Power meter question?

The older quarqs require (or at least highly recommend) you send in the crank and have them replace the chainrings.  It has to do with the calibration of the unit.  Even if you were successful in swapping out the chainrings, the power it spits out may be unreliable unless you are familiar with the calibration process.  I think the Qalvin app (for iphone/ipad) in conjunction with an ANT+ sensor can be used, but that's beyond my expertise.

I would highly suggest you call Quarq (they have excellent customer service) and ask them how to proceed.  The fact that you removed the outer chainrings means you have some careful work to do even if you want to put the ring back on and have it work properly.  I think it's just a matter of properly tightening down the bolts.

The newer Quarq cranks (Riken and Elsa versions) do not rely on the chainrings to measure power, so you can change those rings without re-calibration.

I'm not sure if it's possible to swap the crank arms.  My first thought is no...but I don't know for sure.

1. Send your Quarq back to them to have it re-calibrated once you get the chainring thing figured out.  Their customer service is top notch and they will send it back almost as soon as they get it.  There is a way to re-calibrate it yourself but it may be cost prohibitive unless you already have an ANT+ dongle to fit into your smart phone and an accurately measured weight.  See this page for self-calibration: http://www.quarq.com/qalvin

2. My understanding is that you can change crank arms if the crank arms are the same make and model and the switch is to change lengths or replace a broken crank arm.  Changing to completely different cranks isn't going to work as the Quarq is attached to a crank specific spider.  That's my understanding.

"Compact" when speaking of cranks is in regard to the chainrings, not the length of the crank arms.  Standard cranks are usually 53/39......compact cranks are 50/34. (number of teeth)

Here ya go:

http://www.stevehoggbikefitting.com/cycling/2011/05/compact-cranks-vs-standard-cranks-gearing-implications/

There is not a correlation of shorter crank arms to smaller rings. There is just what manufactures believe people will by using the standard and compact BCD. There is no reason any crank arm size can't be available in either size BCD. What you are looking to do (as best I can tell?) seems kind of complicated because of the powermeter being built into the crank set. Generally one would sell the whole thing and get the size crank & BCD they wanted. The chainrings do have some limitations as to what will fit, but there are some options for each BCD size.

I called Quarq and they said changing a 53/39 round chainring to a 53/39 asymmetric q ring with only have a difference of 0.6 slope. So It would not need to be recalibrated. Which he said isnt a big enough change in slope. But make sure to tighted the bolts to 10nm if steel bolts and 8nm if aluminum. The power meter I own was discontinued in 2010 and it is not capable of having the cranks changed. The cranks will need to be removed to put on the Q rings which he said can be easy but Im thinking about letting my bike shop take care of that.