I think Ray's post was with regards to Vector and not P2M, i.e. Vector seems to assume constant angular velocity. I remember P2M explicitly stating on their site that non-circular chain rings would not influence the power readings on their power meter. However, if that is true and they truly are continuously measuring cadence (and torque) throughout the pedal revolution, I don't know.

Uh-oh. Okay, I followed the thread back more carefully and it looks like you're right, dammit.

I just looked up what P2M says on their site now:


"Changing cranks or chain rings does not affect calibration. You can also use oval chain rings (e.g., Rotor Q-Rings, Osymetric, etc). "


http://www.power2max.de/support_faq.php?lang=en


Strictly speaking I guess they don't say oval chain rings won't affect the accuracy, but it seems a bit implied. Who knows.

Well, oval chain rings wouldn't affect calibration. They affect the angular velocity, so you wouldn't detect any difference with standard torque checks.

In short, yes. The negative torque is much less than the positive torque you generate through the down stroke. Hence as your power increases your torque efficiency increases. What is interesting is observing differences between each leg. I'm right handed and I can see from torque efficiency that the neuro-muscular control of my right leg is better than my left.

Interesting...so I guess that means that they too didn't understand the particulars of the problem when making the statement that it isn't affected.

So, at this point in time at least, Vector will inflate power values with non-round rings.

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http://bikeblather.blogspot.com/

Possibly...it all depends on how Polar decided to "sum it all up"...and the particulars of the Polar power calculation algorithm have never been quite clear. They DID require a cadence trigger to get power values.

Of course, I know someone who has a working Polar power unit and an Osymmetric ring on hand who could figure this out ;-)

You know...I wonder if the non-round ring giving a bit of a "forcing" function into the chain might help a Polar work more reliably on a trainer? :-)



...not only for the above reasons, but also mainly for the reason that the rear wheel speed is going to be fairly steady simply due to it being "coupled" to the rider mass (assuming freewheel is engaged) and the resultant large inertia.

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http://bikeblather.blogspot.com/

What about Stages? Google-fu couldn't tell me if Stages makes the CV assumption....

>In short, yes. The negative torque is much less than the positive torque you generate through the down stroke.

I'm still a little confused. Your wording let me to believe that you were using:

positive torque = downstroke
negative torque = upstroke


But Watt Matters defines it as

positive torque = assisting in forward movement of crank, anywhere in the cycle
negative torque = opposing torque...torque which slows down the crank anywhere in the cycle.

But I could be wrong, and you're really using the same definition as him.

Don't quote me on this...but my recollection is that it does.

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http://bikeblather.blogspot.com/

They had trouble getting cadence right to begin with, let alone instantaneous rotational velocity.

Power2Max have not adequately answered the question of measurement of rotational velocity, IMO.

I need convincing that accelerometers can really get high frequency crank velocity samples with sufficient sample by sample accuracy.

Since torque is vector quantity, conventionally defined by the right hand rule, its direction is normally defined in relative to the direction of angular motion of whatever you are applying torque to. It matters not where in the rotation the force creating the torque is applied.

e.g. on a fixed gear bike one can apply negative torque on the forward foot downstroke. It can hurt like mofo after hard riding, but that's an example of where downstroke and upstroke have no relevance to the definition of positive and negative torque. Likewise on the rear foot upstroke, it can apply negative or positive torque, or sweet bugger all.

Where and how you apply torque at various positions of crank rotation may or may not be all that interesting/relevant to performance, but that's for another discussion.

Dear all,

Nicolas from power2max here. Just found this thread. To answer the question about data: we sample cadence and torque at 50 Hz. The statement about being able to change chain rings is also related to another characteristic of our power meter. power readings are not affected by the chain rings or exact chain ring bolt torques, making it safe to switch chain rings without affecting your readings.

Regarding our distribution that Ray mentions: check our website this coming Monday, things are moving and there will be interesting news for you guys in the States. Not saying more for now :-).

Cheers
Nicolas

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power2max
http://www.power2max.com/northamerica
official power meter of Movistar Team

Nicolas, thanks. So, just to clarify, how does the Power2Max use the angular velocity and torque samples to produce power? ANT+ allows for transmission at 4Hz, right? Are you sending a different value for power and cadence each quarter of a second? Or do you average the angular velocity samples and average the torque samples over some longer time interval (like, a second) and then multiply them to get the average power over the interval and then transmit them?

Hi rchung,

Unfortunately I cannot go into the details of how the electronics exactly work, since it is proprietary. We are aware of the issue you mention above and have taken it into account in our approach. We also test our power meters both with round and Q-rings against the Cyclus2, which is independently calibrated and measures differently, and find that our results are consistent.

Best wishes,
Nicolas

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power2max
http://www.power2max.com/northamerica
official power meter of Movistar Team

I apologize for not having been initially more clear about this point in my query to Ray in this thread!

I should have specified that my concerns with Power2max are the 110bcd spider and it's structural compatibility with O-Rings (which to my understanding isn't an issue with their 130BCD meters), while my concerns with the other meters was the Velocity issue.

Personally, I'll probably keep both meters running (the P2max on my road / dedicated training bike, the Vector's on my race bike to run Aerorings) because the p2max has been an absolutely faultless meter through 6 months of heavy use. I trust it's numbers implicitly for training, and can't see shifting it off.

Thrilled to hear that you guys have gotten the U.S.A distribution issues addressed, it's great to see a solid product at that price point being made available more widely.

For anyone who hasn't dealt with Power2max: their customer service is fantastic, and hence my vocality about them here!

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Proud member of the GUCrew
Twitter: @tripigeon Blog: Ironpigeon.com
Thoughts on AG sponsorships / community involvement: http://bit.ly/1dQlVDy

Then without disclosing proprietary information, can you clarify a couple of things?

1. When you say your results with round and non-round rings "are consistent" are you saying that they are exactly the same, and have you tested rings of different degrees of eccentricity?

2. I'm just trying to clarify that your use of the term "samples" is the same as my use of the term. You say that you "sample" cadence at 50Hz. Are you saying that you know how far the crank has rotated in the previous 1/50th of a second and thus know the crank's exact position 50 times a second? Or are you saying that you check every 1/50th of a second to see whether the crank has passed a particular fixed point?

Hi,

re 1: "exactly the same" is difficult to prove / disprove because one cannot use round and oval rings at the same time but has to repeat tests to make comparisons. We find that both with round and oval rings results of these tests are within the margin of error. We regularly use Q-Rings and I have also run Osymetric, but cannot say off the top of my head if we have run all the same tests with both.

re 2: We don't use fixed points such as magnets to determine position, like some other systems, so we don't check for an event of passing a point.

Best
Nicolas

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power2max
http://www.power2max.com/northamerica
official power meter of Movistar Team

We already know that, and is not addressing the question asked.

It's pretty simple. Either the P2M is:
i. measuring the crank rotational velocity one per revolution and using that to calculate power, or
ii. it's measuring crank rotational velocity many times per revolution and using these more frequent crank velocity samples to calculate power.

If it's the latter (ii.), we'd appreciate more information on that, at least what you can say without release of proprietary information, e.g. how many times per second or per revolution is crank rotational speed sampled, and to what level of accuracy. Although we might be interested, we are not asking how P2M might do that, which would be the proprietary information I'd have thought.

If however it's (i.), then we know that the P2M will have the same accuracy issues with non-round rings as other crank based power meters, and a claim that accuracy is not affected would be incorrect.

Further, if (i.) applies, I think it's important to then explain the accuracy of determining crank rotational velocity once per revolution, as if it checks say only every 1/50th of a second, then to help understand how the P2M avoids a sample by sample precession "error" in power calculations.

 

>Don't quote me on this...but my recollection is that it does.

I got a response from Stages:

"Oval rings or odd shaped rings for that matter do not affect the power reading. Since we use an accelerometer in our power meter that determines the position of the crank itself so the power meter reading would not be effected. Let me know if you have any other questions."


This is slightly vague - like the Power2Max guy - but I take it to mean that they sample velocity at a higher rate than once per revolution (presumably a significantly higher rate), which is certainly possible with an accelerometer.

But he flat out says "do not affect." No mealy-mouthed claim there.

Cool. That's certainly unambiguous.

Hi all,

I am sorry that my response is not more in-depth, but unfortunately I am not at liberty to go into the technical bits because it always touches on the "how" and that is proprietary. As I said we have tested round and oval rings (Q and I got confirmation that Osymetric, too) against the Cyclus2 and there is no deviation when the oval rings are used.

Thanks for your understanding and happy training and racing,

Nicolas

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power2max
http://www.power2max.com/northamerica
official power meter of Movistar Team

OK, well that's a disappointing response but thanks anyway. We can only ask.

TriPigeon:
Totally agree with you on P2M.
Those guys are awesome and the PM(s) I have from them have been outstanding.
I don't blame Nick for answering as he has done either.
-YT