Kimmo 

It's just a cutout. Pretty sure the cheaper ally rings were missing the hole behind the sticker, like the steel rings... just look at the back of the ring - either there'll be metal where a hole should be, or you'll see the back of the sticker covered in crud.

GreenNeedle

Funny you going that way. I am looking at going the other way and trying some 170s. No problems with the 175s but just to try.

I'll have a look later on. They are back on my 1981 Sun Solo with the rest of the groupset at the moment.

Decadence

A recent roadie resto-project of mine came with an exage/biopace crankset, and while I haven't been up any steep hills yet, the biopace rings feel good and shift smoothly. They probably aren't super-light but they're totally functional

Lemond1985

One of their selling points BIDT, used to be that Biopace rings were "designed by a computer". And since we all know that computers pretty much infallible, that's probably why they shift so good.

popeye

Here today gone tomorrow. Reminds me of the latest fad in road bikes.

Kimmo 

If they're ally forget the weight, NBD. But a plain big just doesn't cut it next to one with ramps and pins, especially if it's properly indexed to the small ring, and Shimano (I use SRAM rings on one bike because ramps and pins by anyone is way better than none, the rest is diminishing returns). The Biopace big rings are pretty close to round, anyway; they don't do much.

To index a modern big ring against a Biopace small ring, you'd need to have a matching small ring to find a position to mount the big ring so the small ring's teeth are in the same spot relative to it. Since it won't be a 40t, you should be able to get it close enough.

canklecat

I had some shifting issues when I tried to combine the 52T Biopace big ring with a conventional 38T little ring. But the chain drops may have been the usual problem with too much difference in diameter, not specifically a non-round chainring issue. I tried it for only a week and didn't care for the gear steps anyway -- with a 7-speed freewheel the steps are more critical.

I switched back to the 42 Biopace mostly because I like the feel on slow cadence climbing. It's subtle but I can feel a little mechanical advantage that suits my raggedy cadence. And I plan to try some 172.5 or 175 crank arms instead of the 170 on the bike now.

I may swap out the 52 big ring Biopace and try a round 52 or 53 big ring. As other folks noted, there's less elongation with the 52 Bioopace and it doesn't feel like it offers the same advantage in slow cadence, higher effort pedaling. It'll be interesting to see whether a 53T ramped and pinned round big ring gets along okay with the 42 Biopace.