Does swapping round chainrings to oval on SRM require recalibration?

Discuss light weight issues concerning road bikes & parts.
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ninjanoir78
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Joined: Sat Jul 19, 2008 8:14 pm

by ninjanoir78

what do you think? and I guess the accuracy wil be affected?
thanks
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pritchet74
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by pritchet74

Yes, accuracy is effected and no a calibration won't fix it. This has to do with the SRM measuring the forces applied at a fixed distance from teh center of the BB spindle. With non-round chainrings that offset slightly changes. The early studies on the Osymetric chainrings showed increased power output but it was later determined that the rings showed false power increases. There was a guy on a different forum back in 2005 that put a SRM and a PowerTap on the same bike (he calibrated the SRM to show the exact same power as his PowerTap) and did some hillclimb intervals using round rings and the Osymetric rings. His first tests confirmed this.

If you like oval rings then go for it - your power numbers once you switch will be comparable to each other - just don't think that you are putting more power to the bike just because you have oval rings.
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by Weenie


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ms6073
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by ms6073

While power discrepancies have already been explained above, to answer the OP's question, with the latest SRM offerings, switching chainrings does not require calibration, but if you have the ability, a cycle computer that will update the offset (Garmin Edge 520/820/1000/1020/1030 etc), and a calibrated weight, the process takes less than 10-minutes to complete.
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ninjanoir78
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by ninjanoir78

ms6073 wrote:
Mon Mar 12, 2018 4:31 pm
While power discrepancies have already been explained above, to answer the OP's question, with the latest SRM offerings, switching chainrings does not require calibration, but if you have the ability, a cycle computer that will update the offset (Garmin Edge 520/820/1000/1020/1030 etc), and a calibrated weight, the process takes less than 10-minutes to complete.
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Ninja

kwakekeham
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by kwakekeham

Oval rings change both the lever arm as mentioned, but also the rotational velocity. The way a spider based unit works is it senses actual torque so distance of the chainring doesn't matter. This is how they can switch from big to little rings without problem. The source of the problem is that most PM's use a constant velocity assumption for rotation. There is math out there (I think alex's cycling blog has some of it) that show that on average this results in calculatable to a percentage depending on ovality. The inertia, both linear and rotational, makes this constant velocity assumption relatively valid for round chain rings, but by using an oval rings there are predictable variances during the pedal stroke in rotational velocity.

Because the size of the chainring varies, the calibrated weight method will not compensate for the velocity change during the large torque portions. You can still check torque reading using the hanging weight method, but again it won't compensate for cyclic velocity profile caused by the ovality.

Last I had a PM6 open it had a single reed switch, so constant velocity assumption. When I had a PM7 open it had 2 reed switches. However, that is insufficient to compensate. Original Quarqs used 5 but only 1 for timing the rest for direction and possible reed switch death / redundancy. I haven't had an SRM origin open yet, but short of using a gyro it's likely not compensating for the varied velocity and I believe Origin still requires a magnet stuck t the frame somewhere.

by Weenie


TobinHatesYou
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by TobinHatesYou

What kwakekeham said and NOT what pritchet74 said. The shape of the chainrings do not matter, your cranks are still moving around a fixed radius.

It's all about the angular velocity polling rate and very few PMs actually measure more than once per rotation. Verve Infocranks should be able to measure accurately in magnetless mode. PowerTap P1s claim to measure angular velocity 40x per rotation. Quarq claims they will release a non-round chainring compatible firmware...still waiting...

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