sam83

Looking for a 42 chainring and found a biopace.

But it is supposed to mount to the "inside". I need to mount it to the outside. That makes the orientation tab wrong.

I've oriented it so that it looks like the same, but does anyone know if it can be mounted this way and achieve the same geometry orientation?

fuzz2050

I'm a little confused, you should be able to mount the chainring in either position. Maybe if you posted a picture it would be easier to visualize the problem.

sam83

Never mind. The biopace rings are not round. They have a combination elliptical design and has a locator to show you orientation (line up with crank arm).

Mine was designed to be mounted inside (bolt recess). I traced the ring on cardboard, flipped the ring and attempted to match an orientation when reversed. Almost, but no cigar.

TallRider

I don't think that biopace rings are symmetrical (i.e., flipping it around will have a different functional curve).
just mount it on the outside and don't worry that there's not a recessed spot for chainring bolts. they will still bolt on correctly.
is this for a singlespeed?

Bianchigirll 

that makes no sense. if the chainring fits on the inside of the spider there no reason why is should not fit on the outside. it does not matter whether the chainring is on the inside or outside the 'tab' should line up with the crankarm.

fuzz2050

I'm still not quite clear as to the problem. It sounds like you can just simply move the chainring to the outer position without flipping it and everything should work out fine. I suppose the bevel might be off a little, and maybe the ramps and pins won't be in exactly the right spot, but if it's designed for an inner, they won't work quite right anyway.

Jeff Wills

On the high-quality Biopace chainrings, there's a recess the chainring nut fits into. If the "inner" ring is mounted in the "outer" position, this recess is up against the chainring spider if the chainring is in the correct orientation.

As Bianchigrrl pointed out, this makes no real difference. As long as the chainring bolts keep the chainring lined up with the spider, there won't be any issues.

cnnrmccloskey

It sounds to me like what the op is talking about the pin which sticks out on the inside of the largest chain ring, which if you moves the chain ring onto the inner side of the spider might run into the chainstay/downtube.
If thats what your talking about you should just grind that pin down it serves to no function that I know of.

sam83

This is for a 1 X 9 commuter. This biopace chainring was designed as an inner ring and has no ramps or pins (42T, 5-hole, 130 BCD). These rings have a little “tab” that orients with the crank arm when mounted on their intended side of the spider. To get the chainline to line up in the middle of the cassette (without changing the bottom bracket), I need the chainring to mount to the outside of the spider.


Mounting the ring with the bolt head recesses against the spider (wrong way) certainly preserves the intended orientation, but since it was not made to be mounted like that I had originally decided against it. After flipping it around to place bolt head recesses appropriately and placing it on the outside of the spider, I was able to rotate the ring by 2 holes and got what appeared to be something very close to the intended orientation.

I took it for a 30-mile moderately hilly ride. I had to go to my easiest combination a few times to keep things “comfortable” on some of the steeper climbs, but that did not seem unusual at all. I felt no “advantage”, my average speed seemed a little down, and afterwards my legs felt unusually tired for such a short, easy ride. This got me to wondering if my “best fit” was not good enough and potentially working against me. And, if so, perhaps the right orientation would actually prove beneficial (never used these voodoo devices before).

After removing the chainring and tracing the shape on cardboard, I flipped the ring over and compared it to the pattern on the cardboard. It was easy to see that the odd shape could not be exactly duplicated when inverted, regardless of bolt hole pattern.

As suggested, I’ll mount it to the outside with the recesses against the spider, thus preserving the intended orientation.

In general, are there any negatives regarding mounting any ring this way?

Also, I have toyed with the idea of putting this ring on my fixed gear. Sheldon Brown said it will work fixed and after thinking about it, I can see how it theoretically makes sense. And my rear derailleur does not pivot back and forth as the crank spins on the 1 X 9. Thought it would be fun on the fixed just to make people scratch their heads!

Thanks for the feedback.

cnnrmccloskey

I'm not sure if your saying what I think youare but it sounds like you rotated the chain ring on the spider which If you think about the design of a biopace chainring could actualy produce a negative impact.

Fixed gear biopace is awesome I ran one with a 52t chainring and I found the difference in chain tension noticable and a bit annoying but I switched down to a smaller one (42 or 44t) and switched to a smaller cog to preserve the ratio and I loved it.

I also ran into the problem your describing with the recesses in the chainring and I just slapped it on in spite and I never had any problems.

joejack951

I would recommend fitting some thin washers to fill in the recess on the other side. The thinner material with no support behind it is at risk of shearing when you tighten down the chainring bolts. Try to find washers that just barely lift the chainring off the spider and you'll eliminate that potential issue.

davidad

Most people don't notice any real difference with the bio-pace so the orientation is not that important. It's another one of shimano's screw-ups that quietly died because it didn't work.

Bianchigirll 

I am going to have to look around and see if I can find a bio pace ring and check this out. I just can understand why this ring can't be flipped.

also I think the shortlived bio pace design predates any kind of SG design with guide ramps and pins.

sam83

When I moved this inner ring to the outside, I flipped it so that the bolt head recesses would still be properly used.

The orientation (shape) was waaaaay off if I lined up the locator with the crank arm. But I could "index" or rotate the ring a couple positions and get a very close intended orientation.

I think the problem is that this ring has a complicated, non-symmetrical design, and that flipping it over may have had a slight negative impact. Or maybe that was just in my head. Regardless, it would not work as intended.

sam83

The jury is out with me, but I think the theory makes sense. Some have an opinion different from yours: https://www.sheldonbrown.com/biopace.html

Gonzo Bob

Yes, you can flip a Biopace ring and still get it in the "correct" orientation, but the tab will not line up with the crank arm when you do this. I flipped a Biopace outer ring and put it on as an inner. See this thread...

https://www.bikeforums.net/showthread...hlight=biopace

sam83

Correct orientation, yes. Exact same geometry, no.

The wide and narrow WILL match up, but the issue is that the radius on one side of the "peak" is different than the radius coming down the other side. That makes it non-symetrical.

If you don't look closely at these areas when you compare the inverted chainring to the tracing, you can miss it; These areas are off (plus and minus ~1/8" on the 42t).

While the effect of this is difference is way beyond my puny brain, I can fit square parts in square holes and round parts in round holes.