# What's the correct position to assemble eliptic (Biopace, Rotor, etc) chainrings?

If you take a regular, "classic" road chainring (130bcd, five arms), you have five possible positions to assemple it in the crank arm. This can even be used to "rotate" between different positions over the chainring lifetime and get a more homogeneous teeth wear.

But with elliptical chainrings, there should be only ONE position, shouldn't it? I mean, it was designed to have the larger chainring radius in a given position, so the question is: which position was it designed to work best?

Aditional question: would it be possible/wise to "tune" the performance for a given riding style, possibly going "against" what the manufacturer designed? It looks like RotoR and Biopace have different theories about that...

• This debate is one of the reasons that folks largely gave up on such rings -- the "theoretical" orientation never quite matched what "just seems to work". Mar 27, 2013 at 21:23
• Many modern circular chainrings are designed to only have one "correct" position on the crank arm. The ramps and pins are placed to facilitate shifting at certain points in the crank's rotation. Mar 28, 2013 at 5:55
• Some related analysis here: bikedynamics.co.uk/blog/?p=53 Mar 28, 2013 at 12:27
• From what I know it's quite personal (ie, varies widely between individuals). It might be easier to experiment using a 4-bolt crank and chainring so the 90 degree rotations are simpler. It means cutting your own chainrings but these days that is not too hard - you could probably get any of the CNC manufacturers to design and cut a one-off for a couple of hundred bucks.
– Móż
Jun 25, 2013 at 23:15

According to the ROTOR FAQ - http://www.rotorbikeusa.com/support.html - the recommended orientation of the chainring maximizes the effective chainring diameter at the point of maximum power (consistent with theory 1 in Daniel Hicks response). Contrary to Daniel Hicks reply, theory 1 requires the longer axis of the ellipse/oval to be PERPENDICULAR to the cranks rather than parallel. The effective diameter at any point in the pedal stroke is at the point where the chain wraps onto the chainring at the top.

They claim that this happens at just past 3 o'clock. They actually vary this slightly for the inner and outer rings, based on the notion that that varies a bit when you're climbing, presumably in the smaller inner chainring. This is what the default starting position (position 3 in their setup) sets as the peak: 18º for the 53t (outer ring); 23º for the 40t (inner ring)

This is degrees past horizontal/3 o'clock. Rotor chainrings allow you to vary this in 5º to whatever you want. So you could more or less recreate the biopace effect, putting the peak closer to 6 o'clock if you choose.

Rotor does encourage riders to adjust that based on their riding position and their pedal dynamics within a 8º-33º range. The recommended starting point for triathletes/time trialers sitting way forward in the saddle moves the peak position down by another 5º for both chainrings.

Conversely, as noted, you could mount a Biopace ring 72º or 144º off how it is "supposed" to be mounted and achieve something like the Rotor Q-ring effect.

There have been some studies based on actual comparisons of power output. Studies for a 1K time trial show about a 3% increase in average power with Rotor rings. Another study for a 10K time trial with the same Rotor rings show improvement for some of the test subjects, but overall, no significant improvement. There is another study that does computer modeling of muscles and force through the pedal stroke that concludes that the "optimal" shape is like the Rotor ring but with higher eccentricity. They conclude that increasing the effective gearing where you are able to generate more force increases the average power output by increasing the time you spend in your high force zone.

Practically speaking, I am now experimenting with the combination of an outer 53T Q-ring mounted in position 3 (18º peak) and a rotated Biopace inner 42T ring. The inner ring puts the peak closer to a 10º peak (again, angle measured down from horizontal), but it's not easy to compare directly as the shapes are different. The biopace ring has a broader zone that stays at the larger diameter and then the transition to a smaller diameter is quicker.

I found that both were pretty easy to get used to compared to a round ring and that the effect is relatively subtle. With both, I find that it does seem easier to maintain a smooth pedal stroke all the way around.

• "The effective diameter at any point in the pedal stroke is at the point where the chain wraps onto the chainring at the top." Yep, I'll admit that I forgot that point. Jun 26, 2013 at 0:07

IIRC, there are at least 3 conflicting theories:

1. The rings should be oriented to extract the maximum energy from the most powerful part of the stroke (from roughly 3-oclock to 6-oclock, viewed from the right).
2. The rings should be oriented to maintain a (relatively) constant forward force/velocity, despite the change in leg force through the rotation.
3. The rings should be oriented to maintain "smooth" leg motion. (Whatever that means.)

(1) would have the largest diameter parallel to the cranks, (2) at right angles, (3) somewhere else.

Update: I have 1 and 2 backwards -- see jae's answer on that point.

• That's more or like what I think. Sheldon Brown has said something about the easier sector being use to ease the beginning of the downstroke, so that when the leg has already gained momentum, there comes chainring sector. That makes some sense, and would flatten the peak the peak of muscular force, while theoretically maintaining or improving the net power through the stroke. It would be interesting to check, with a round chainring, if the peak of muscular force (resultant force on the pedal) and power are in phase with each other, but that's harder to achieve without instrumentation. Mar 27, 2013 at 23:28
• In the end, the Biopace design claims to have used empirical measurements, while RotoR apparently used speculative, inductive reasoning. Mar 27, 2013 at 23:30
• I had Biopace rings on my old bike, and experimented with installing them contrary to the Biopace scheme (though I can't remember what that was). Felt strange, and no obvious benefit. Mar 27, 2013 at 23:39
• Daniel nailed it pretty succinctly here. I can't really say it any better, but I will offer you my opinion. You're getting into the world of pseudo-science and snake oil when you start talking oval chainrings. There's a reason that they pop up every decade or so but never gain any footing. Mar 28, 2013 at 23:05
• @jm2 I sort of agree, and I am aware of the "world" I am getting into. Anyway, the bike that will receive the ring is odd enough by itself, and the Biopace, if not for the performance gain, at least will fit very fine in the overall weirdness picture... Nice to start a chat with veteran riders... ;o) Apr 1, 2013 at 12:39

According to another oval chainring maker, Osymetric, the Bio-Pace folks got it wrong by placing crank in line with the large edges of the chainring. According to them, with Bio-Pace you actually lost power. So their own recommendation is to assemble crank aligned with the shortest of diameters.