Crank based power meter questions

Discuss light weight issues concerning road bikes & parts.
TobinHatesYou
Posts: 3547
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

I recently added a DZero (barely used) to my stable of PMs and the results were interesting upon install. For some reason it took 3-4 rides to fully "settle," which isn't something I'd expect on a spider-based PM where the chainrings haven't been touched since they were taken off the previous owner's bike. Nonetheless, the first time I performed a zero offset on the DZero it returned an offset of 197. I immediately zeroed again, and it returned 192. At the end of the first ride, I zeroed once more and it returned 217. Before the next ride, I got 216. In the middle of that ride I backpedaled 4x and checked the offset again...208. On the 3rd and 4th rides, the offsets were identical at 220 and the power numbers were finally within 1-2W of my Vector 3s.

Very odd. I wonder what was settling exactly, the NDS crankarm + GXP bolt interface??

I did check the torque on the chainring bolts, and everything was plenty tight. I did not check the 8-bolt DS/spider interface, but maybe I should have. Anyway it seems fine now, so it probably wasn't that.

robertbb
Posts: 898
Joined: Thu Jul 23, 2009 3:35 am
Location: Melbourne, Australia

by robertbb

I thought PM's were supposed to return a value of '0' after a successful calibration? (i.e. hence the term zero offset)

by Weenie


TobinHatesYou
Posts: 3547
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

robertbb wrote:
Sun Dec 09, 2018 11:42 pm
I thought PM's were supposed to return a value of '0' after a successful calibration? (i.e. hence the term zero offset)

It depends on the power meter. Some PMs will always display 0. Some PMs will display an actual offset value. Some PMs will just display the real-time torque in newton-meters, which should basically be zero.

In the case of Quarq, it shows an integer up to +/-1000, which corresponds to a static N-m offset. The Qalvin app will show both.

robertbb
Posts: 898
Joined: Thu Jul 23, 2009 3:35 am
Location: Melbourne, Australia

by robertbb

TobinHatesYou wrote:
Sun Dec 09, 2018 11:52 pm
robertbb wrote:
Sun Dec 09, 2018 11:42 pm
I thought PM's were supposed to return a value of '0' after a successful calibration? (i.e. hence the term zero offset)

It depends on the power meter. Some PMs will always display 0. Some PMs will display an actual offset value. Some PMs will just display the real-time torque in newton-meters, which should basically be zero.

In the case of Quarq, it shows an integer up to +/-1000, which corresponds to a static N-m offset. The Qalvin app will show both.
Interesting. The Assioma Duo's always display 0 when they are calibrated without any weight (crank vertical in 12/6 oclock position). They'll show a torque value if, for example, you do calibration process while pedals are weighted...

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