Dual side Ultegra (& Dura Ace) are inaccurate because the asymmetrical drive-side arms react unpredictably to strain, making it nearly impossible to get an accurate torque reading from a strain gauge. It doesn't just affect L/R balance, it's overall inaccurate. You'd be just as well off to have a Left-Side only Stages or 4iiii.

The best budget dual-side that fits a Shimano BB is the Power2Max NG Eco with Rotor Aldhu 24 cranks.

You can get the 4x110 version and transfer your Shimano chainrings, or for an extra ~$100 you can get the 5x110 or 5x130 with Praxis chainrings.

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"They're made of latex, not nitroglycerin"

https://www.dcrainmaker.com/...ter-buyers-2018.html

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80/20 Endurance Ambassador

Thanks for that link and yes, i have read that. I cant use pedal-based power meters (am a Speedplay user), so that leaves me with crank-based only. Since these reviews, he has posted an article about accuracy issues on the right side of Shimano-based PMs. And yet his reviews also mention how accurate the Stages and the 4iiis are.

I wanted to get some more info on those, as well real-world experiences (eg, my real-world opinion of Vector 3s, based on my wife's set, is that they are far too unreliable and need too-frequent battery replacement).

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Those who are slower than me suck.
Those who are faster than me dope

The thing is, most people's "real world experience" with a dual-side Ultegra PM doesn't give them any indication that the power data is inaccurate. Only when used in conjunction with one or more other power sources trusted to be accurate does the inaccuracy become evident. That's the value of the info folks like DC Rainmaker and GPLama bring to the table.

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"They're made of latex, not nitroglycerin"

Thanks - looks like 4iii and Stages are off the radar for now.

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Those who are slower than me suck.
Those who are faster than me dope

Aaand i am confused again. Here is what 4iii just replied to me:


4iiii Powermeters utilize 3D strain gauge technology to fully capture the power being applied to the crank arm. By measuring the strain directly, and not relying on algorithms, our technology is able to map more complex strain fields. Recent reviews have referenced challenges based on the current generation of asymmetric crank arms being produced by Shimano (R7000, R8000 and R9100). Our engineering team identified this in early stages of development of these cranks and used our 3D strain gauge technology together with an improved calibration process to maintain our high standard of 1% accuracy of our powermeters.

We at 4iiii believe in truth in advertising and because of this, have pursued independent, third-party testing at an academic institution to provide the most thorough analysis of our powermeters. In collaboration with Dr Rodger Kram at the Locomotion Lab, University of Colorado Boulder, we have published two white paper reports demonstrating our powermeter accuracy on Shimano cranksets. These studies used fundamental physics to evaluate powermeter accuracy, providing the most comprehensive way of evaluating performance. You can read these reports at the following link:

https://4iiii.com/gold-standard/


Thoughts?

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Those who are slower than me suck.
Those who are faster than me dope

Well it might not be a great option, seeing as how you say you have at least 4 bikes and may have tons of wheels sets, but the old PowerTap hub is stone cold reliable and mighty accurate. I have seen them for $200 on FleaBay recently.

You might also wait for a bit and see what Wahoo does with Speedplay. I think it is a good bet that they will bring a power meter to market.

I just bought that exact setup from P2M and had put it on my new Premier Tactical. Previous cranks were Ultegra, which I sold. Factoring in the sale of the Ultegra cranks, the total cost was around $700. I have had two previous Power2Max power meters and the only thing I had to do over about four years worth of use on two bikes was change batteries around 2x a year. I highly recommend Power2Max, and their customer support is also great to deal with - NA office is based in Vancouver, so friendly Canadians to chat with if needed for advice or anything else.

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Blog: http://262toboylstonstreet.blogspot.com/
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I also believe in truth... but not advertising.

- The data I see doesn't match their accuracy quotes on Shimano based cranks. It's damn close on their FSA Carbon crank though. So their gauges and tech is good. It's the Shimano crank letting them down. Their lab numbers show this too. The margin of error with non Shimano cranks is a lot lower.

- Their testing protocol wasn't against other power meters. It was tested against a treadmill and physics. That's totally fine... But.... They didn't report left/right power (the key problem with the Shimano crank as a base unit). They didn't go over 350W. They only tested to 85rpm. They didn't have a tertiary power meter to test against. They didn't use a combination of ERG/SIM mode. It wasn't tested with vibrations. When you pull apart their testing it's pretty easy to see it's VERY light on detail. It's a great start... It's far from what anyone would consider normal use.

- Of course they'll defend their product. That's business. There's no way they'll admit the Shimano right crank is a turd. That's business.

- Even Shimano can't get their power meter right on their own cranks. It wasn't designed to be a power meter.

- All power meter companies using Shimano cranks and externally glued on gauges are on borrowed time. Shimano only need to put some kind of asymmetric ridge/groove along the inside of the crank arm and call it done. Then they can go about releasing their own power meter - using a much better design. One that's designed as a power meter from the ground-up.

- I have no skin in this game. I don't sell power meters. I just wanted answers as to why all L/R Shimano cranks with slapped-on gauges didn't pass my testing protocol, and why I couldn't rely on them to compare other meters or smart trainers with. It took me a while to pinpoint the issue.... and here we are.

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Shane Miller - GPLama
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I used to use Powertap earlier - but yeah, I do swap wheel around between bikes, so PT wouldnt work.

I am actually not holding out hope that they'll be able to make Speedplay-based PM pedals - IIRC, the company behind Vector first started out promising a Speedplay-based power and missed deadlines for a few years before being acquired by Garmin and switching to Look-style pedals. I wonder if it has something to do with the nature of Speedplay pedals and their dual-sided nature. But if they do, yeah, it will be fantastic.

So i did more research on Power2Max - the cranks from the US site work out to about $750. For around $850-900, I can probably get another Quarq D-Zero (to go with my existing D-Zero, Elsa and Riken). I may just go the Quarq route in that case, as it has worked very well for me. That said, i am trying out my wife's Assiomas today on a spare set of shoes. If I can get the foot shim and clear angle dialed in correctly, maybe I'll use that.

Btw, GPLama - thanks for your informative and data-driven review. That was what prevented me from pulling the trigger on the Stages or 4iii - while I'd prefer not to spend more than I need to, I'd also rather spend a few hundred bucks more and get something reliable/low fuss.

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Those who are slower than me suck.
Those who are faster than me dope

I just read dc rainmakers review of the shimano lr and it was favorable. He did mention gplama's review and something to be wary about, but his power traces looked to be in good agreement between each other.

True. I was on the fence for a while - in the end, i decide to suck it up and get another Quarq. After flipping my Shimano crank and not having to pay it international shipping to send them halfway across the world, it was only around $150 more. I have had great luck with Quarqs on all my bikes, and this lets me use them interchangeably without too much variation in power.

I remember getting a Stages single side arm many years ago and it was like, 10% higher than my Quarq. Great for my ego, not so good for training. I am sure a dual-sided PM will be closer, but why take the change, right?

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Those who are slower than me suck.
Those who are faster than me dope

I bought one based off Ray's Review and after an initial, obvious issue (which Stages quickly fixed), it's been solid.
This is both on the trainer (Cycleops H2 that I've ridden a lot with SRM and Quarq PMs) and riding outside.

At this point, I wouldn't hesitate to do the Stages again.

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I talk a lot - Give it a listen: http://www.fasttalklabs.com/category/fast-talk
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The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
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