, but has hardly been ridden. I think it might have gotten a few bumps from being moved form place to place with the last owner.

The part that is farthest out is opposite the crank.

As stated on the yellow sticker on the bottom of the picture, that is a Shimano Biopace chainring. It was a gimmick started in the late 1980s by Shimano that was supposed to improve pedaling efficiency and get rid of the the "dead spot" at the bottom the of the pedal stroke. Some people that they looked too weird and replaced them, others experienced knee pain after using them, and some recumbent cyclists find them beneficial.
If you are not experiencing any mechanical or physiological problems that arise from the chainrings themselves, it would be okay to leave them and use them until they wear out.
Summary: They're supposed to be like that.

Cheers
lverhagen

Wow, that's hilarious. I would have never guessed that it was deliberate, though I was perplexed because there was no sign of anything having gotten bonked.

Thanks!

ok, having read further, it seems like anyone who was cycling through the 80s and 90s is well aware of the Biopace. What is really interesting is the level of emotion that pops up in almost every internet discussion about these chainrings. Things often degrade into insults and personal attacks...
In any case, Sheldon like them https://www.sheldonbrown.com/biopace.html

Interesting too that some professionals are now using elliptical chainrings apparently.

Rotor, a Spanish company has a different oval, and a crankset with a cam
to move your feet past BDC with each pedal rotation,
no power at the bottom of the stroke..

**** you.

OK, guess the thread is complete. Kidding, but on a serious note, if you never noticed it riding, then there's no problem. I use a converted MTB with Biopace for commuting/utility-riding and always forget about it.

Biopace was a sound design but a marketing fail for Shimano and now we have seen the return of non round chain rings at the professional level and this might trickle down for regular users.

This is what Shimano did not do with Biopace... most of their stuff starts at the top and trickles down and Biopace was sold at every level and think they had a very strong belief that this would revolutionize chain wheels and that everyone should have it.

It is a good system that works in a wide range of applications... it is great for climbing and at very high rpm can smooth out one's pedal stroke and they are not any harder on your knees than round chain rings and can actually help people with knee issues.

Their design unloads the pedal at it's highest stress / load point and allows you to maintain a consistent crank speed and I have more than a few clients who have had knee issues and surgeries who love their Biopace rings.

I blew out my knee 4 years ago on the trail and could ride a Biopace long before I could ride with a standard round chain wheel... it wad definitely easier on the knee.

There were copycat non-round rings from other companies, such as the Sugino Cycloid series:

The chainring holes look a little strange.... what BDC is that? I have never seen a 7 hole ring with a U socket.

I never actually used that 28T ring but I probably got it when I bought a middle/40-something Cycloid ring that I used for a while as part of single-ring setup with an early-90's Shimano LX crankset, which was 110 BCD with, what, 74 for the granny?
I just measured it, and it's about 44mm, which makes it 74 BCD.

WHOOSH! (That's Aquakitty's comment flying overhead. )

LOL. There's no way.

I remember being pretty impressed by these back in the day... back in the 80s I imagined they were marvels of engineering.

These biopace rings have served to reveal the ways in which we're not entirely rational in our understanding of our world.
We assume that they're not effective simply because they weren't exclusive to expensive bicycles.
We assume that they are bad for everyone's knees because my knees hurt when I used them.
Biopace is a gift from Gaia, not so much to help us cycle, but to help us look into our own hearts.



On another (serious) note, I heard that they're Egged shaped, not oval. Wouldn't that make the peddaling experience different for each leg??

There's no real egg-shaped rings - any non-round rings that aren't just ellipses (like Biopace) have a shape that's repeated each 180°.

And they're too close to round to make much difference. Useful amounts of ellipticality (?) start to cause problems with the FD.

Interestingly, Biopace rings are made so the ratio's lower during the power phase of the stroke, and almost all the rest are the other way around... IMO the latter makes more sense.

Idk about that. I was just comparing a 53T Biopace to a 52 and 53 round ring, putting them back to back. The Biopace ring got "smaller" than the 52 ring in some spots, and larger than the 53 in others. So I'd say it has a range of maybe 51-54 teeth. And that's on the largest ring, which are the roundest looking. I don't have any little round rings for comparison, but the little 26 and 28T Biopace rings are really elliptical as seen in the photo above of the Sugino ring. They probably change several teeth in each direction.

Sheldon's article on Biopace explains why the power stroke has the lower ratio.

Whether Biopace is a marvel of engineering or an unsightly blemish on Shimano's history, some will never agree on. But I like my Biopace.

I came across a study linked in an earlier thread; went right into the whole deal. Rings need to be seriously wonky before being good for an extra 6-7% max. Biopace rings were in the range of around 1% when rotated IIRC (earlier ones were less round and good for a tad more). That ring is Sugino thus not Biopace.

Sadly, couldn't find the article the next time these came up... : (