NoWhammies

Happy 4iiii camper over here.

Barry2 

modern Power Meters are extremely accurate.
I went with the Stages due to better temp compensation.

My coach claimed my Wahoo Kickr Bike and my outdoor road bike with Stages dual were reading differently.

So I borrowed a pair of Garmin Vector 3’s and did a comparison.

rode Wahoo Kickr Bike with Vectors and compared the built in PM with the Vectors.
Rode my outdoor Bike with Stages dual and Vectors.

All three were within 3% (vector end 3% higher in both tests)

So yes……. Expect them to read the same!

Barry

Doomrider74

Presumably though a single sided crank arm power meter won't necessarily agree with the Kickr PM purely because it has to average across both legs, which aren't always equal. I'm pretty certain my left left is weaker than my right, so I won't expect it to read exactly the same as the Kickr.

RChung

This is from here, which was a 2017 check on power meters out in the hands of actual riders. Since this was done a few years ago, there weren't many pedal power meters in the study. Nonetheless, while you may expect them to read the same, you really actually have to check to know for sure.

Doomrider74

If I'm reading that correctly, it doesn't look too complimentary towards the Stages power meters, which seem to have a much wider range of deviation.

Seattle Forrest

I have a guy telling me power is impossible to measure on a bike.

RChung

You're reading that correctly.

To be fair, back when that study was done, I think the Stages may have been the only power meter in that study that was left-side only, so another way to look at these would be a single-sided power meter compared to a bunch of devices that read total power generated from both sides.
I feel your pain. I have guys telling me it's impossible to measure aerodynamic and rolling drag on a bike.

Barry2 

Regarding my PM, I’m a lot more interested in precision than accuracy.
A lot of people believe precision/accuracy are the same.

200 lb Man jumps on a bathroom scale 3 times
250.0001 lb
250.0003 lb
250.0002 lb

That scale is not accurate, but it sure is precise.

I’ll take precision numbers over accurate, after all I don’t want to compare to others, I just need to compare to my prior measurements.

Barry

RChung

You know what's precise? A wristwatch. You'll get very precise numbers over time, and it's a lot cheaper than a power meter.

Barry2 

Sorry… but in my part of the world we have changing wind speed and direction.
But each to their own

Barry

colnago62

lots of riders at the velodrome don’t use a power meter. A stopwatch and heart rate monitor tell them everything they need to know.

Doomrider74

I'm sure that's fine indoors on the velodrome, but aren't there too many variables outdoors to make that a reliable source of data?

RChung

Oh, so you *do* care about accuracy, and not just precision.

colnago62

Yes, outdoors does create some issues. Around here, I train late evening. The wind usually has died down and the velodrome is still holding heat.

Barry2 

Doomrider74

Ive just watched the Shane Llama lab test where he discovers the Stages Left-sided PM doubled is more accurate than the LR combined. Interesting. No such issues with the 4iiii, apparently.

RChung

The problem is with the right side crank, not the left -- more precisely, the problem is with the right side Shimano DuraAce, Ultegra, and 105 cranks. More more precisely, it's with putting a strain gage on the right side Shimano DuraAce, Ultegra, and 105 crank arms. SRAM, FSA, Rotor, and other right side crank arms appear to be okay, as are crank spiders.

But you said you had a 105 crank set. So that's why I said *if* you wanted a left-side-only crank arm power meter, you should probably consider that a non-upgradeable purchase, and don't think about possibly buying the right side crank in the future "as an upgrade." It wouldn't be.

Doomrider74

Understood, thanks. As you say, it doesn't seem worth the money. My only reservation is that my left leg is weaker than my right, so I'd expect the numbers to be lower when compared to Zwift. However, could I use that to my advantage? Would having a left sided only PM help me even out the imbalance? If nothing else, it might help me mentally, by forcing me to lead with my left more. Is that a possibility?

bobin

I want to get a power meter for outdoor rides. Any reason to spend more for a double sided crank when I can just get a new/used one arm Stages/Precision?

smashndash

Leg imbalances exist. They also aren't constant. Depending on your current power output and fatigue level, your imbalance might change. This can make maintaining a certain power level by feel difficult.

That's about it.

RChung

I suppose it's a possibility, but a lot depends on why your left is weaker: some things may be correctable, others may not be. I have a friend who's a cycling coach, pretty well known, who had to have his lower left leg amputated after a cycling accident. After a long and arduous recovery, he was able to *exceed* his pre-amputation FTP although his L/R balance was, obviously, unbalanced. There is no way he can "pedal circles" or "pull up" or "scrape the bottom of the stroke": he'd yank his prosthesis off his stump if he tried any of that. All he can do is stomp with it, so he stomps down as hard as he can, and he only pays attention to his total combined one-and-a-half-leg power. Below the knee amputation, so maybe one-and-two-thirds. A dozen or so years ago, I had a very unpleasant experience with sciatica, and my right leg went dead. I'd already had several years of power data so I didn't need separate L/R data to tell me that my right leg was dead and my pedaling was asymmetric -- my total power dropped and I couldn't control my right foot. Over the course of six or nine months I slowly recovered, and I was able to match my power from before. I wasn't specifically concentrating on strengthening my right leg, I was just riding and walking and trying to get back to where I was. I have no idea whether my L/R power production is symmetric now, and I don't care. All I care about is whether I can achieve my goals with as many legs as I have available.

In a deep sense, if you can go faster and/or achieve your goals, do you really care if it was because of your left leg rather than your entire aerobic system, however it is bilaterally allocated? (I don't have an answer for that -- I think it will vary by individual).

Doomrider74

You may want to look at my current thread, which is about exactly the same thing. It might be of use.

Doomrider74

Makes sense. With me, I'm naturally right foot dominant so there's no physical injury to deal with, but there is a mental bias that convinces me to rely on my right when the power needs to be put down.

Seattle Forrest

Can you give me an example of accurate but not precise?

asgelle

Today’s date is 2021.