Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

Hmm, I had given up over a week ago..

Go straight to Verve directly for support via here: http://support.vervecycling.com/support/login

I had a similar issue to you and they went above and beyond. I only had to cover postage to Austria for repair. They sent me a set of loan cranks while repair was being carried out.

What are my options for Cannondale Hollowgram cranks? P2M sell a spider - presumably that just means I can use the existing cranks but I'd have to ditch the Spider Rings?

Yes.

You can get a stages arm....

blueeeghhh etc

but yea pretty much everyone makes a spider for them, erm, P2m and SRM at least...

Gotcha. Would the Spider Rings fetch much second hand?

Not really, seem to go for about £80-£100 as unused take offs

Cheers - they are quoting €30 plus postage, but I guess I don't have any other options..

My Power2Max classic has got the death symptoms according to P2M Germany. Solid red light in the battery compartment. 7 years old and they are offering half off a new unit if mine can't be salvaged. Pretty good service.

I've just paid for one of those Magene two-sided ultegra based power meters. Wish me luck.

Thats really good

Finally fixed my power meter woes so I'm shifting some stuff

Rotor 3D30 170mm 110bcd cranks with NDS power meter - good knick, works perfectly and shockingly light (first bit of Rotor kit I've played around with) £225

Quarq DZero 110bcd 5 bolt spider with 170mm BB30A alloy cranks - never been used, comes with full warranty registration bits and soforth. £350

I can also add a Rotor 4130 pressfit BB to get both of those cranks fitted on a BB86 or similar frame for £20. Never fitted, in the packet. They're both the longer axle style, so will fit in wider bottom brackets without a spacer or in a BB30 frame with a spacer.

Based in Hackney / Hatton Garden and happy delivering or posting at cost.

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An odd question... would putting a longer crank arm on a PM impact the power reading?

Scenario: strapped a 170mm Stages Left arm to a Quarq Dzero crank with a 165mm arm. The difference between the two readings was substantial. Could this have influenced the Quarqs reading?

Does the 165mm arm read less?
My understanding is that you need a slightly higher cadence for the same power/speed on a shorter crank, so with a matched cadence, the Stages might read more?
Also as the Quarq assumes the upstroke to approximate the power in the left crank, the fact that this is a longer length might cause some calculation inaccuracy.

You are correct, on a short test there was an approx 17% difference with the Quarq reading consistently lower. LR Balance was close too. Smoothing out the data to 10s averaging, the difference is pretty big, e.g. 267w/301w at the top and 81w/110w at the low end.

I'm just trying to help my other half understand if she has an underlying illness that's coincided with swapping to the Quarq,if the readings are now much lower on it, or a bit of both. I'm not a doctor so I can only help with the PM stuff - I think the Quarq is working OK, presuming the longer left arm may have reduced the numbers and she will just need to do a fresh RAMP test when feeling better.

What are the values you are getting?

The 170mm arm should read slightly higher, due to the increased torque.

Bear in mind however that neither is 100% accurate. Most power meters are +/- 2% , so you could have one with -2% accuracy and one with +2 % accuracy.

Also the stages is only measuring power from one leg - a discrepancy of a couple of percent between legs is also common, sometimes considerably more and typically not consistent - it can very between legs at different power levels and fatigue.

the stages is only measuring power

It's not even doing that. While Stages has the potential to be precise for a given rider who makes no set-up changes, at the fundamental design level there is no possibility of its being accurate.

The 170mm arm should read slightly higher, due to the increased torque.

Wouldn't that be factored into the algorithm?

If you apply the same force to a 165mm lever vs a 170mm lever, the longer lever will generate more torque.
Under normal usage the body just increases the rpm's to compensate, so power remains the same (as power is proportional to torque x rpm).
Given that the 165mm is attached to the 170mm crank in this case the RPM must be the same, so the power would be higher for the 170mm crank.

Hah, true.

Makes far too much sense

If you do want to compare the two units, you should do the test with only the left leg pedalling, then you can ignore the crank length discrepancy and the body's power imbalances and you should only be left with the discrepancy due to each systems accuracy.

I think the route of the problem is that single sided power meters make too many assumptions to be of any real use.