Originally Posted by
abhirama
Power meter question: I have a PT but I want to get a crank based sometime in the next couple of months (if i get a good deal) because I'm building up a nice set of training-*** race wheels and would like to have power while riding them. So, when you have a quarq with a certain crank arm length, are you stuck with that? I ask because I thought that was the case until I recently saw that P2M sells spider-only and you can use compatible rings (any) and crank arms (any length I assume). Can you change to different length cranks without sending it back to the factory for calibration with P2M? What about quarq and srm? I know that with quarq you can now change chainrings without recalibrating but crank lengths?
I ask because i can't decide if i like 170s or 172.5s more. I also want to try 175s at some point.
so... a couple of things. you can switch crank arm length for some cranks, but not all. for this you need a design that does not have a captive spider. for example, specialized and hollowgram (cannondale) crank arms come to mind; shimano and FSA cannot.
once you've got a design where the arms are separate from the spider, you then need to worry about calibration. the electronics are contained in the spider. in theory, swapping a spider should not require recalibration.
in my experience, quarq designs (despite the marketing material) have always required a recalibration of slope during any ring change -- not only from one size to another but from one brand to another. the arm change length is another area that should not require the recalibration...but i would not trust a quarq that has not been slope-checked. whether you send that in or do it yourself (it's easy) is another matter.
SRM tends to be less affected by changes in rings. (i always check, but i'm obsessive -- a change from 53/39 -> 56/44 results in a 0.5% change for me...essentially nothing.) arm length never requires recalibration (i swap hollow gram and specialized cranks often for myself and friends/customers).
just my experience, but with quarq there is no way i would change anything about the system without checking slope.
can't speak to P2M as while i've seen them i just haven't used/serviced them extensively. power2max does not allow the end user to check slope, so it's a non-starter for me. i'm also not a fan of the way it handles zero offset and the temperature issues it had in the past. in theory it's good, but i've got some scenarios where i believe it provides data that
may be problematic.