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Power Meters and Oval Chainrings

Oval chainrings have been around for many years in various forms and configurations. The concept is simple enough. Oval chainrings aim to minimize the “dead spot” in the pedal stroke and lengthen the power phase. The result is a more efficient, powerful pedal stroke. However, there is a long running debate in the cycling community regarding power meters and oval chainrings. The question is, to what extent, if any, oval chainrings impact power meter accuracy. So if you’re a fan of oval chainrings and you train with a power meter, this article is for you.

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Power Meters and Oval Chainrings

Oval Chainrings

With a standard round chairing, when your feet come across 6 and 12 o’clock during your pedal stroke, your power drops significantly. Oval chainrings, also called non-round or elliptical rings, are shaped in a manner as to minimize this dead spot. The non-round shape means your pedal stroke passes through this dead spot more quickly. Moreover, the non-round shape emphasizes 3 and 9 o’clock, your power position, allowing you to produce maximum power here. By allowing the legs to do work where they are at their strongest, and less where they are at their weakest, the theory is you will have a more efficient, powerful pedal stoke.

The majority of oval rings have a relative difference of about 10% from the strong spot to the weak spot. For example, a 53-tooth ring feels like a 58-tooth in the power phase, and a 48-tooth in the dead spot.

ROTOR Q-Rings

Image of two ROTOR Q-Rings

absolute BLACK Oval Ring

Image of an absolute BLACK Oval Ring

Do They Work?

While not the subject of this article, I felt compelled to at least touch on this (hotly debated) topic. Researchers have examined both the physiological and performance differences between oval and round rings on several occasions. Some reports conclude that oval rings are indeed effective. Furthermore, numerous professional riders including elite cyclists such as Bradly Wiggins and Chris Froome have used oval rings. (Chris Froome still does and swears by them.) Surely they know something…right? On the contrary, you can find reports and studies that don’t show any real performance gains. Likewise, it’s easy to find someone who has used oval rings and has found their way back to round rings. In fact, reports are that Bradly Wiggins is back to round rings.

It’s probably fair to say, if they help, the gains are marginal. But this is a sport that is all about marginal gains…so that’s not a put-down. Ultimately, you need to give them a try and decide for yourself. Remember, it’s not always a clear-cut ‘they work or they don’t’ situation. What works for one, might not work for another and vice-versa. In addition, it’s all about how you feel and rider preference. If oval rings feel better and more natural to you, use them. Lastly, I would mention that since professional cyclists typically have very efficient pedal strokes with minimal dead-spots, it has been theorized that oval chainrings may be more helpful to the amateur cyclist.

 

Power Meters and Oval Chainrings: Are they Compatible?

Ok, now to the good part.

There is a long running debate in the cycling community regarding power meters and oval chainrings. The question is, to what extent, if any, oval chainrings impact power meter accuracy. The issue is that most power meters make the assumption that your pedaling velocity is constant. In other words, power meters assume that you turn the pedals at steady speed throughout each pedal stroke revolution. The problem is, oval chainrings cause your crank velocity to change during each pedal stroke, as they limit the time in the dead spot and lengthen the time in the power phase, as discussed above.

The net result is that oval rings can cause power numbers to be inflated, or so the theory goes. The more non-round your chainring is, the more your power numbers will be inflated. Most studies say you can expect an increase of anywhere from 1% to 4%.

 

The Counter Argument

However, there are those who say this is not the case, rather that power meters and oval chainrings don’t skew or inflate your power data. Take the folks at SRM for example.

“We have had an increasing number of customers use Q-rings with their SRM Power Meter. There is no difference in wattage accuracy between using a regular chain ring or an elliptical chain ring. The important thing is that the SRM power meter is calibrated with a regular round chain ring to determine the slope of the power meter, which is exclusive to that power meter. Once this is determined, any type of chain ring can be used and wattage accuracy is maintained. Think of it in these terms, the elliptical chain ring only adds more leverage, which in turn can allow for more torque per pedal revolution. This is no more different than changing the crank arm length, which has no effect on the accuracy of the power meter. Torque is torque…”

 

Our Take

Both of these arguments seem valid. We’re not engineers and we don’t pretend to have exceptional insight into the inner workings of power sensors when paired with an oval chainring. We can only go off of what the experts in this field say, and clearly there is a difference of opinion. What we can say is that, while we acknowledge the potential for inflated power numbers, we don’t feel it is cause for major concern.

Why? Because if an oval ring indeed results in higher power numbers for you, the difference is likely to be slight. In addition, remember that perhaps more important than accuracy, is consistency. If your power meter gives you consistent power data day-in and day-out, then it’s largely irrelevant if it reads 204 watts when you are actually producing 200. We think if you are more comfortable with oval rings on your bike, then go for it. Just keep in mind the impact your oval rings might have on your power numbers.

 

Power Meter Specific Notes

ROTOR 2INpower

There’s no crankset-based power meter more Q-Ring compatible than the ROTOR 2INpower MAS power meter. Not only is the ROTOR 2INpower certified accurate with a Q-Ring, but the Micro Adjust Spider (MAS) allows you to adjust your Q-Ring position with a great degree of precision. By fine-tuning the position of your Q-Ring, you can set your pedal stroke to best take advantage of your Q-Rings and get the most power and pedal efficiency out of your crank.

 

PowerTap P1 Pedal

The PowerTap P1 perfectly supports non-round rings according to PowerTap. PowerTap says, “The P1s take 40 individual angular velocity measurements and subsequently 40 power measurements per pedal stroke. Any acceleration/deceleration within a pedal stroke would be measured.” In other words, the P1 Pedals account for the fact that your pedaling velocity changes when using an oval ring, and therefore your power data remains precise. Pretty slick!

 

PowerTap G3 Hub

Since the PowerTap G3 measures power at the hub, your pedaling velocity is irrelevant. A hub-based power meter doesn’t see or care what type of chainring you’re using, rather it simply measures the power or torque at the rear wheel. Therefore, the G3 works great with an oval ring.

 

Verve InfoCrank

Note that due to the thickness or depth inherent in some oval rings, the oval chainrings can interfere with the RF module cover on the inside of the Verve InfoCrank drive side crank arms. We have had some customers actually drill out an area on the chainring for the module to fit.

 

Garmin Vector

Garmin states the following: “[The Vector pedals] assume constant angular velocity within a single crank revolution. For most riding conditions and bike set-ups, this assumption allows the Vector systems to report power within its stated accuracy. Garmin has not determined how variable the angular velocity is when using the elliptical rings in real riding conditions. Because of this, we are unable to state what impact would appear on power values. For the best accuracy, it is advised that when using a Vector system, it should be installed on a circular ring set.”

This is just a thorough way of saying they cannot attest to what impact, if any, oval rings might have on accuracy. I would suspect that the impact with Garmin Vector’s wouldn’t be much different than that with any other power meter. In fact, we have a customer that has compared their Vector’s with a Q-Ring to a PowerTap G3 hub. He reports that the difference was negligible.

 

Stages Cycling

Mechanically, the Stages power meter will work fine with oval chainrings. However, they note that their power measurement is “event based, where as an event is one complete pedal revolution”. Like Garmin, due to the angular velocity inherent in oval rings, they note that power will likely be skewed higher. They estimate it can be as much as 4-5% higher. They emphasize that it is indeed fine to use oval rings with a Stages power meter. However the rider simply needs to take this difference into account when using their power meters and oval chainrings.

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