Is your 4iiii single or dual?

Maurice

they are both single

Someone else might chime in, sorry no experience with either.

Maurice

Assuming they're both Shimano Left units, yes. They should be very comparable. To know for sure you'll need something to compare them to, either a set of power pedals or a smart trainer you can trust.

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Shane Miller - GPLama
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Mickison,

It would very, very difficult to tell. I think Stages claims +-1-2% and 4iiii claims +-1.5%. So, if they are both telling the truth, the worst case scenario would be 3.5% off. Interestingly your position might cause more of a difference in power creating than the variability in the devices.

I'm always leery of getting into these discussions with triathletes - lots of type A folks who are troubled when two athletes running the same course have GPS readings that are 1/10th of a mile apart (a satellite is the size of a minivan and it's between 1,200 miles and 20,000 miles above the planet but please tell me more about the inaccuracy of your 10k).

Ian

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Ian Murray
http://www.TriathlonTrainingSeries.com
I like the pursuit of mastery
Twitter - @TriCoachIan

One is an FSA and one is Shimano soooo...

I’m just trying to get a sense of if they would give numbers that are way off. I wouldn’t assume so. Ideally I’d get power pedals and use those on both bikes.

Power pedals would be the way to go with different cranks. I have the same type cranks on my tri and road (ultegra/105 same length Shimano) bike so I switch my left side crank arm PM instead of pedals but with different cranks pedals are the best. I found it cheaper to just switch my crank on my second bike to match my power meter than to buy pedals. I am going to test my 4iii to my kinetic inride later this week which I have heard is pretty accurate. I will try to chime in.

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2019 T-Rex Tri Series

A simple way to figure out their difference would be to hang the same weight off the left pedal or crank arm at the 9 o'clock position while holding the right crank arm at the 3 o'clock. Take power measurements from both. Also ensure you have to have both PMs at the same temperature.

BUT be careful about extrapolating this information out too far because to truly understand the differences, you'd need to use multiple weights to get an accurate %difference.

As Ian pointed out though, understanding the differences in PMs just tells you one of the many potential reasons that the power readings may be different.

Justin

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USA Triathlon Level 2 Coach
Slowtwitch Master Coach
Head Coach, TriCoach Colorado, LLC

You mean force? If the cranks aren't moving there is no power.

Sell both of your power estimators, and buy one actual power meter. Do a stomp test to determine it's inherent accuracy.

those are good ideas. Yeah. I definitely understand the power readings can be affected by position, road vs tri, among other things. I'm certainly not looking for perfect just pretty good comparable accuracy. And closer to my races I spend more time on my tri bike and my coach has me do some workouts leading up to races to dial in the goal race power and I always do those on my tri bike.

I'm definitely leaning towards doing that.

You're absolutely correct. There will still be a "power" reading transmitted from the PM because we're deflecting the strain gauge in the crank arm.

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USA Triathlon Level 2 Coach
Slowtwitch Master Coach
Head Coach, TriCoach Colorado, LLC

My power meters all wait for a few pedal strokes before they start sending power. Which kind of makes sense because the power calculation requires a distance measurement (power = (force * distance)/time). In the case of crank power meters the distance is the angular distance of rotation of the crank. Which is why accelerometers or magnets are used to measure cadence. I don't see how there could be a power reading without movement of the crank.