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Slight side comment, but I've been using a Quarq since late october and swapping from road bike to TT bike 2-3 times a week. Swapping takes about 2 minutes (can do in 1 but I tend not to turn it into a race). Having decent power and WKO3 is really helping me plan training and on club rides I am way way quicker as a result of going easier on the early parts of hills, meaning I recover and regain any distance dropped with interest in the second half of the climb.
So definite thumbs up to power in general, and there are options out there.
I must admit I've been put off using a power meter up to now because most systems restrict you to pretty much one bike or set of wheels. The Garmin system looks good for ease of switching between bikes, although it's probably more vulnerable to damage in a (very expensive) spill. I hadn't thought about doing that with the Quarq though - I guess if you have compatible bottom brackets on both bikes it's a straightforward switch. Although it wouldn't work on my fixie.
It's an expensive gadget however you do it though.
Second BB for the road bike was $40 IIRC. Ie peanuts.
Regarding the left right reading then be careful. It doesn't work how you think. As it's a unidirectional strain guage, then how does the Garmin work out what direction the force is in. Down, forward, slightly back, even up. And how to account for a down force when your foot is in the rear half of the cycle (ie moving up). So the 45% reading you could in theory get doesn't really tell you the whole story.
Not saying it's a bad system, but think hard about how it will help you.
SRAM 975 is a single bolt on the non drive side. So with a decent ratchet and a long torque wrench it's a doddle and way easier than the vectors will be to swap between bikes.
Hope this doesn't count as spam (I've no connection to them other than they are sponsoring the race I'm putting on) but these guys pedalpowermeters.com specialise in pedal power measurement - might be worth calling them as they should know the pros and cons of the different systems.
Plus if you do and you tell them you saw them sponsoring my race they might sponsor it again next year !
Cougie / FB - a sort of +3.
If you use it as a bike computer then it's a total waste of money. You do need to be a bit geeky, and religiously download and analyse the data. You also need to buy / read Coggan's book and that's possibly the best place to start BEFORE you buy the meter. £15 before £1500. But then as with most people on here, then we spend a bit too much time in front of the PC compared to actually out training, or once you've done a long session then 10 minutes to download and analyse.
It's not right for everyone, and you do need to approach training with a scientific attitude rather than going on feel, but I know I'm quicker on my £700 bike with £1500 power meter than I would be on a £2200 bike. Quicker by a margin approaching 'value' at £1500? Different question...