Get the Rotor power meter crankset with q rings and kill 3 birds with 1 purchase.

I thought that but it looks like I can't get 165s. Am I missing something?

Rotor 3D and 3D+ go down to 165mm; the 3D+ may actually go down to 155mm, iirc

But the power meter may not be available at that length.

Plus, the non-round rings will make the powermeter inaccurate, which is just crazy for rotor to make a powermeter with that issue.

On their site the power meter crank lengths only go to 170. I don't know if that means you can't go shorter or would just have to buy a set of 165s separately. The only way to go shorter with a pm on the Rotor site is with the SRM.

You could not buy a set of 165 arms for the powermeter, because the arms themselves are what contains the strain gauges that measure power. Maybe the 165s are to short to add the strain gauge.

A few things. If you could get your hand on a Rotor Quarq, it will most definitely take 165mm crankarms that you can buy from Rotor. I know b/c i owned one of those. Basically buy crank arms, have it shipped to Spearfish with the Rotor Quarq, and have the swap done. Quarq used to sell Rotor Quarqs, but those have been phased out.

No idea about the power 2 max (it seems the 3D+ can be taken down to 150mm). Maybe they require a specially machined crank arms for the 3D, maybe they don't. You can always email P2Max to find out. As for the 3D+, Rotor and Wheel Manufacturing make all sorts of BB adapters so that you can run the 3D+ on a standard BSA frameset.

As for the SRM, the SRM Rotor 3D requires a specially machined set of crank arms, so you can't supply your own, however, it does come in 165 mm even if it's not listed on the website (I know b/c i just asked). You can supply your own arms for the 3D+

The thing about non-round rings is, that they may actually make your cadence more even (after all, they're intended to give you more leverage where your leg geometry is weaker, and less leverage where you have the advantage), and as such oval rings may give intrinsically more accurate power on most crank/pedal based powermeters that assume constant cadence through the pedal stroke. It depends on your pedaling style and on the type of resistance you're up against. This has been reasonably well documented and analyzed by multiple parties.

Even if non-round rings cause a bias... it will be the same from one ride to the next, and so a 10% improvement in FTP over time, is still 10%. Position changes that cause a change in power output, will still cause the same proportional change and the power meter will tell you what that is.

Last: The difference in reported power, according to several analyses, is on the order of 4%, at the most. That is double the quoted accuracy of most commercial power meters, but they will only stay within that accuracy spec if you zero out torque offsets frequently (more than once per ride on some power meters) and regularly check the scale factor calibration using weights that have a known weight... if you're off on that weight, you'll introduce a power error that's proportional to the weight error.

So use whatever shape rings that make you ride best; and use whichever power meter suits your bike(s) best, bearing in mind that it's as much about how you use the power meter as the PM itself.

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Less is more.

So you are saying technique will change how accurate the powermeter would be with round rings, but it will be consistent from one ride to the next? I do not know how both of those could be true.

Can you show me these studies that show how it could be more accurate? I have never seen that, although I could see how it could happen on a very low inertia trainer.

echapp's got it right.

as for the inaccuracy part, i can offer 2 thoughts:
1) if you train on only one set of cranks, there really is no issue. as said above, a 10% increase in FTP is still a 10% increase not matter what the shape of the rings.

2) if you have multiple PMs (sounds like you don't) or you switch to round rings one day, you need to know that crank-based PMs that depend on cadence for reporting power tend to overstate power by ~5%. no big deal--you just need to know why you "lost" power.

there is no "free lunch", despite what manufacturers of oval rings might have one believe. sure, they may be preferable to round rings for a whole host of valid reasons, but adding power, instantly, is not legitimate.

if you need help with SRM's version of the rotor PM, feel free to contact me. (/disclosure: i sell SRMs but also sell quarqs)

Sorry. Should have said "If your riding style stays the same, the accuracy will stay the same".

One problem is that without a really accurate crank rotation encoder, you can't tell how much instantaneous cadence variation there is, and therefore you don't know how much power meter error there might be, regardless of the type of crank rings are being used.

I didn't say 'studies', since that would imply a peer-reviewed paper or some such thing. Dan Connell did put out a piece on his blog doing a mathematical analysis that showed that the power error was proportional to the square of the cadence variation. If that's correct then 10% cadence variation (regardless of the cause) would result in 1% power error. The new super-elliptical rings from Rotor have about 20% ellipticity; worst case you'd therefore expect about 4% error.

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Less is more.

Powertap doesn't have this problem.
Just sayin =)

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For us Quarq owners, where can we go about getting a 162.5 or 165mm crank without having to buy the entire unit?

I'm calling BS on the above...even at low speeds, the total inertia of a bike+rider is fairly high, and thus as long as you are pedaling such that your freewheel doesn't disengage, the rear wheel rotational velocity is fairly steady. By definition then, the crank rotational velocity will NOT be with a non-round ring, and thus the power output WILL be artificially inflated on a PM that uses the assumption of constant crank angular velocity as part of the power calculation.


No "ifs" about it...it WILL be inflated with a power meter that assumes constant angular crank velocity, and it will be proportional to the degree of ovality and will vary with the effort level.

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http://bikeblather.blogspot.com/

i'm assuming you are talking about Rotor Quarq, as the Riken should go down to 155 mm

for 165 mm, you can just get the crankarms from any retailer that sells Rotor products

for 162.5 mm, only if you have a 3D+ unit.

Thanks echappist. I have the Quarq s975 170mm. What are my options on shorter cranks?

Have you considered http://www.power2max.com? It's a relatively new company but I've had mine for a few months and have been extremely happy with the purchase. There are a number of other forum users that have similar thoughts on this PM. DC Rainmaker has a nice review on them as well. One of the most affordable crank based PMs on the market today.

Sram Red or Elsa. I don't know much about the new cranks, but it's possible that the arms would be backward compatible with your current unit. And sorry about what i wrote earlier; i thought Quarq had units going down to 155, but that's apparently note the case

I have looked at it briefly before, and just read DC Rainmaker's review. I'd say this is on the list with the only downside being a little more work to switch from one bike to the next. This seems like a relatively affordable way to do everything that I'm trying to do with crank length and QRings.

Thank you for the suggestion, this feels like a pretty good option and comes in cheaper than expected.

Any other suggestions? My only issue with the P2M is that I can't buy it from my lbs. I think I would feel a little guilty since my fitter is the one who recommended and had me try shorter cranks during my fitting.

Rotor 3D goes down to 155.

Is it assumed or proven that velocity changes throughout the pedal stroke?

Or as some non round ring producers say is it force that changes (ie formation patterns through out the pedal stroke) and velocity remains constant?

I don't know, but I would like to.

We have done a bit of testing on erg setting with a Velotron and it looks 1-2% better on q-rings, but we never got to the point where we had the equipment to measure velocity,

I am actually thinking of hooking up a "known" constant velocity out put (ie a motor and a gear box) to prove or disprove this.

Thoughts?

Maurice