Weird power meter issue

I’ve experienced very low numbers lately and during my last ride I was sure the power meter was bad. It’s five years old and is single sided (left) 4iiii. It’s been working flawless for around 2500 hours I’d say in all conditions. I did a final test: put it on another bike and all the numbers went back to normal.

I think the issues with the power meter started when I fitted it with a new right crankarm and new chainrings and when I put it back with the crankset it belongs too it works fine.
I have calibrated, zero-offset, changed batteries, factory reset and re-installed and re-torqued the bolts.

It’s a 4iiii power meter on a Shimano 105 crankarm that works with Shimano 105 right crankarm with Ultegra 52/105 36 chainring.
It doesnt really work with a Shimano Ultegra 8100 right crankarm with Rotor 53/39 chainrings.

I panic ordered a new power meter that arrives today which I have second thoughts about now. I was planning to upgrade a lot of other components first so I somewhat hope I can return it.

Have anyone came across this issue, is there a logic explanation?

Are these rotor chainrings circular or ovalized.

There is a known power discrepancy when using ovalized chainrings that alters the numbers slightly as the calculation for the actual chain velocity vs force applied is altered with the different chain path.

This is usully subtle though, a few %, not “very” low.

Personally I’ve never had a problem swapping a crank arm to different crank spiders and getting reliable numbers with a 4iiii.

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Yeah @dfquigley, I think the chainrings are oval since they are Rotor :slight_smile:

If that’s the case @Nicleza, then that’s probably why you are seeing power discrepancies.

Here is a nice article that may be helpful: 4iiii Precision Power Meter In-Depth Review | DC Rainmaker

There’s a section that talks about pedaling style causing accuracy issues, and if the rotors are oval then you may have an answer :slight_smile:

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I use a 4iiiiiiiii with oval MTB rings without noticeable issues. It is very sensitive to torque and how deep it sits on the crank spindle. I’d say it was the specific torque on the PM and the BB / Spacers used.

It’s actually a big round chainring and the small one is oval, which I barely use where I am now.

I don’t get it. Last week I rode in 40 km/h in a crosswind and the power meter said 200 Watts and my body said 320 Watts.
Then I put it on another bike and the wattage feels more to normal even if I’m in a very bad form right now.