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On mountain bikes of a certain age, "Biopace" chainrings are ubiquitous. These chainrings are (purposely) not circular. It seems that most people today agree that Biopace rings didn't live up to the hype. They were a fad, and they're no longer popular.

Despite this, a new type of non-circular chainrings, "Rotor cranks", are currently in vogue. Professional road riders use them. What are the key differences between rotor cranks and Biopace? In other words, how can Biopace be bad and Rotor cranks be good? Or are they both bad or both good?

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Hmm .. Did not know about RotoR. Am about to fit biopace cranks on front of my bike and test. From reading Sheldon, he evaluates both concepts, and reckons biopace is the right geometry, despite, theoretically, not making sense. But there you go. RotoR just does the same thing that has been historically thought to be correct. I am going to explore Sheldon's viewpoint, and wee where it takes me. :) –  user3450 Feb 11 '12 at 12:24
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4 Answers 4

I think "Biopace" chainrings and "Rotor's Q-rings" are similar products (i.e. ovoid chainrings). Q-rings is what the cervelo guys use. I think the benefits of these types of chainrings are still up for discussion. Some riders like them, some don't. I like the reasoning behind them, and I think the benefit is real (this wikipedia citation is really good) but they probably cost more, and introduce issues with front-shifting. For now I'd say this is mainly a matter of personal taste.

Note that some ovoid rings have their longer axis parallel to the cranks while others have the longer axis perpendicular to it. The reasoning behind the first option is that you have maximum leverage when you can exert maximum force, the reasoning behind the second option is that you have smallest effective diameter of your chainring at the top and bottom of your stroke, allowing you to quickly pedal through these dead-zones with minimum exertion. Biospace is the first variant, I don't know which the Rotor Q-rings are.

The Rotor Crank varies the angle between the two cranks during the stroke rather than fixing it at 180 degrees to reduce the time the pedals spend in the dead spots at the top and bottom of the stroke. Wikipedia cites weight as a major disadvantage of the Rotor crank. Also, the movement is really different, so it takes some getting used to, and you'd probably want the system on all your bikes, which is expensive.

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I would disagree with your statement that they are basically the same product because, in my experience and from what I have gathered, they are not. I won't edit that statement but I would suggest you might want to. I would note that your following two paragraphs suggest vast differences between the two products thus negating that first statement. –  tplunket Oct 21 '10 at 11:43
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They (biospace and rotor q-rings) are the same in that they are "ovoid chainrings", which is what I state. I also think that the reasoning behind both is similar though they reach different conclusions. The rotor crank is a really different thing, and I also state that. –  jilles de wit Oct 22 '10 at 7:56
    
But they are not the same, which is what I am getting at. Shimano and SRAM chainrings, while both round, are far from the same. They are only the 'same' if you consider the common thread of being a chain ring sufficient to be the same. The reasoning behind the two products (biopace/RotoR) is the direct opposite with both supposedly reaching the same conclusion (less knee strain, greater traction etc). It is correctly spelled biopace. –  tplunket Oct 22 '10 at 10:34
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@tplunket: What I meant to convey was that both types of ovoid chainrings basically try to change the shape of circular chainrings to attain a certain benefit whereas the Rotor Crank completely changes the mechanical operation of the chainring/crank system. So compared to the Rotor crank the two different versions of ovoid chainring could be considered "the same". I changed my wording to "similar". –  jilles de wit Oct 22 '10 at 11:44
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If you are looking at the Biopace/Rotor/O-Symmetric relationship as similar due purely to aesthetics, or their similarity due to their lack of similarity to round chainrings then, yes, they are similar products.

But, that said, from the RotoR website "The Q-Rings are elliptical; the Biopace and O.SYMETRIC chainrings are asymmetrical.". And Sheldon Brown points out that Shimano called Biopace "point-symmetric egg curve". Which really only gives them the similarity of being different from the norm which in this case, is a round chainring, but if you go a step further, they are quite different.

So, to try and answer the question posed above.

  • The first difference between RotoR and Biopace is elliptical vs. asymetrical.
  • The second is the inherent purpose which, as previously stated, is that the Biopace chainrings placed the maximum tooth size at the dead spot while Q-Rings (RotoR) have the minimum tooth size at the dead spot. RotoR claims this is to increase your ability to get through the inefficient spot of your pedal stroke. Biopace is supposed to speed your way through the power portion (less teeth) and use 'stored' energy to power through the dead spot (more teeth)
  • Which leads to the third difference which is the adjustable nature of Q-rings (RotoR). Since my dead spot could be different from your dead spot (and would ideally be analyzed with some sort of spin scan) you can adjust where the minimum tooth size sits in relation to the crank arm.

So, to simplify it (perhaps too much) RotoR claims that it is more efficient and better for your knees etc. to minimize the tooth size at the dead spot and maximize the tooth size where you generate the most power and are the most efficient.

Having installed a set and seen the power file from a MTB powertap I can vouch for the fact that it appears to 'smooth' the application of torque while mountain biking. The thought behind this is that by smoothing out your application of torque you will be able to ascend technical climbs ... better. That is one coaches take on the Q-rings and why he sees benefit.

So, that all said. Have a gander at Sheldon Browns take on the issue of Biopace. Essentially, to try and summarize his thoughts, Biopace is striving to achieve the exact things that RotoR is trying just by going about it in what appears to be the completely opposite way. In fact, he even mentions the application of Biopace to mountain biking and how it evens out the application of power to the cranks.

IMO it is all very confusing. RotoR does trot out some scientific studies and I can attest to a difference in torque application on the mountain bike. In my experience, what is universal with RotoR and O-Symmetric is that the shifting is ABSOLUTELY terrible. Add mud to the mix (MTB) and it goes from terrible to non-existant so whatever perceived benefit there is ... I think you might lose with poor shifting performance.

What is helpful from Sheldon's take is that it seems Biopace failed more due to clumsy marketing and poor communication than it did from being a poor design.

Lastly, I don't think any of them are 'GOOD' or 'BAD'. They are different and might work very well in some applications and poorly in others (Ie. RotoR and mud). But the Biopace choice is pretty much a moot point because ... you can't buy them new anyways.

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Interesting that both systems purport to solve the same problem by stretching the chainring in opposite directions: but the compromise between these (i.e. good ol' round) is considered sub-optimal? –  Andrew Vit Oct 21 '10 at 21:52
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While it might seem the round is considered sub optimal ... it always seems to go back to round. Shifting performance is such a huge part of cycling (evidenced by SRAMs XX chainrings, shimanos DI2 etc) and I think it can be said that, without a doubt, shifting performance on round chainrings is far superior while shifting on the non-rounds is sub-optimal at best. –  tplunket Oct 21 '10 at 22:14
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Elliptical chainrings have been around pretty much forever (my old Nishiki ca 1979 had one). How does RotoR get to claim that they're "new"? –  Daniel R Hicks Feb 11 '12 at 13:25
    
In practice, a persistent rider could assemble the chainring in the position he wants after having tested the different five possibilities, because either rationale (RotoR's and Shimano's) seem to be much more inductive and speculative than deductive and evidence-based. And I guess different riders could get the "sweet spot" of cadence, torque or power with quite different setups... –  heltonbiker Mar 27 '13 at 21:06
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I have both Rotor q, and Biopace, I traced the oval of all the rings on paper and discovered they have the same oval shape. I agree with Rotor Q's method, so on my road bike I mounted the Biopace rings two bolt holes over from the normal location(5 bolt crank), and the orientation is very close to that of The Rotor Q's on my MTN bike. The biggest performance change I noticed is that I can stand and peddle longer with the oval rings.

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As User1376 was the first to point out, you can simply rotate the Biopace chainrings 2 bolt holes from its original setup to mimic what the ROTOR cranks do. Worth a try.

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