Fixed Gear w/BioPace: Differences Between Ring Sizes?
 

Will different sizes of BioPace rings produce differing amounts of variation in chain tension?

I recently came upon two BioPace rings: one around 40 teeth and the other around 50. My intuition tells me that using the 40 tooth ring with a smaller rear cog would result in less variation in chain tension than using the 50 ring with a larger rear cog. If I'm right, would the variation be enough to warrant choosing the smaller BioPace ring?

For that matter, theoretically, does a large BioPace ring produce a larger performance gain relative to a small BioPace ring?

What would you choose: (large BioPace ring and large cog) or (small BioPace ring and small cog)?

bio pace!

damn. I didn't know this was the 1980s technology forum!

I would think that you would want to go with the small ring. however, i don't think it would be to good of an idea because the difference in chain tension could be damaging to the chain itself and throw the chain off if you get the rpm's up.

biopace....damn are transformers on?

I know some people that swear by biopace.

I am not sure of the answer here but i would reckon that there is a constant ratio between the two axes of the ellipse of a biopace ring so that the difference between the two would be greater on the larger ring and therefore the slack would be more. i run a 42 tooth biopace ring and i have not had any problem with it. however i am a wuss and use a brake so if you are an avid skipper or whatnot that YMMV.

OK I'll step up to the plate. I run a 48 tooth biopace chainring and a 17 tooth cog. Its my main ride. The trick is to set the chain tension a tad bit tight, with the cranks vertical. That is where you'll get the most tension. Then turn the cranks 90 degrees and you'll see that its a lot of slack. Its not a problem. I rarely use my brake, but I don't skid or skip. I've been riding this setup for about 2 months and have yet to have a single problem. I wouldn't worry about it. Just run what you want!

Biopace chainrings on a fixed gear bike where chain tension can be the difference between awesome and face on the ground seems really stupid to me.

Plus they just look awful.

I am aware of Sheldon's article on biopace; in fact, I linked to it in my original post on this thread. Therefore, I am also aware of the fact that it is possible to use biopace on a fixed gear bike; that's not what I'm asking. I do not believe Sheldon's article addresses the questions I originally posed.

I've never heard of anyone having any problems with biopace on a fixie; Sheldon even rides with them! Also, unless you really have a sharp eye, I think it's pretty hard to tell the difference between a biopace ring and a regular ring, especially from far away.

it looks like he addresses it here: "People are often astonished to learn that I ride Biopace chainrings on fixed-gear bikes. They imagine that there will be tremendous changes in chain tension as the chainring rotates. In practice, this is not the case. A 42 tooth chainring will generally engage 21 teeth against 21 chain rollers, regardless of its shape.

There is a sligth variation in tension resulting from the varying angle between the two straight runs of chain as the axis of the chainring rotates, but this has not generally been of a sufficient magnitude to cause any problem in practice for me."

or am i misunderstanding your question?

repeatedly stressing and unstressing a chain, hmm sounds like a fatigue test i ran in college. i'm with $0.00/g on this one.

I think you are. While the number of teeth engaging the ring is constant during rotation, the chain tension is not constant. Sheldon and rykoala are just saying that the change in tension is negligible and therefore not a problem for fixed gear riding.

My question is the following: will a large (high 50s-tooth) biopace ring have a higher variance in tension than a small (low 40s-tooth) biopace ring?

I realize that both rings should be perfectly fine for fixed riding; I'm just wondering if there are any significant differences besides the number of teeth. For example, besides tension variance, would a large biopace ring theoretically provide a larger performance gain relative to a small biopace ring?

Okay, the way I see it, when the biopace ring has the long axis vertical, it's engaging the same amount of teeth as when the long axis is horizontal. So, if that's true, why is there any difference in chain tension at all?

Distance from cog to far side of chainring is greater when the chainring is "fat" rather than "tall", that's what effects the chain tension.

that depends on whether or not the ratio (long axis vs short axis) is constant on all biopace rings.

Wow, that's exactly right (assuming they're perfect ellipses). I'll measure them when I get a chance (I'm away for the next couple days).

I'm no math or any other sort of guru, but I have run both the 38 tooth ring on a 15 tooth cog and the 48 tooth ring on a 17 tooth cog. I notice a little bit more difference in chain tension on the 48.

My stock cyclone chainring appears to be round but it isn't The loose section of it worries me a little bit, but I've never had a problem. The thought of using a chainring that is even less round than the one I use now sounds insane to me. I've seen many biopace chainrings and you can see clearly that they're not round. Stupid idea. F sheldon brown.

Biopace from Shimano was made to ease the pains of the knee. They came out in 1988.

Some people like them ,I didn't they suck especially on a track bike. It didn't feel right.

Messengers I know used them on the street as well as racing them seen the light and got rid of them. Shimano seen the light to and got rid of them too.

Use caution.

0.00, don't "F" a man due to him stating his opinion because only in the long run you will be "F" yourself.



S/F,
CEYA!

I think you're confusing roundness with concentricity.

Your Cyclone is round, but is not centered perfectly.

This is quite common, and often is not too difficult to fix.

I explain how to minimize the problem at: http://sheldonbrown.com/fixed.html#tension

Sheldon "Biopace Works Fine For Me" Brown

I ride a 53t biopace with a 17t EAI cog.
I set the tension at 1/8" at the tightest spot and about 5/8" at the loosest spot.
That puppy won't come off no matter what I do.

It takes the same number of watts to climb a hill or push the wind out of the way, no matter what kind of chainring a rider has.
That said, I find the biopace much more elegant and expecially so on hills where I have to get out of the saddle.
I only wish the biopace came in more offerings.