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I did wonder if a new power tap hub was due. As they seem to be a little thin on the ground. You are correct about the disc too.
Ok I’ll have a look at those.
Are the accuracies (or in) as bad as they seemingly are made out?
Also does it really matter? Presumably consistency of reading is more important for training.
GoatandTricycle-
Which one is inconstant? From what I gather all those mentioned (P2M, Quarq and Powertap) have a good reputation.
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Do you still have just the one bike? A disc powertap will mean you'll only be able to use that wheel on frames that can accommodate them. Which is fine if you don't plan on n+1, but Powertap pedals would be a good choice if you want to do a turbo session on your TT bike (for example) one day and then go on a long Sunday ride on your disc road bike the next.
spinnnout
pifko
I wouldn't say it has to be much more spendy. Have a look at the P2M NGeco, if you buy spider only it's comparable with some Left only units. Also look at Quarq, the Dzero crank is sub £500 on CRC.
If you do decide on hub, expect a new disc version G4 to be out soon (ok maybe early 2019). I think the current G3 requires proprietary rotors.